Neutrality remake batch: from-scratch field derivation with project parameters and derived fields
---
title: HVAC Sound Attenuators
category: Mechanical / Air Distribution
−toc_depth: 3
description: >
− When to use: Prefabricated in-duct sound attenuators (duct silencers) that reduce
− airborne fan, equipment, and cross-talk noise propagating through supply, return,
− exhaust, and outdoor-air ductwork in commercial, institutional, healthcare,
− laboratory, performing-arts, and industrial ventilation systems where a mechanical
− engineer or acoustical consultant has set noise criteria (NC/RC) for occupied
− spaces. Covers straight absorptive rectangular and round silencers, elbow
− silencers, splitter inserts, and pod-type cylindrical attenuators characterized by
− dynamic insertion loss, generated noise level, and pressure drop per ASTM E477.
− Not intended for: duct liner and flexible duct wrap (use [[sync/hvac-ductwork]]);
− duct fittings, access doors, and turning vanes (use [[sync/duct-accessories]]);
− grilles, registers, and diffusers (use [[sync/hvac-air-distribution-devices]]);
− VAV terminal units and their integral sound packages (use
− [[sync/variable-air-volume-terminals]]); makeup-air-unit casing noise isolation
− (use [[sync/makeup-air-units]]); equipment vibration isolators and flexible
− connectors; room and envelope acoustic treatments; and active noise cancellation
− systems.
+ When to use: prefabricated in-duct sound attenuators, also called duct silencers, installed in supply, return, exhaust, outdoor-air, and relief ductwork to reduce fan and equipment noise reaching an occupied space, and in transfer ducts to reduce room-to-room cross-talk. This standard establishes the rating basis and test evidence the performance data must carry, the room noise criterion and space sound level estimation procedure the selection answers to, the attenuation credits the selection is allowed to take, the casing and acoustic media construction, the approach and flanking conditions that protect the rated performance in the field, and the post-installation verification that confirms it.
+ Not intended for: duct liner, duct wrap, and externally applied acoustic lagging, which belong to the duct assembly ([[sync/hvac-ductwork]], [[sync/mechanical-insulation]], [[sync/acoustic-insulation]]); dampers, turning vanes, access doors, and other duct-mounted accessories ([[sync/duct-accessories]]); fire dampers, smoke dampers, and combination dampers ([[sync/fire-and-smoke-dampers]]); diffusers, registers, grilles, and linear slot outlets ([[sync/hvac-air-distribution-devices]]); the integral sound package of an air terminal unit ([[sync/air-terminal-units]], [[sync/variable-air-volume-terminals]]); casing-transmitted and breakout noise from air handling units, fans, and makeup air units ([[sync/air-handling-units]], [[sync/hvac-fans]], [[sync/makeup-air-units]]); vibration isolators, flexible connectors, and seismic restraint hardware ([[sync/vibration-isolation-and-seismic-restraint]]); room and envelope acoustic treatment ([[sync/acoustic-wall-panels]], [[sync/acoustic-ceiling-clouds-and-baffles]]); grease-laden kitchen exhaust and particulate-laden collection ductwork ([[sync/kitchen-exhaust-systems]], [[sync/dust-and-fume-collection]]); engine, generator, and combustion exhaust silencers; and active noise cancellation equipment.
---
# Scope {toc}
−## This standard covers prefabricated in-duct sound attenuators, also called duct silencers, installed in HVAC air distribution systems to reduce airborne noise propagating through supply, return, exhaust, and outdoor-air ductwork. {note}
+## This standard covers prefabricated in-duct sound attenuators installed in HVAC air distribution systems to reduce airborne noise travelling through the duct from a fan or other equipment to an occupied space, and to reduce noise travelling between spaces through a shared duct or transfer path. {note}
−## Equipment covered includes straight absorptive (dissipative) silencers in rectangular and round/circular configurations, elbow silencers, splitter inserts, and pod-type cylindrical attenuators. {note}
+## An attenuator is a passive acoustic device with no moving parts and no control function. It dissipates or reflects acoustic energy in the duct while air passes through it, and everything it does is described by three quantities measured together in a laboratory: the dynamic insertion loss it provides in each octave band, the sound power it generates from its own airflow, and the total pressure it costs the fan. {note}
−## A duct silencer is a passive acoustic device, not a mechanical accessory: it attenuates fan and equipment noise by absorbing acoustic energy in fill media as air passes through, and its performance is characterized by three coupled quantities measured per ASTM E477 in laboratory conditions. {note}
+## The three quantities are coupled, and an attenuator specified on one of them alone is specified wrong. Insertion loss rises with length and with narrower passages between baffles; both of those raise pressure loss, and narrower passages raise the air velocity between the baffles, which raises generated sound power roughly as the fifth to sixth power of that velocity. A device that meets an insertion loss target on paper can be the loudest thing in a quiet room. {note}
−### The three performance quantities are dynamic insertion loss (DIL), the reduction in sound power the silencer provides in each octave band; generated noise level (GNL or self-noise), the sound power the silencer itself produces from airflow; and pressure drop, the static pressure the silencer adds to the system at design airflow. {note}
+## This standard applies to new construction and to mechanical renovation on any project where a noise criterion has been established for one or more spaces, whether by the Engineer of Record or by an acoustical consultant. {note}
−## This standard applies to new construction and to major mechanical renovation on projects where a mechanical engineer or acoustical consultant specifies noise criteria (NC or RC levels) for occupied spaces. {note}
+## Per-attenuator quantities live on the schedule, and project-wide policy lives in the datasheet. Airflow, insertion loss, generated sound power, pressure loss, face dimensions, and length differ for every tag on a project and are scheduled by the designer. The rating basis, the criterion metric, the estimation procedure, the credits the selection is allowed to take, and the construction of the device are the same across the project and are selected once in the datasheet. {note}
−## The following are outside the scope of this standard. {note}
+## Attenuators furnished under this standard shall comply with the duct construction, sealing, and leakage requirements of [[sync/hvac-ductwork]] for the duct system in which they are installed, except where this standard states a more stringent requirement.
−- Duct liner materials and flexible duct wrap, which are part of the duct assembly and are covered by [[sync/hvac-ductwork]].
−- Duct fittings, access doors, turning vanes, and other general duct accessories, covered by [[sync/duct-accessories]].
−- Air terminal devices such as grilles, registers, diffusers, and fan-powered mixing boxes, covered by [[sync/hvac-air-distribution-devices]].
−- Variable air volume terminal units and their integral sound packages, covered by [[sync/variable-air-volume-terminals]]; the VAV box specification owns its own attenuator and casing details.
−- Makeup-air-unit fan and casing-transmitted noise isolation, covered by [[sync/makeup-air-units]].
−- Vibration isolators and flexible duct connectors for equipment vibration, which are mechanical isolation accessories rather than acoustic duct devices.
−- Building-envelope and room acoustic treatments such as wall panels and ceiling clouds, which are outside the mechanical scope entirely.
−- Active noise cancellation (ANC) systems, which are niche electronic equipment not covered by standard HVAC mechanical specifications.
+## Where a requirement of this standard and a requirement of [[sync/hvac-ductwork]] address the same subject, the requirement of this standard shall govern.
