1 Scope

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. (1.1)

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. (1.2)

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. (1.3)

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. (1.4)

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. (1.5)

1.6 Attenuators furnished under this standard shall comply with the duct construction, sealing, and leakage requirements of HVAC DuctworkHVAC DuctworkResolves to the current edition.sync/hvac-ductwork for the duct system in which they are installed, except where this standard states a more stringent requirement.

1.7 Where a requirement of this standard and a requirement of HVAC DuctworkHVAC DuctworkResolves to the current edition.sync/hvac-ductwork address the same subject, the requirement of this standard shall govern.

NOTE The following are outside the scope of this standard: (1.8)

Duct liner, duct wrap, and externally applied acoustic lagging, which are part of the duct assembly and are covered by HVAC DuctworkHVAC DuctworkResolves to the current edition.sync/hvac-ductwork, Mechanical InsulationMechanical InsulationResolves to the current edition.sync/mechanical-insulation, and Acoustic InsulationAcoustic InsulationResolves to the current edition.sync/acoustic-insulation
Dampers, turning vanes, access doors, flexible connectors, and other duct-mounted accessories, which are covered by Duct AccessoriesHVAC Duct AccessoriesResolves to the current edition.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 Fire And Smoke DampersFire and Smoke DampersResolves to the current edition.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 HVAC Air Distribution DevicesHVAC Air Distribution DevicesResolves to the current edition.sync/hvac-air-distribution-devices
The attenuator section integral to an air terminal unit, which is furnished as part of the unit under Air Terminal UnitsAir Terminal UnitsResolves to the current edition.sync/air-terminal-units and Variable Air Volume TerminalsVariable Air Volume Terminal UnitsResolves to the current edition.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 Air Handling UnitsAir Handling UnitsResolves to the current edition.sync/air-handling-units, HVAC FansHVAC FansResolves to the current edition.sync/hvac-fans, and Makeup Air UnitsMakeup Air UnitsResolves to the current edition.sync/makeup-air-units
Vibration isolators, flexible duct connectors, and the seismic restraint hardware itself, which are covered by Vibration Isolation And Seismic RestraintVibration Isolation and Seismic RestraintResolves to the current edition.sync/vibration-isolation-and-seismic-restraint
Room-side and envelope acoustic treatment such as wall panels, ceiling clouds, and baffles, which are covered by Acoustic Wall PanelsAcoustic Wall and Ceiling PanelsResolves to the current edition.sync/acoustic-wall-panels and Acoustic Ceiling Clouds And BafflesAcoustic Ceiling Clouds and BafflesResolves to the current edition.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 Kitchen Exhaust SystemsKitchen Exhaust SystemsResolves to the current edition.sync/kitchen-exhaust-systems and Dust And Fume CollectionDust and Fume Collection SystemsResolves to the current edition.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

2 Referenced Standards

2.1 Equipment, materials, testing, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.

2.2 Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.

Standard	Title
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

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. (2.3)

3 Submittals

3.1 Performance and Product Submittals

3.1.1 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

Performance and Product Submittals Requiredcheckbox

☑ 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

3.2 Test Evidence Submittals

3.2.1 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

Test Evidence Submittals Requiredcheckbox

☑ 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

3.3 Closeout Submittals

3.3.1 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

Closeout Submittals Requiredcheckbox

☑ As-installed record documentation

☑ Operation and maintenance data with permitted cleaning methods

☐ Post-installation sound level verification report

☑ Executed manufacturer's warranty

4 Rating Basis and Test Evidence

4.1 Acoustical Rating Test Method

NOTE 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. (4.1.1)

4.1.2 Rated acoustical and airflow performance shall be established by laboratory measurement in accordance with the test method indicated in the datasheet.

Acoustical Rating Test Methodselect

ASTM E477

ISO 7235

ASTM E477 or ISO 7235, either accepted

4.1.3 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.

4.1.4 Calculated, scaled, and analytically modelled performance shall not be submitted in place of measured performance for any model furnished.

4.2 Test Laboratory Accreditation

NOTE 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. (4.2.1)

4.2.2 Performance data shall originate from a laboratory holding the accreditation indicated in the datasheet for the test method used.

Test Laboratory Accreditation Basisradio

● 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

4.2.3 Each test report shall state the accrediting body, the accreditation number, and the date the accreditation is current through.

4.2.4 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.

4.3 Tested Length and Model Correspondence

NOTE 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. (4.3.1)

4.3.2 The correspondence between the performance data submitted and the specimens actually tested shall be as indicated in the datasheet.

Rated Length and Model Correspondenceradio

○ 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

4.3.3 Where interpolated performance is submitted, the manufacturer shall identify the two tested lengths that bracket the furnished length and shall state the interpolation basis.

4.3.4 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.

4.4 Manufacturer Qualification

4.4.1 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.

4.4.2 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.