−## The silencer is selected by the mechanical engineer from noise criteria established by the project's acoustical consultant where one is retained; the engineer translates those criteria into the dynamic insertion loss, generated noise level, and pressure drop values inserted into this specification. {note}
+## The following are outside the scope of this standard: {note}
+- Duct liner, duct wrap, and externally applied acoustic lagging, which are part of the duct assembly and are covered by [[sync/hvac-ductwork]], [[sync/mechanical-insulation]], and [[sync/acoustic-insulation]]
+- Dampers, turning vanes, access doors, flexible connectors, and other duct-mounted accessories, which are covered by [[sync/duct-accessories]]
+- Fire dampers, smoke dampers, and combination fire and smoke dampers, including any such damper installed in a duct that also contains an attenuator, which are covered by [[sync/fire-and-smoke-dampers]]
+- Diffusers, registers, grilles, and linear slot outlets, and the sound they generate at the room boundary, which are covered by [[sync/hvac-air-distribution-devices]]
+- The attenuator section integral to an air terminal unit, which is furnished as part of the unit under [[sync/air-terminal-units]] and [[sync/variable-air-volume-terminals]]
+- Casing-radiated and breakout noise from air handling units, fans, and makeup air units, which is controlled at the source under [[sync/air-handling-units]], [[sync/hvac-fans]], and [[sync/makeup-air-units]]
+- Vibration isolators, flexible duct connectors, and the seismic restraint hardware itself, which are covered by [[sync/vibration-isolation-and-seismic-restraint]]
+- Room-side and envelope acoustic treatment such as wall panels, ceiling clouds, and baffles, which are covered by [[sync/acoustic-wall-panels]] and [[sync/acoustic-ceiling-clouds-and-baffles]]
+- Grease-laden kitchen exhaust ductwork and particulate-laden collection ductwork, which carry construction and cleaning requirements this standard does not address and are covered by [[sync/kitchen-exhaust-systems]] and [[sync/dust-and-fume-collection]]
+- Engine, generator, boiler, and other combustion exhaust silencers, which are pressure vessels of a different construction class rated by a different method
+- Active noise cancellation equipment, which is powered electronic apparatus rather than a passive duct device
+
# Referenced Standards {toc}
…4 unchanged lines
| Standard | Title |
|----------|-------|
−| ASTM E477-20 | Laboratory Measurements of Acoustical and Airflow Performance of Duct Liner Materials and Prefabricated Silencers |
−| ASHRAE Handbook — HVAC Applications, Ch. 49 (2023) | Noise and Vibration Control |
−| SMACNA HVAC Duct Construction Standards, 3rd Ed. | HVAC Duct Construction Standards — Metal and Flexible |
−| NFPA 90A-2021 | Standard for the Installation of Air-Conditioning and Ventilating Systems |
−| ASTM E84-24 | Standard Test Method for Surface Burning Characteristics of Building Materials |
−| UL 181-2019 | Factory-Made Air Ducts and Air Connectors |
−| ISO 7235:2003 | Acoustics — Laboratory Measurement Procedures for Ducted Silencers and Air-Terminal Units — Insertion Loss, Flow Noise and Total Pressure Loss |
+| ASTM E477 | Measuring Acoustical and Airflow Performance of Duct Liner Materials and Prefabricated Silencers |
+| ASTM E84 | Surface Burning Characteristics of Building Materials |
+| ASTM C1071 | Fibrous Glass Duct Lining Insulation (Thermal and Sound Absorbing Material) |
+| ASTM C1338 | Determining Fungi Resistance of Insulation Materials and Facings |
+| ASTM A653 | Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process |
+| ASTM A463 | Steel Sheet, Aluminum-Coated, by the Hot-Dip Process |
+| ASTM A240 | Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications |
+| ASTM B209 | Aluminum and Aluminum-Alloy Sheet and Plate |
+| ANSI/AHRI 885 | Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets |
+| ANSI/ASA S12.2 | Criteria for Evaluating Room Noise |
+| ANSI/ASHRAE/IES 90.1 | Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings |
+| ASHRAE Handbook — HVAC Applications | Noise and Vibration Control |
+| ASCE/SEI 7 | Minimum Design Loads and Associated Criteria for Buildings and Other Structures |
+| ISO 7235 | Acoustics — Laboratory Measurement Procedures for Ducted Silencers and Air-Terminal Units |
+| ISO/IEC 17025 | General Requirements for the Competence of Testing and Calibration Laboratories |
+| NFPA 90A | Installation of Air-Conditioning and Ventilating Systems |
+| NFPA 90B | Installation of Warm Air Heating and Air-Conditioning Systems |
+| NIST Handbook 150 | NVLAP Procedures and General Requirements |
+| SMACNA HVAC Duct Construction Standards | Metal and Flexible |
+| SMACNA HVAC Air Duct Leakage Test Manual | HVAC Air Duct Leakage Test Manual |
+| UL 723 | Test for Surface Burning Characteristics of Building Materials |
−## ASTM E477 is the primary governing test method for North American prefabricated silencers, defining how dynamic insertion loss, generated noise level, and pressure drop are measured and reported across octave bands; ISO 7235 is the international counterpart and may be cited in lieu of or alongside ASTM E477 on projects with international design teams or equipment sourced outside North America. {note}
+## ASTM E477 and ISO 7235 both measure insertion loss, flow noise, and pressure loss on a ducted silencer, and both are in commercial use, but they do not use identical terminations, source conditions, or reporting bandwidths, so numbers from the two methods are not interchangeable band for band. Where equipment is sourced from more than one market, holding every bidder to a single method keeps the comparison honest. {note}
# Submittals {toc}
−## The Contractor shall submit the following Action Submittals for review prior to fabrication:
+## Performance and Product Submittals {toc}
−- Product data for each silencer type, model, and size, identifying configuration, casing material and gauge, fill media, and pressure class.
−- Acoustical performance data: dynamic insertion loss, generated noise level, and pressure drop in each octave band from 63 Hz through 8000 Hz, for both forward-flow and reverse-flow conditions where applicable.
−- Test reports from a NVLAP-accredited laboratory documenting ASTM E477 testing for each silencer model proposed.
−- Shop drawings showing silencer dimensions, active length, connection details, baffle arrangement, and orientation relative to airflow.
−- A schedule cross-referencing each silencer mark to its scheduled DIL, GNL, and pressure-drop requirements.
+### The Contractor shall submit the following for the Engineer of Record's review and return, and no attenuator shall be released for fabrication until the submittal covering it has been reviewed and returned:
+- Product data for each attenuator model furnished, identifying the configuration, the casing material and sheet thickness, the acoustic media class, the perforated facing, and the pressure class
+- Octave-band dynamic insertion loss, generated sound power level, and total pressure loss for each model, size, and length furnished, at the scheduled airflow, in the 63 Hz through 8000 Hz bands
+- The same three quantities under reverse flow for every attenuator serving a return, exhaust, or relief duct
+- The space sound level calculation for each space carrying a scheduled noise criterion, showing the source sound power, every attenuation term taken, and the resulting octave-band sound pressure level
+- The air density correction applied to the rated pressure loss and generated sound power, stating the site elevation used
+- Shop drawings showing face dimensions, overall and acoustically active length, baffle arrangement, connection details, weight, and the rated flow direction
+
```datasheet
−label: Action Submittals
+label: Performance and Product Submittals Required
type: checkbox
options:
− - Product data (type, size, casing, fill, pressure class)
− - Octave-band DIL, GNL, and pressure drop (forward and reverse flow)
− - NVLAP-accredited ASTM E477 test reports
− - Shop drawings (dimensions, active length, connections, orientation)
− - Silencer schedule cross-referenced to required performance
+ - Product data for each model furnished
+ - Octave-band insertion loss, generated sound power, and pressure loss
+ - Reverse-flow performance for return, exhaust, and relief attenuators
+ - Space sound level calculation for each scheduled criterion
+ - Air density correction record
+ - Shop drawings with dimensions, weight, and flow direction
default:
− - "Product data (type, size, casing, fill, pressure class)"
− - "Octave-band DIL, GNL, and pressure drop (forward and reverse flow)"
− - "NVLAP-accredited ASTM E477 test reports"
− - "Shop drawings (dimensions, active length, connections, orientation)"
+ - Product data for each model furnished
+ - Octave-band insertion loss, generated sound power, and pressure loss
+ - Reverse-flow performance for return, exhaust, and relief attenuators
+ - Space sound level calculation for each scheduled criterion
+ - Shop drawings with dimensions, weight, and flow direction
```
−## The Contractor shall submit the following Closeout Submittals prior to Substantial Completion:
+## Test Evidence Submittals {toc}
−- Operation and maintenance data describing inspection, cleaning, and fill-media replacement intervals.
−- Record documentation reflecting as-installed silencer marks, locations, and orientations.
+### The Contractor shall submit the following with the performance data they support:
+- The laboratory test report for each model and length furnished, identifying the test method, the test date, the specimen size, and the airflow rates tested
+- Evidence of the testing laboratory's accreditation, stating the accrediting body and the accreditation number, and showing that the accreditation scope covers the test method used
+- Where performance for a furnished length is interpolated rather than measured, identification of the tested lengths that bracket it and the published interpolation basis
+- Surface burning characteristic test data for every material exposed to the airstream, reporting the flame spread index and the smoke developed index
+- Fungal resistance test data for fibrous acoustic media exposed to the airstream
+
```datasheet
−label: Closeout Submittals
+label: Test Evidence Submittals Required
type: checkbox
options:
− - Operation and maintenance data
− - As-installed record documentation
−default: [Operation and maintenance data, As-installed record documentation]
+ - Laboratory test report for each model and length
+ - Laboratory accreditation body, number, and scope
+ - Interpolation basis and bracketing tested lengths
+ - Flame spread and smoke developed index data
+ - Fungal resistance test data
+default:
+ - Laboratory test report for each model and length
+ - Laboratory accreditation body, number, and scope
+ - Flame spread and smoke developed index data
```
−## The Contractor shall submit the following Informational Submittals for projects where independent verification is required:
+## Closeout Submittals {toc}
−- Statement of NVLAP accreditation for the test laboratory, including accreditation number and scope.
−- Manufacturer's certification that exposed fill media meet NFPA 90A flame-spread and smoke-developed requirements.
+### The Contractor shall submit the following before the date of Substantial Completion:
+- Record documentation identifying the as-installed tag, model, size, length, and orientation of every attenuator, and the as-installed flow direction of each
+- Operation and maintenance data covering inspection, cleaning, and media replacement, including the manufacturer's stated cleaning methods and the methods that void the warranty
+- The post-installation sound level verification report, where verification is required
+- Manufacturer's written warranty, executed and dated
+
```datasheet
−label: Informational Submittals
+label: Closeout Submittals Required
type: checkbox
options:
− - NVLAP accreditation statement (number and scope)
− - NFPA 90A flame-spread / smoke-developed certification
−default: [NFPA 90A flame-spread / smoke-developed certification]
+ - As-installed record documentation
+ - Operation and maintenance data with permitted cleaning methods
+ - Post-installation sound level verification report
+ - Executed manufacturer's warranty
+default:
+ - As-installed record documentation
+ - Operation and maintenance data with permitted cleaning methods
+ - Executed manufacturer's warranty
```
−# Quality Assurance {toc}
+# Rating Basis and Test Evidence {toc}
−## Acoustical performance data submitted for each silencer model shall originate from testing performed in accordance with ASTM E477.
+## Acoustical Rating Test Method {toc}
−## Test data shall be produced by a laboratory accredited under the National Voluntary Laboratory Accreditation Program (NVLAP) for acoustical silencer testing.
+### Insertion loss is a comparison, not an absolute property. The laboratory measures the sound transmitted through a duct run with the attenuator in place and again with a plain duct of the same length substituted for it, and the difference is the insertion loss. Because it is measured with air flowing at a stated velocity, the reported value already contains the effect of flow on the attenuation, which is why it is called dynamic insertion loss. {note}
−## NVLAP accreditation of the test facility is the controlling quality lever for this equipment: it distinguishes independently verified performance from catalog values that may be extrapolated or measured at non-accredited facilities, a risk that is most acute for short 3 ft (900 mm) silencer lengths. {note}
+### Rated acoustical and airflow performance shall be established by laboratory measurement in accordance with the test method indicated in the datasheet.
−## Where the specification permits substitution, the Contractor shall submit NVLAP-accredited third-party test reports for the proposed alternate; manufacturer-published data from a non-accredited source shall not be accepted as the sole basis for substitution.
−
```datasheet
−label: Acceptance Basis for Performance Data
−type: radio
+label: Acoustical Rating Test Method
+type: select
options:
− - NVLAP-accredited third-party test report required for all manufacturers
− - Manufacturer-published NVLAP data accepted for named manufacturers; third-party report required for substitutions
− - Manufacturer-published data accepted
−default: Manufacturer-published NVLAP data accepted for named manufacturers; third-party report required for substitutions
+ - ASTM E477
+ - ISO 7235
+ - ASTM E477 or ISO 7235, either accepted
+default: ASTM E477
```
−## Each silencer shall be tested in both forward-flow and reverse-flow directions where the duct system it serves carries noise opposite to the airflow direction, such as return and exhaust runs.
+### Performance shall be measured at not fewer than three airflow rates spanning the model's published operating range, and the reported values shall be those measured at the airflow closest to the scheduled airflow or interpolated between the two measured rates that bracket it.
−## The manufacturer shall be a firm with a minimum of five years' experience producing prefabricated duct silencers with full ASTM E477 NVLAP-accredited test data.
+### Calculated, scaled, and analytically modelled performance shall not be submitted in place of measured performance for any model furnished.
−# Environmental and Service Conditions {toc}
+## Test Laboratory Accreditation {toc}
−## The silencer casing material shall be selected for the service environment of the duct system it serves; standard galvanized steel is suitable for typical conditioned-air systems but corrodes in moist, chemically aggressive, or sanitary environments.
+### Accreditation is what separates a measurement from a claim. An accredited laboratory has demonstrated to a third party that its test chamber, instrumentation, and procedure conform to the method, and it is subject to periodic reassessment. Accreditation says nothing about who owns the laboratory, and several attenuator manufacturers operate accredited chambers of their own. {note}
−### Natatorium, kitchen exhaust, laboratory fume exhaust, coastal outdoor-air, and similar corrosive duct runs require stainless steel or aluminum casing; specifying standard galvanized casing in these environments is a common and costly design omission. {note}
+### Performance data shall originate from a laboratory holding the accreditation indicated in the datasheet for the test method used.
−## The silencer shall withstand the static pressure of the adjoining duct without panel deflection, leakage, or distortion at the system design pressure.
+```datasheet
+label: Test Laboratory Accreditation Basis
+type: radio
+options:
+ - Accredited laboratory under NVLAP or ISO/IEC 17025, manufacturer-owned or independent
+ - Independently owned accredited laboratory only, no manufacturer-owned facility
+ - Non-accredited laboratory data accepted where the test method and apparatus are documented
+default: Accredited laboratory under NVLAP or ISO/IEC 17025, manufacturer-owned or independent
+```
−## Casing material shall be selected from the following based on the service environment.
+### Each test report shall state the accrediting body, the accreditation number, and the date the accreditation is current through.
+### Where the accreditation scope does not cover the test method used for a submitted report, that report shall not be accepted as evidence of rated performance.
+
+## Tested Length and Model Correspondence {toc}
+
+### Attenuator catalogs commonly publish a continuous family of lengths from a smaller set of tested specimens, filling the gaps by interpolation. The error this introduces is smallest in the middle of a tested family and largest at its ends, and it is largest of all in the low-frequency bands of the shortest lengths, where the physical specimen is only a few wavelengths deep and the measured curve changes quickly with length. {note}
+
+### The correspondence between the performance data submitted and the specimens actually tested shall be as indicated in the datasheet.
+
```datasheet
−label: Casing Material
+label: Rated Length and Model Correspondence
type: radio
options:
− - Galvanized steel (G90), standard conditioned air
− - Aluminum, lightweight or mildly corrosive service
− - Stainless steel 304, corrosive or sanitary service
− - Stainless steel 316, severe corrosive or coastal service
−default: Galvanized steel (G90), standard conditioned air
+ - Measured test data for every model and length furnished
+ - Measured test data for the model family with published interpolation between tested lengths
+ - Measured test data for the model family with published interpolation and extrapolation beyond tested lengths
```
−## The silencer casing pressure class shall match the static-pressure class of the adjoining duct run.
+### Where interpolated performance is submitted, the manufacturer shall identify the two tested lengths that bracket the furnished length and shall state the interpolation basis.
+### Where the parties disagree whether a submitted data set corresponds to the model and length furnished, the Engineer of Record shall make the initial determination.
+
+## Manufacturer Qualification {toc}
+
+### The manufacturer shall maintain published octave-band insertion loss, generated sound power level, and pressure loss data covering every model, size, and length furnished under this standard.
+
+### The manufacturer shall furnish the test report underlying any published value for a model furnished under this standard within fourteen days of the Engineer of Record's written request for it.
+
+# Acoustic Criteria and Selection Basis {toc}
+
+## Room Noise Criterion {toc}
+
+### A room noise criterion is a single number naming a family of octave-band sound pressure limits, so that a design target for a space can be written as one value instead of eight. The metrics in use differ in what they penalize: the Noise Criterion family compares the measured spectrum against tangent curves and reports the highest curve touched, while the Room Criterion Mark II family fits a reference slope and adds a separate quality descriptor identifying a rumbly or hissy spectrum that the single number alone would hide. Where low-frequency rumble is the complaint a project is trying to avoid, a metric that reports spectrum quality separately makes that outcome visible before construction. {note}
+
+### The room noise criterion metric applicable to this project shall be as indicated in the datasheet.
+
```datasheet
−label: Casing Pressure Class
+label: Room Noise Criterion Metric
type: select
options:
− - 1/2 in. w.g.
− - 1 in. w.g.
− - 2 in. w.g.
− - 3 in. w.g.
− - 4 in. w.g.
− - 6 in. w.g.
−unit: in. w.g.
−default: 2 in. w.g.
+ - Noise Criterion, NC
+ - Room Criterion Mark II, RC Mark II
+ - Balanced Noise Criterion, NCB
+ - A-weighted sound pressure level, dBA
+default: Noise Criterion, NC
```
−## The operating temperature range of the silencer and its fill media shall accommodate the design airstream temperature of the system it serves.
+### The criterion value applicable to each space served shall be as indicated in [[drawing: the contract documents]].
+### A criterion applies to the total sound from the mechanical systems serving the space with all of them operating at design airflow, not to the contribution of any single attenuator, duct run, or terminal device. Meeting it is a property of the whole path from the fan to the room, and an attenuator selection is only one term in it. {note}
+
+## Space Sound Level Estimation {toc}
+
+### The space sound level for each space carrying a scheduled criterion shall be estimated in accordance with the procedure indicated in the datasheet.
+
```datasheet
−label: Airstream Temperature Range
−type: range
−min: -20
−max: 250
−step: 5
−unit: °F
−setpoints: [-20, 40, 105]
+label: Space Sound Level Estimation Procedure
+type: select
+options:
+ - ANSI/AHRI 885
+ - ASHRAE Handbook HVAC Applications, Noise and Vibration Control
+ - Manufacturer's published calculation program with the calculation basis submitted
+default: ANSI/AHRI 885
```
−# Acoustic Performance {toc}
+### The estimate shall begin from the sound power level of the source at the point where it enters the duct system, in each octave band from 63 Hz through 8000 Hz.
−## Acoustic performance is the reason the silencer exists, and it is defined by three coupled quantities that must all be specified together: dynamic insertion loss, generated noise level, and pressure drop. Specifying any one in isolation produces an undersized, noisy, or energy-wasting selection. {note}
+### The estimate shall include the generated sound power of the attenuator itself and of every downstream device that generates sound in the airstream.
−## Dynamic Insertion Loss {toc}
+### The estimate shall be performed for each octave band separately and shall not be reduced to a single-number comparison before the criterion is applied.
−### Dynamic insertion loss (DIL) is the reduction in transmitted sound power, in decibels, that the silencer provides in each octave band under design airflow; it is the silencer's core rated capability. {note}
+### An estimate that stops at the attenuator outlet has answered a different question than the one the criterion asks. What reaches the listener is the sound power that leaves the attenuator, less whatever the remaining duct run takes out of it, plus whatever the terminal device adds back, spread into the room by its absorption. Every one of those terms can be larger than the difference between two candidate attenuators. {note}
−### Fan and VAV system noise is predominantly low-frequency, concentrated in the 63 Hz through 250 Hz octave bands; specifying strong mid- and high-frequency DIL while neglecting the low bands produces a silencer that is undersized for the actual noise problem. {note}
+## Attenuation Credits {toc}
−### The silencer shall provide the scheduled dynamic insertion loss in each octave band from 63 Hz through 8000 Hz at the design airflow, measured per ASTM E477.
+### Attenuation credits are the terms in the estimate that reduce sound without an attenuator doing the work. Each is real, and each carries its own uncertainty: {note}
−### For return and exhaust silencers, the silencer shall provide the scheduled dynamic insertion loss under reverse-flow conditions as well as forward-flow conditions.
+- End reflection loss is the low-frequency energy reflected back up the duct where the duct discharges into the much larger room, and it is significant only at the low bands and only for small terminations
+- Sound power division at a branch takeoff is the share of the power that goes down the other branch, and it depends on the airflow split actually achieved rather than the one scheduled
+- Elbow and fitting attenuation is the loss at turns and transitions in the run downstream of the attenuator, which varies with whether those fittings are lined
+- Room absorption is the difference between the sound power arriving in the space and the sound pressure at the listener, which depends on the finishes and furnishings actually installed
+- Duct breakout transmission loss is the energy that leaves the duct through its walls before reaching the outlet, which reduces the level at the outlet while raising it wherever the duct passes
−```datasheet
−label: Required Dynamic Insertion Loss — 63 Hz
−type: range
−min: 0
−max: 30
−step: 1
−unit: dB
−setpoints: [6, 12]
−```
+### The attenuation credits the selection is permitted to take shall be as indicated in the datasheet.
```datasheet
−label: Required Dynamic Insertion Loss — 125 Hz
−type: range
−min: 0
−max: 40
−step: 1
−unit: dB
−setpoints: [10, 15]
+label: Attenuation Credits Permitted in the Selection
+type: checkbox
+options:
+ - End reflection loss at the room termination
+ - Sound power division at branch takeoffs
+ - Elbow and fitting attenuation downstream of the attenuator
+ - Room absorption effect at the receiver
+ - Duct breakout transmission loss along the run
+default:
+ - End reflection loss at the room termination
+ - Sound power division at branch takeoffs
+ - Room absorption effect at the receiver
```
−```datasheet
−label: Required Dynamic Insertion Loss — 250 Hz
−type: range
−min: 0
−max: 45
−step: 1
−unit: dB
−setpoints: [15, 25, 45]
−```
+### An attenuation credit not indicated in the datasheet shall not be taken in the space sound level estimate.
−```datasheet
−label: Required Dynamic Insertion Loss — 500 Hz
−type: range
−min: 0
−max: 50
−step: 1
−unit: dB
−setpoints: [25, 40, 50]
−```
+### Each credit taken shall be shown as a separate line in the estimate with the value taken in each octave band.
−```datasheet
−label: Required Dynamic Insertion Loss — 1000 Hz
−type: range
−min: 0
−max: 50
−step: 1
−unit: dB
−setpoints: [30, 45, 50]
−```
+### Taking breakout transmission loss as a credit at the outlet also asserts that the energy left the duct somewhere along its run, so it reduces the calculated level in the served space while raising the level in whatever space the duct passes through. Where the duct runs above a ceiling in an occupied area, the same energy has been moved rather than removed. {note}
−```datasheet
−label: Required Dynamic Insertion Loss — 2000 Hz
−type: range
−min: 0
−max: 50
−step: 1
−unit: dB
−setpoints: [30, 45, 50]
−```
+## Generated Sound Power Limit Basis {toc}
−```datasheet
−label: Required Dynamic Insertion Loss — 4000 Hz
−type: range
−min: 0
−max: 45
−step: 1
−unit: dB
−setpoints: [20, 35, 45]
−```
+### Generated sound power, also called self-noise, is the sound the attenuator makes from its own airflow: the air accelerates through the narrow passages between the baffles, separates at the baffle noses and tails, and radiates broadband noise downstream. It rises very steeply with the velocity between the baffles, so a device made more effective by narrowing its passages becomes louder at the same time. In a quiet space at high face velocity the self-noise can be the level that governs. {note}
+### The basis on which the generated sound power of each attenuator is limited shall be as indicated in the datasheet.
+
```datasheet
−label: Required Dynamic Insertion Loss — 8000 Hz
−type: range
−min: 0
−max: 35
−step: 1
−unit: dB
−setpoints: [12, 25, 35]
+label: Generated Sound Power Level Limit Basis
+type: radio
+options:
+ - Octave-band sound power level scheduled for each attenuator tag
+ - Contribution to the served space at or below the scheduled criterion, computed by the selected estimation procedure
+ - A stated margin below the residual system sound power at the attenuator outlet
+default: Octave-band sound power level scheduled for each attenuator tag
```
−### The flow direction for which the scheduled DIL applies shall be identified.
+### The generated sound power shall be evaluated at the scheduled airflow through the attenuator's own face dimensions, not at the velocity in the adjoining duct.
+### Where an attenuator is larger in cross-section than the duct it serves, the velocity through it is lower than the duct velocity and its self-noise is correspondingly lower, but the transitions that make the change add pressure loss and can add their own noise. Face area is the variable that trades self-noise and pressure loss against space and first cost. {note}
+
+# Scheduled Performance {toc}
+
+## Each attenuator shall comply with the airflow, dynamic insertion loss, generated sound power level, total pressure loss, face dimensions, and overall length scheduled for its tag on [[drawing: the sound attenuator schedule]].
+
+## The scheduled dynamic insertion loss shall be provided in every octave band from 63 Hz through 8000 Hz at the scheduled airflow.
+
+## The scheduled generated sound power level shall not be exceeded in any octave band from 63 Hz through 8000 Hz at the scheduled airflow.
+
+## The scheduled total pressure loss shall not be exceeded at the scheduled airflow, and the reported loss shall include the entrance and exit losses at the attenuator's own face dimensions.
+
+## Attenuators serving a return, exhaust, or relief duct shall meet the scheduled insertion loss, generated sound power level, and total pressure loss with the airflow in the reverse direction.
+
+## Fan noise in a return or exhaust duct travels toward the fan while the air travels away from it, so the sound and the flow move in opposite directions. Baffle noses shaped to meet the flow gently in one direction meet it abruptly in the other, and the insertion loss and pressure loss both change. A forward-flow rating applied to a return attenuator describes a condition the device is not in. {note}
+
+## The rated total pressure loss and generated sound power level shall be corrected for the air density at [[parameter: altitude]] using the manufacturer's published correction factors.
+
+## The scheduled total pressure loss of every attenuator shall be included in the fan system pressure used to demonstrate compliance with the fan power limits of [[parameter: adopted-energy-code]].
+
+## An attenuator's pressure loss is paid by the fan for every hour the system runs, and it is paid whether or not anyone reserved it. Where the loss was not carried in the fan schedule, the shortfall surfaces during construction as a fan that cannot make its airflow, and the choices left are all expensive. Where it is carried, it is visible in the fan power calculation while the selection can still be changed. {note}
+
+## The Contractor shall verify, before releasing any attenuator for fabrication, that the scheduled face dimensions and overall length fit the space provided, clear of structure, piping, conduit, and the approach and discharge lengths required by this standard.
+
+## The Contractor shall report any conflict found by that verification to the Engineer of Record before releasing the affected attenuator for fabrication.
+
+# Airstream Service Conditions {toc}
+
+## The attenuator, its acoustic media, its facing, and its casing shall be rated by the manufacturer for continuous service at the design airstream temperature of the system in which it is installed.
+
+## Acoustic media sealed inside a polymer film barrier carries a lower continuous temperature limit than the same media used bare, because the film softens and loses its seal well below the temperature the fibrous media itself tolerates. Where an airstream runs hot, that limit, and not the media, is what bounds the choice. {note}
+
+## The casing, the perforated facing, the media barrier, and every fastener in the airstream shall be resistant to the moisture, condensate, and chemical constituents of the airstream in which they are installed.
+
+## Attenuator interiors stay wet longer than the duct around them, because the media holds moisture and the perforated facing slows it drying. Natatorium exhaust, laboratory exhaust, coastal outdoor-air intakes, and any duct that runs below its dew point in service are the conditions in which a casing selected for conditioned air corrodes from the inside, where nobody sees it until the performance is already gone. {note}
+
+## The attenuator shall withstand the greater of the design static pressure of the duct system and the fan's shutoff pressure without permanent deformation of the casing, separation of a seam, or displacement of the acoustic media.
+
+## Shutoff pressure, not design pressure, is what a duct system sees when a fan starts against a closed damper or when a filter bank loads to the point of collapse. It is a condition every duct system experiences, and it is higher than the pressure the system was sized for. {note}
+
+# Attenuator Configurations {toc}
+
+## Attenuator configurations differ in how they fit rather than in what they do. Each of the following is a distinct family: {note}
+
+- A rectangular straight attenuator holds parallel splitter baffles across the duct, and it is the family with the widest published performance range because both baffle thickness and passage width can be varied
+- A rectangular straight attenuator with a lined casing and no splitter baffles has a much lower pressure loss and much less low-frequency attenuation, since the only absorbing surface is the wall
+- A round straight attenuator holds an annular layer of media inside a cylindrical casing, connecting directly to round duct without transitions
+- A round straight attenuator with a center body adds an absorbing pod on the axis, which narrows the annular passage and raises both attenuation and pressure loss
+- A rectangular elbow attenuator combines a turn with attenuation in the space a plain elbow would occupy, taking advantage of the fact that sound striking the outer wall of a turn is already partly absorbed there
+- A round elbow attenuator does the same for round duct
+- A plenum attenuator is a lined expansion chamber that attenuates by absorption and by the abrupt area change at inlet and outlet, and it is the family that performs best at low frequency for a given length
+- A transfer-duct attenuator is a short unit serving a room-to-room path at low or zero airflow rather than a fan noise path
+
+## The attenuator configurations permitted on this project shall be as indicated in the datasheet.
+
```datasheet
−label: Rated Flow Direction
−type: radio
+label: Attenuator Configurations Permitted
+type: checkbox
options:
− - Forward flow only (supply)
− - Reverse flow only (return/exhaust)
− - Both forward and reverse flow
−default: Forward flow only (supply)
+ - Rectangular straight with parallel splitter baffles
+ - Rectangular straight with a lined casing and no splitter baffles
+ - Round straight with annular media
+ - Round straight with annular media and a center body
+ - Rectangular elbow with absorptive turning baffles
+ - Round elbow
+ - Plenum attenuator
+ - Transfer-duct attenuator
+default:
+ - Rectangular straight with parallel splitter baffles
+ - Round straight with annular media
+ - Round straight with annular media and a center body
+ - Rectangular elbow with absorptive turning baffles
+ - Transfer-duct attenuator
```
−## Generated Noise Level {toc}
+## The configuration furnished for each tag shall match the shape of the duct it connects to and shall fit within the space shown for it without altering the routing of the duct system.
−### Generated noise level (GNL), also called self-noise, is the sound power the silencer itself produces from air passing through its baffles; a silencer can meet its insertion-loss target yet become the dominant noise source in a quiet space if its self-noise is uncontrolled. {note}
+## A configuration other than the one scheduled for a tag shall not be furnished unless the Engineer of Record accepts in writing a revised space sound level estimate covering every space that attenuator serves.
−### Specifying required insertion loss without also specifying a maximum generated noise level is the single most common error in silencer specification; at high face velocities the self-noise can exceed the residual fan noise the silencer was installed to suppress. {note}
+# Casing Construction {toc}
−### The generated noise level of the silencer at the design face velocity shall not exceed the scheduled sound power level in any octave band.
+## Casing Material {toc}
−### For return and exhaust silencers, the generated noise level under reverse-flow conditions shall not exceed the scheduled limit.
+### The casing material shall be as indicated in the datasheet.
```datasheet
−label: Maximum Generated Noise Level Target
−type: radio
+label: Casing Material
+type: select
options:
− - NC-25 equivalent (noise-critical spaces)
− - NC-30 equivalent
− - NC-35 equivalent (general occupied space)
− - NC-40 equivalent (back-of-house / industrial)
−default: NC-35 equivalent (general occupied space)
+ - Galvanized steel
+ - Aluminized steel
+ - Aluminum
+ - Stainless steel Type 304
+ - Stainless steel Type 316
+ - Carbon steel with a factory-applied corrosion-resistant coating
+default: Galvanized steel
```
−## Pressure Drop {toc}
+### Galvanized steel casings shall conform to ASTM A653 with a G90 coating designation.
−### Pressure drop is the static pressure the silencer adds to the system at design airflow; it is paid for by the fan in energy and must be reserved in the fan schedule.
+### Aluminized steel casings shall conform to ASTM A463 Type 2.
−### Specifying a long, narrow-baffle, high-performance silencer without reserving adequate static pressure in the fan selection forces fan re-selection during construction and generates avoidable requests for information; the pressure-drop budget must be coordinated with the mechanical schedule at design.
+### Aluminum casings shall conform to ASTM B209.
−### The total pressure loss across the silencer at the design airflow and face velocity shall not exceed the scheduled maximum.
+### Stainless steel casings shall conform to ASTM A240 in the type indicated in the datasheet.
+### Carbon steel casings with a factory-applied coating shall carry that coating on the interior and exterior of the casing, on the perforated facing, and on every fastener exposed to the airstream, and the manufacturer shall identify the coating and its cured thickness in the submittal.
+
+### Cut edges, welds, and fastener penetrations made after coating or galvanizing shall be repaired with a coating compatible with the casing finish.
+
+## Casing Pressure Class and Sheet Thickness {toc}
+
+### The casing pressure class shall be as indicated in the datasheet.
+
```datasheet
−label: Maximum Total Pressure Drop
+label: Casing Static Pressure Class
type: range
−min: 0.05
−max: 0.50
−step: 0.01
+derived: "the static pressure class of the duct system in which the attenuator is installed"
unit: in. w.g.
−setpoints: [0.05, 0.1, 0.15]
+min: 0.5
+max: 10
+setpoints: [0.5, 1, 2, 3, 4, 6, 10]
+default: derived
```
−## Face Velocity {toc}
+### The casing shall be constructed to the sheet thickness, reinforcement, and transverse joint requirements of the SMACNA HVAC Duct Construction Standards for the selected pressure class and the attenuator's face dimensions.
−### Face velocity, the air velocity entering the silencer cross-section, is the dominant driver of both self-noise and pressure drop; lowering it reduces both at the cost of a larger silencer cross-section. {note}
+### The casing of an attenuator installed in a duct system of a higher pressure class than the attenuator is built to becomes the weakest section of that run, and it deflects, leaks, and drums at exactly the point the design placed there to make the system quiet. Matching the class costs sheet thickness and nothing else. {note}
−### General supply and return silencers are selected at 1000 to 1500 FPM face velocity; noise-critical spaces such as recording studios, executive conference rooms, and performance venues require 500 to 800 FPM to suppress self-noise to NC-25 or quieter. {note}
+## Casing Sealing and Leakage {toc}
−### The silencer shall be sized so that the face velocity at design airflow does not exceed the scheduled maximum.
+### Casing seams, joints, and connections shall be sealed to the SMACNA seal class required for the duct system in which the attenuator is installed.
+### Attenuators shall be included within the duct sections leakage-tested under [[sync/hvac-ductwork]] and shall not be isolated or bypassed for that test.
+
+### Leakage past an attenuator is not just lost air. Sound follows the leak, and a gap at a transverse joint is an unattenuated path around the very baffles the project paid for, radiating directly into whatever space the attenuator sits above. {note}
+
+## Duct Connections {toc}
+
+### The duct connection method shall be as indicated in the datasheet.
+
```datasheet
−label: Maximum Face Velocity
−type: range
−min: 500
−max: 2500
−step: 50
−unit: FPM
−setpoints: [500, 1000, 1500, 2500]
+label: Duct Connection Method
+type: select
+derived: "the transverse joint system of the duct to which the attenuator connects"
+options:
+ - Companion-angle flange
+ - Integral rolled transverse flange
+ - Slip-and-drive edges
+ - Plain ends for field connection
+ - Van Stone rotating flange
+ - Beaded end for spiral duct coupling
+default: derived
```
−# Silencer Type and Configuration {toc}
+### Where the attenuator face dimensions differ from the adjoining duct, a transition shall be provided at each face.
−## The silencer type is selected by duct shape, available space, and required low-frequency performance; rectangular and round straight absorptive silencers cover most installations, with elbow silencers used where a turn coincides with the attenuation point. {note}
+### A transition upstream of the attenuator shall not exceed 15° per side.
−## The silencer type shall be as scheduled and shall match the duct shape and orientation it serves.
+### A transition downstream of the attenuator shall not exceed 30° per side.
+### A transition steeper than these limits separates the flow before it reaches the baffles, and the attenuator then operates in a velocity profile nothing like the uniform one the laboratory used. Both the insertion loss and the self-noise move away from the rated values, and always in the unhelpful direction. {note}
+
+## Outdoor and Wet-Service Construction {toc}
+
+### The construction required for outdoor exposure shall be as indicated in the datasheet.
+
```datasheet
−label: Silencer Type
+label: Outdoor Exposure Construction
type: radio
options:
− - Straight absorptive, rectangular (splitter baffles)
− - Straight absorptive, round/circular (annular fill, center pod)
− - Elbow silencer (90-degree, absorptive)
− - Splitter insert (field-installed in plenum)
− - Pod-type cylindrical attenuator
−default: Straight absorptive, rectangular (splitter baffles)
+ - Not required, indoor installation
+ - Weather-resistant casing with sealed seams and a drainable base
+ - Weather-resistant casing with an intake hood and a bird screen
+default: Not required, indoor installation
```
−## The silencer performance configuration shall be selected to match the application: low-pressure-drop where fan energy and static budget are constrained, or high-insertion-loss where attenuation governs and space permits a longer, denser unit.
+### The provisions required for condensate and wash-down service shall be as indicated in the datasheet.
```datasheet
−label: Performance Configuration
+label: Condensate and Wash-Down Provisions
type: radio
options:
− - Standard (balanced DIL and pressure drop)
− - Low-pressure-drop (wide baffle spacing, bell-mouth noses)
− - High-insertion-loss (narrow baffle spacing, extended length)
−default: Standard (balanced DIL and pressure drop)
+ - None
+ - Sloped baffle closures with a casing drain connection
+ - Wash-down construction with an impervious media barrier and a casing drain connection
+default: None
```
−## Active Length {toc}
+### A casing drain connection provided under this article shall be piped to a drainage point under [[sync/condensate-drainage-piping]] and shall not discharge into the space or onto the structure.
−### Active length is the acoustically treated depth of the silencer in the direction of airflow; greater length yields greater low-frequency insertion loss but must be reconciled against the plenum, shaft, or mechanical-room space available.
+### Water that reaches acoustic media stays there. Fibrous media holds it against the perforated facing, loses absorption while it is wet, and provides the moisture that microbial growth needs in a duct that is otherwise dry. An attenuator in an outdoor-air intake downstream of a rain louver is the position where this happens most often, because the louver is the only thing between the media and the weather. {note}
−### Silencer active lengths of 5 to 10 ft (1500 to 3000 mm) are frequently not flagged on mechanical drawings and conflict with structure, piping, or shaft height in the field; the selected active length must be coordinated against available space during design.
+## External Insulation of the Casing {toc}
−### The silencer active length shall be as scheduled.
+### Responsibility for external thermal insulation and vapor retarder on the attenuator casing shall be as indicated in the datasheet.
```datasheet
−label: Active Length
−type: select
+label: Casing External Insulation Responsibility
+type: radio
options:
− - 3 ft (900 mm)
− - 5 ft (1500 mm)
− - 7 ft (2100 mm)
− - 10 ft (3000 mm)
−unit: ft
−default: 5 ft (1500 mm)
+ - Field-applied continuously with the adjoining duct insulation
+ - Factory-applied external insulation and vapor retarder
+ - Not required
+default: Field-applied continuously with the adjoining duct insulation
```
−## Round Silencer Diameter {toc}
+### Where external insulation is field-applied, it shall be continuous across the attenuator casing and its connections without interruption at the transverse joints.
−### For round and pod-type silencers, the casing diameter shall match the adjoining round duct.
+### An uninsulated attenuator in an insulated cold-air duct is a condensing surface with a large area sitting above a ceiling. The casing is metal on both faces, the media inside it is not a vapor barrier, and the transverse joints are the coldest line on the run. {note}
−```datasheet
−label: Round Silencer Diameter
−type: range
−min: 6
−max: 60
−step: 2
−unit: in.
−drawing_ref: "round duct schedule"
−default: deferred
−```
+# Acoustic Media and Baffles {toc}
−# The silencer is a sheet-metal casing enclosing absorptive baffles filled with sound-absorbing media; casing construction, baffle construction, and fill media each carry their own requirements. {note}
+## The acoustic media class is the decision that carries the most consequence in this standard, because it sets what the attenuator can do at low frequency, whether anything fibrous is exposed to the air, how the device can be cleaned, and what it costs. {note}
−# Casing {toc}
+## Acoustic Media Class {toc}
−## The casing shall be constructed of the scheduled material and gauge, with airtight seams suitable for the scheduled pressure class.
+### The acoustic media class shall be as indicated in the datasheet.
−## The casing gauge shall follow SMACNA HVAC Duct Construction Standards for the applicable pressure class, with a minimum of 22 ga for sizes up to 24 in. wide, 20 ga for 25 to 48 in., and 18 ga for sizes over 48 in.
−
−## Casing seams and joints shall be sealed to achieve the duct leakage class of the adjoining duct system.
−
−## Connection flanges or slip-and-drive edges shall be compatible with the connection method of the adjoining duct.
−
```datasheet
−label: Connection Type
+label: Acoustic Media Class
type: radio
options:
− - Flanged (companion angle / TDC)
− - Slip-and-drive
− - Raw edge for field connection
−default: Flanged (companion angle / TDC)
+ - Fibrous media behind a perforated facing
+ - Fibrous media with a fiber-retention barrier behind a perforated facing
+ - Fibrous media fully encapsulated in a sealed impervious barrier
+ - Packless construction with no fibrous media in the airstream
+default: Fibrous media behind a perforated facing
```
−# Baffles and Fill Media {toc}
+### The four classes differ in what stands between the media and the air: {note}
−## The acoustic baffles shall contain the scheduled fill media, retained against erosion into the airstream at the design face velocity.
+- Fibrous media behind a perforated facing exposes the media surface to the airstream through the perforations, which is the arrangement the published broadband performance is measured on
+- A fiber-retention barrier is a thin acoustically transparent cloth or scrim between the media and the facing, which holds fibers back at higher passage velocities and costs a small amount of high-frequency absorption
+- A sealed impervious barrier encloses the media entirely, which keeps the media dry and cleanable and removes most of the absorption above about 2000 Hz because the sound no longer reaches the fibers at those wavelengths
+- Packless construction contains no fibrous media at all, attenuating instead through tuned chambers, membranes, or resonant cavities, which gives strong attenuation over the bands it is tuned for and much less outside them
−## Fill media shall be glass fiber or mineral wool at a minimum density of 1.5 lb/ft³ (24 kg/m³).
+### Where the airstream serves a space in which airborne fibers or cleanability govern, and where the noise to be attenuated is concentrated in a band a tuned device can cover, packless construction removes the fibrous media from the question entirely. Where the noise is broadband, the same construction requires substantially more length to reach the same insertion loss. {note}
−## Where fill media are exposed to the airstream, they shall meet NFPA 90A Section 5.2, with a flame-spread index not exceeding 25 and a smoke-developed index not exceeding 50 when tested per ASTM E84.
+## Fibrous Acoustic Media {toc}
−## The fill-media facing shall be selected for the service: a perforated metal facing protects the fill in standard systems, while encapsulation isolates the fill entirely for hygienic and erosion-sensitive systems.
+### Requirements in this article apply where a fibrous acoustic media class is indicated in the datasheet.
−## Open (unencapsulated) fiberglass fill shall not be used in healthcare, cleanroom, laboratory, or kitchen exhaust systems; NFPA 90A and infection-control requirements mandate encapsulated fill or no exposed fibrous media in these applications. {note}
+### The fibrous media material shall be as indicated in the datasheet.
−## The fill-media encapsulation shall be as scheduled.
−
```datasheet
−label: Fill Media Encapsulation
−type: radio
+label: Fibrous Media Material
+type: select
options:
− - Perforated-face liner (standard, fill protected)
− - Fully encapsulated fill (no exposed media)
− - Filmless / no-fill acoustic baffle (cleanroom)
−default: Perforated-face liner (standard, fill protected)
+ - Glass fiber
+ - Mineral wool
+ - Not applicable, packless construction indicated
+default: Glass fiber
```
−## The fill media density shall be as scheduled.
+### Glass fiber media shall conform to ASTM C1071.
+### Mineral wool media shall be inorganic, shall be free of slag inclusions that abrade the facing, and shall carry the manufacturer's published nominal density and continuous service temperature.
+
+### Fibrous media shall have a density not less than the value indicated in the datasheet.
+
```datasheet
−label: Fill Media Density
+label: Minimum Fibrous Media Density
type: range
−min: 1.5
−max: 6.0
−step: 0.5
unit: lb/ft³
−setpoints: [1.5, 3, 6]
+min: 1.5
+max: 8
+setpoints: [1.5, 2, 3, 4, 6, 8]
```
−# Testing {toc}
+### Where no minimum media density is indicated in the datasheet, the manufacturer's standard density for the model furnished shall govern, and the manufacturer shall state that density in the submittal.
−## Acoustical and airflow performance shall be established by laboratory test per ASTM E477 at a NVLAP-accredited facility; field testing of installed silencers is impractical and is not a substitute for accredited laboratory data. {note}
+### Density is a means, not an end. The rated insertion loss is measured on the media the manufacturer actually uses, so raising the specified minimum above what the rating was measured on changes the device without improving the number it was selected against. Density is worth specifying where erosion or settlement, rather than absorption, is the concern. {note}
−## Each silencer model shall be tested per ASTM E477 for dynamic insertion loss, generated noise level, and pressure drop in each octave band from 63 Hz through 8000 Hz.
+### Fibrous media shall be tested for fungal resistance in accordance with ASTM C1338 and shall show no growth.
−## Testing shall be performed at a minimum of three airflow rates spanning the silencer's rated operating range.
+### Fibrous media shall be retained without erosion, migration, or fiber release at a passage velocity not less than the value indicated in the datasheet.
−## Silencers serving return or exhaust ducts shall be tested in both forward-flow and reverse-flow directions.
+```datasheet
+label: Media Erosion Resistance Rating
+type: range
+unit: FPM
+min: 2000
+max: 8000
+setpoints: [2000, 3000, 4000, 5000, 6000, 8000]
+default: 4000
+```
−## Test reports shall state the NVLAP accreditation number and scope of the testing laboratory.
+### The passage velocity between baffles is roughly double the face velocity at the attenuator, because the baffles occupy about half the cross-section. An erosion rating compared against face velocity is therefore compared against the wrong number by a factor of about two. {note}
+### Fibrous media in a baffle installed with its length vertical shall be supported at intervals that prevent a settlement void from forming at the top of the baffle.
+
+### Settlement is a slow failure with no symptom. The void opens at the top of a vertical baffle, the insertion loss falls first in the low bands where the project needed it most, and nothing about the installation looks different from the outside. {note}
+
+## Packless Acoustic Media {toc}
+
+### Requirements in this article apply where packless construction is indicated in the datasheet.
+
+### Packless attenuators shall contain no fibrous media in contact with or exposed to the airstream.
+
+### Packless attenuators shall carry measured performance data from the same test method and the same accreditation basis required of fibrous attenuators by this standard.
+
+### The manufacturer shall identify, in each octave band, the attenuation the packless attenuator provides, so that the bands it is tuned for and the bands it is not are both visible in the submittal.
+
+### A tuned device does its work over a limited range of frequencies by design. Where the source spectrum shifts, from a fan speed change, a different fan selection, or a variable-speed system operating away from its design point, the attenuation follows the tuning and not the source. Reading the octave-band curve rather than a single number is what makes that visible while it can still be acted on. {note}
+
+## Perforated Facing {toc}
+
+### The perforated facing material shall be as indicated in the datasheet.
+
```datasheet
−label: Test Standard
−type: radio
+label: Perforated Facing Material
+type: select
+derived: "the casing material indicated for the attenuator"
options:
− - ASTM E477-20 (North American)
− - ISO 7235:2003 (international)
− - ASTM E477 or ISO 7235 (either accepted)
−default: ASTM E477-20 (North American)
+ - Galvanized steel
+ - Aluminized steel
+ - Aluminum
+ - Stainless steel Type 304
+ - Stainless steel Type 316
+default: derived
```
+### The perforation pattern and open area shall be those used in the specimen from which the submitted performance data were measured.
+
+### The facing shall be secured to the baffle frame so that it cannot deflect into the airstream, buckle, or separate at an edge under the pressure difference across it at the scheduled airflow.
+
+### Perforated facing is acoustically transparent only because its open area is large enough that the sound passes through to the media behind it. Reducing the open area to gain strength, or substituting a different pattern for availability, changes the absorption the rating was measured with, most noticeably at the high frequencies where the hole size is comparable to the wavelength. {note}
+
+## Surface Burning Characteristics {toc}
+
+### Acoustic media, facings, barriers, adhesives, and sealants exposed to the airstream shall have a flame spread index not exceeding 25 and a smoke developed index not exceeding 50 when tested in accordance with ASTM E84 or UL 723.
+
+### The attenuator assembly shall comply with NFPA 90A, or with NFPA 90B where the system it serves falls within that standard's scope.
+
+### The flame spread and smoke developed limits apply to the material as it is installed in the attenuator, including any barrier, adhesive, or coating applied to it. A media that passes bare can fail once it is bagged in a film, so the test data submitted have to match the assembly furnished. {note}
+
+# Transfer-Duct Attenuators {toc}
+
+## A transfer-duct attenuator serves a different problem from a fan noise attenuator. The path is room to room through a shared opening or a short duct above a ceiling, the airflow is low or zero, and what governs is speech privacy rather than a background level. Because the flow is negligible, self-noise and pressure loss are close to irrelevant and insertion loss across the speech bands is nearly the whole requirement. {note}
+
+## Requirements in this article apply to attenuators serving a room-to-room transfer path.
+
+## Transfer-duct attenuators shall provide the scheduled insertion loss in the 250 Hz through 4000 Hz octave bands with no airflow through the attenuator.
+
+## Transfer-duct attenuators shall be sized so that the pressure loss at the design transfer airflow does not exceed the value scheduled for the tag.
+
+## Where a transfer-duct attenuator passes through a fire-rated or smoke-rated assembly, the damper required for that penetration shall be provided under [[sync/fire-and-smoke-dampers]].
+
+## A transfer attenuator installed in an opening whose surrounding construction is weaker than the attenuator has not improved the privacy between the rooms. The path with the least resistance governs, and where the partition stops at the ceiling the transfer opening is rarely that path. {note}
+
+# Identification {toc}
+
+## Each attenuator shall carry a permanently attached label stating the tag, the manufacturer, the model, the face dimensions, and the overall length.
+
+## Each attenuator shall carry a permanent arrow indicating the flow direction for which it is rated.
+
+## Labels and flow direction arrows shall remain legible and visible from the access side after the attenuator is installed and insulated.
+
+## Attenuator identification shall additionally comply with [[sync/mechanical-identification]].
+
+## An attenuator installed backwards looks correct in every way an inspection notices. The face dimensions match, the connections fit, and the airflow is right. The only evidence is a performance shortfall measured after the ceiling is closed, which is why the arrow has to survive the insulation. {note}
+
# Installation {toc}
−## A silencer that is correctly specified can still fail in the field if it is installed with turbulent inlet conditions or with acoustic bypass paths around its body; installation requirements protect the rated performance. {note}
+## Approach and Discharge Conditions {toc}
−## Inlet and Outlet Conditions {toc}
+### Rated performance is measured with fully developed flow entering the attenuator. A fan discharge, an elbow, a takeoff, or a partly closed damper leaves the flow separated and swirling, and an attenuator placed in that flow both attenuates less and generates more. The measured effect is largest at the highest velocities, which is exactly where a designer is most likely to have placed the attenuator tight to the equipment to save space. {note}
−### Turbulent flow entering the silencer raises its self-noise and reduces its effective insertion loss; a silencer placed immediately downstream of a fan discharge or elbow performs below its rated values. {note}
+### Each attenuator shall be installed with not less than three equivalent duct diameters of straight duct at its approach face, unless the manufacturer's published data establish the rated performance under the actual approach condition present.
−### The silencer shall be installed with a minimum of one to two equivalent duct diameters of straight duct upstream of the silencer face, unless the manufacturer's published data establish performance under the actual inlet condition.
+### A damper, takeoff, or abrupt transition shall not be located within one equivalent duct diameter of the attenuator's discharge face, unless the manufacturer's published data establish the rated performance under that condition.
−### Inlet and outlet transitions shall be gradual and shall not exceed the divergence angles recommended by the manufacturer.
+### Where the parties disagree whether an approach condition is covered by the manufacturer's published data, the Engineer of Record shall make the initial determination.
+## Flanking and Bypass Paths {toc}
+
+### Insertion loss describes only the sound that goes through the attenuator. Anything that gets past it arrives at full strength, and the strongest attenuator on the project cannot reduce a path it is not in. Gaps at the casing, unsealed penetrations, and shared plenum walls are the paths that most often defeat an otherwise correct selection. {note}
+
+### Gaps between the attenuator casing and surrounding construction shall be sealed airtight over the full perimeter.
+
+### Where an attenuator passes through a wall or floor separating the noise source from a space served, the penetration shall be closed with a resilient seal that maintains the acoustic performance of that assembly.
+
+### Where the assembly penetrated is fire-rated, the penetration shall be firestopped under [[sync/firestopping]].
+
+### Duct, transitions, and offsets between the attenuator and the space served shall not be connected to the noise source by any unattenuated branch, bypass, or open plenum path.
+
+### Sound also leaves a duct through its walls rather than its ends. Where a duct downstream of an attenuator crosses an occupied space, the level in that space can be set by what breaks out of the duct rather than by what reaches the outlet, and the treatment for that is on the duct itself under [[sync/hvac-ductwork]]. {note}
+
+## Support and Seismic Restraint {toc}
+
+### Each attenuator shall be independently supported from the building structure.
+
+### An attenuator shall not impose any part of its weight on the adjoining duct or on its transverse joints.
+
+### Supports shall be sized for the operating weight of the attenuator including its acoustic media, its factory-applied insulation where furnished, and any water retained in wash-down service.
+
+### Supports shall be spaced so that the casing does not deflect enough to open a transverse joint or displace a baffle.
+
+### Seismic restraint of attenuators shall be as indicated in the datasheet.
+
```datasheet
−label: Minimum Upstream Straight Duct
−type: range
−min: 0
−max: 4
−step: 0.5
−unit: duct diameters
−setpoints: [1, 2, 4]
+label: Seismic Restraint of Attenuators
+type: radio
+derived: "[[parameter: seismic-design-category]] and the component exemptions of ASCE/SEI 7"
+options:
+ - Not required
+ - Required for attenuators exceeding the weight exempted by ASCE/SEI 7
+ - Required for all attenuators regardless of weight
+default: derived
```
−## Flanking and Bypass Paths {toc}
+### Seismic restraint hardware, attachment to structure, and the associated calculations shall be provided under [[sync/vibration-isolation-and-seismic-restraint]].
−### Insertion loss is defeated by any unlined path that lets sound bypass the silencer body, such as unlined plenum transitions, gaps around the casing, or shared plenum walls. {note}
+### An attenuator is heavier than the duct on either side of it by a wide margin, and its weight is concentrated over a short length. That concentration is what puts it above the component weight exemptions that the surrounding duct falls under. {note}
−### Plenum transition drawings shall require flanking paths to be sealed and lined.
+### Attenuator supports and hangers shall additionally comply with [[sync/hangers-and-supports]].
−### Plenum transitions, offsets, and connections immediately adjacent to the silencer shall be acoustically lined where shown to prevent flanking transmission around the silencer.
+## Orientation and Location {toc}
−### Gaps between the silencer casing and surrounding construction shall be sealed airtight and acoustically.
+### Attenuators shall be installed in the positions and orientations shown on [[drawing: the mechanical plans]].
−### The silencer shall be installed in the orientation and location shown, with the rated flow direction matching the system airflow. [[drawing: silencer location and orientation]]
+### Each attenuator shall be installed with its rated flow direction matching the direction of airflow in the duct.
−## Support {toc}
+### Attenuators shall be installed so that casing joints and duct connections remain accessible for disassembly after all adjacent work is complete.
−### Silencers shall be independently supported from the structure and shall not impose load on the adjoining duct.
+# Field Verification of Space Sound Levels {toc}
−### Support spacing and method shall accommodate the silencer weight, including fill media, without casing deflection.
+## A space sound level estimate is a calculation, and a measurement is the only thing that closes it. Where a project carries a contractual noise criterion, measuring it is what converts the criterion from an intention into an accepted condition; where it does not, the measurement has no threshold to be compared against and produces a number nobody can act on. {note}
+## The extent of post-installation sound level verification shall be as indicated in the datasheet.
+
+```datasheet
+label: Post-Installation Sound Level Verification
+type: radio
+options:
+ - Not required
+ - Measurement in a representative sample of spaces carrying a scheduled criterion
+ - Measurement in every space carrying a scheduled criterion
+```
+
+## Where verification is required, measurements shall be taken in accordance with ANSI/ASA S12.2 with all mechanical systems serving the space operating at design airflow.
+
+## Measurements shall be taken after the systems serving the space have been balanced and accepted under [[sync/testing-adjusting-and-balancing]].
+
+## The verification report shall state, for each space measured, the octave-band sound pressure levels, the criterion value applicable to that space, the measured background level with the mechanical systems off, and the resulting criterion rating.
+
+## Where a measured level exceeds the criterion applicable to the space, the Engineer of Record shall make the initial determination of the cause.
+
+## Where the cause is determined to be an attenuator that does not deliver the performance scheduled for it, the Contractor shall bear the cost of correcting the deficiency and of one re-measurement of every space affected.
+
+## Where the measured background level with the mechanical systems off is within 3 dB of the criterion in any octave band, the measurement in that band shall be reported as inconclusive rather than as a pass or a failure.
+
# Delivery, Storage, and Handling {toc}
−## Silencers shall be delivered with casing ends protected and fill media kept dry, since wet fill media lose acoustic performance and can support microbial growth.
+## Attenuators shall be delivered with the manufacturer's protective covers in place over both open faces and shall be shipped with those covers sealed.
−## Silencers shall be delivered with factory protective covers over open casing ends and shall be stored indoors, off the ground, and protected from weather and construction moisture.
+## Attenuators shall be stored indoors, off the ground, and protected from weather, standing water, and construction dust until they are installed.
−## Silencers shall be handled and rigged at designated lift points so that the casing is not racked or the baffles displaced.
+## Protective face covers shall remain in place until the attenuator is connected to the duct on both faces.
−## Silencers showing wet, contaminated, or damaged fill media shall be rejected and replaced.
+## Attenuators shall be lifted and rigged only at the lift points the manufacturer identifies, and shall not be racked, dragged, or lifted by the perforated facing.
+## An attenuator delivered or stored with wet, soiled, corroded, or physically damaged acoustic media shall be replaced at no cost to the Owner.
+
+## Media that has been wet is not restored by drying. The fibers mat where the water carried them, the density is no longer uniform, and the absorption the rating was measured on is gone in a way no inspection of the assembled device can detect. Rejecting it before installation is the only point at which it is cheap. {note}
+
# Warranty {toc}
−## The manufacturer shall warrant each silencer against defects in materials and workmanship, including acoustic and structural performance, for the scheduled warranty period from the date of Substantial Completion.
+## The manufacturer shall warrant each attenuator against defects in materials and workmanship for the period indicated in the datasheet.
```datasheet
label: Warranty Period
−type: select
−options:
− - 1 year
− - 2 years
− - 5 years
+type: range
unit: years
−default: 2 years
+min: 1
+max: 10
+setpoints: [1, 2, 3, 5, 10]
+default: 1
```
−## The warranty shall cover replacement of fill media that erode, settle, or degrade below rated acoustic performance within the warranty period under normal operating conditions.
+## The warranty period shall begin on the date of Substantial Completion.
+## The warranty shall cover erosion, migration, settlement, delamination, and corrosion of the acoustic media, the media barrier, the perforated facing, and the casing under the service conditions specified for the attenuator.
+
+## The warranty shall cover failure of the attenuator to deliver its rated insertion loss, generated sound power level, and pressure loss under those service conditions.
+
+## An attenuator repaired or replaced under warranty shall carry a warranty for a full term from the date of the repair or replacement, or for the remainder of the original term, whichever is longer.
+
+## The manufacturer shall bear the cost of removing, repairing or replacing, and reinstalling an attenuator repaired under warranty, and of restoring the insulation, ceiling, and adjacent duct disturbed by that work.
+
+## Warranty administration and the Owner's remedies shall additionally comply with [[sync/warranties-and-bonds]].
+
# Spare Parts {toc}
−## Because silencers are sealed prefabricated assemblies with no moving parts, spare-parts provisions are minimal and typically limited to replacement fill panels for systems with erosive or corrosive airstreams. {note}
+## An attenuator has no moving parts and no consumables, so spare parts provisions are narrow. What does wear is the media, and it wears only in the airstreams that erode or corrode it, which is where a spare is worth stocking against a lead time measured in weeks. {note}
−## Where required, the Contractor shall furnish replacement fill panels or baffle sections for silencers serving erosive or high-velocity airstreams.
+## Spare acoustic media modules shall be furnished as indicated in the datasheet.
```datasheet
−label: Spare Fill Panels Furnished
+label: Spare Acoustic Media Modules Furnished
type: radio
options:
- None
− - One replacement baffle set per silencer type
− - Replacement fill panels for erosive/corrosive systems only
+ - One replacement media module for each attenuator model furnished
+ - Replacement media modules for attenuators in erosive, corrosive, or wash-down service
+ - A full replacement media set for each attenuator in erosive, corrosive, or wash-down service
default: None
```
+
+## Spare media modules shall be delivered in the manufacturer's sealed packaging, labelled with the attenuator tag and model they serve, and turned over to the Owner before the date of Substantial Completion.