5 Acoustic Criteria and Selection Basis

5.1 Room Noise Criterion

NOTE 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. (5.1.1)

5.1.2 The room noise criterion metric applicable to this project shall be as indicated in the datasheet.

Room Noise Criterion Metricselect

Noise Criterion, NC

Room Criterion Mark II, RC Mark II

Balanced Noise Criterion, NCB

A-weighted sound pressure level, dBA

5.1.3 The criterion value applicable to each space served shall be as indicated in the contract documents.

NOTE 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. (5.1.4)

5.2 Space Sound Level Estimation

5.2.1 The space sound level for each space carrying a scheduled criterion shall be estimated in accordance with the procedure indicated in the datasheet.

Space Sound Level Estimation Procedureselect

ANSI/AHRI 885

ASHRAE Handbook HVAC Applications, Noise and Vibration Control

Manufacturer's published calculation program with the calculation basis submitted

5.2.2 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.

5.2.3 The estimate shall include the generated sound power of the attenuator itself and of every downstream device that generates sound in the airstream.

5.2.4 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.

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. (5.2.5)

5.3 Attenuation Credits

NOTE 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: (5.3.1)

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

5.3.2 The attenuation credits the selection is permitted to take shall be as indicated in the datasheet.

Attenuation Credits Permitted in the Selectioncheckbox

☑ 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

5.3.3 An attenuation credit not indicated in the datasheet shall not be taken in the space sound level estimate.

5.3.4 Each credit taken shall be shown as a separate line in the estimate with the value taken in each octave band.

NOTE 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. (5.3.5)

5.4 Generated Sound Power Limit Basis

NOTE 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. (5.4.1)

5.4.2 The basis on which the generated sound power of each attenuator is limited shall be as indicated in the datasheet.

Generated Sound Power Level Limit Basisradio

● 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

5.4.3 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.

NOTE 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. (5.4.4)

6 Scheduled Performance

6.1 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 the sound attenuator schedule.

6.2 The scheduled dynamic insertion loss shall be provided in every octave band from 63 Hz through 8000 Hz at the scheduled airflow.

6.3 The scheduled generated sound power level shall not be exceeded in any octave band from 63 Hz through 8000 Hz at the scheduled airflow.

6.4 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.

6.5 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.

NOTE 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. (6.6)

6.7 The rated total pressure loss and generated sound power level shall be corrected for the air density at AltitudeAltitudeParameterEach project supplies its own value.altitude using the manufacturer's published correction factors.

6.8 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 Adopted Energy CodeAdopted Energy CodeParameterEach project supplies its own value.adopted-energy-code.

NOTE 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. (6.9)

6.10 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.

6.11 The Contractor shall report any conflict found by that verification to the Engineer of Record before releasing the affected attenuator for fabrication.

7 Airstream Service Conditions

7.1 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.

NOTE 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. (7.2)

7.3 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.

NOTE 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. (7.4)

7.5 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.

NOTE 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. (7.6)

8 Attenuator Configurations

NOTE Attenuator configurations differ in how they fit rather than in what they do. Each of the following is a distinct family: (8.1)

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

8.2 The attenuator configurations permitted on this project shall be as indicated in the datasheet.

Attenuator Configurations Permittedcheckbox

☑ 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

8.3 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.

8.4 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.

9 Casing Construction

9.1 Casing Material

9.1.1 The casing material shall be as indicated in the datasheet.

Casing Materialselect

Galvanized steel

Aluminized steel

Aluminum

Stainless steel Type 304

Stainless steel Type 316

Carbon steel with a factory-applied corrosion-resistant coating

9.1.2 Galvanized steel casings shall conform to ASTM A653 with a G90 coating designation.

9.1.3 Aluminized steel casings shall conform to ASTM A463 Type 2.

9.1.4 Aluminum casings shall conform to ASTM B209.

9.1.5 Stainless steel casings shall conform to ASTM A240 in the type indicated in the datasheet.

9.1.6 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.

9.1.7 Cut edges, welds, and fastener penetrations made after coating or galvanizing shall be repaired with a coating compatible with the casing finish.

9.2 Casing Pressure Class and Sheet Thickness

9.2.1 The casing pressure class shall be as indicated in the datasheet.

Casing Static Pressure Classrange

in. w.g.

0.51234610

Derived — the static pressure class of the duct system in which the attenuator is installed (by default)

9.2.2 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.

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. (9.2.3)

9.3 Casing Sealing and Leakage

9.3.1 Casing seams, joints, and connections shall be sealed to the SMACNA seal class required for the duct system in which the attenuator is installed.

9.3.2 Attenuators shall be included within the duct sections leakage-tested under HVAC DuctworkHVAC DuctworkResolves to the current edition.sync/hvac-ductwork and shall not be isolated or bypassed for that test.

NOTE 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. (9.3.3)

9.4 Duct Connections

9.4.1 The duct connection method shall be as indicated in the datasheet.

Duct Connection Methodselect

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

Derived — the transverse joint system of the duct to which the attenuator connects (by default)

9.4.2 Where the attenuator face dimensions differ from the adjoining duct, a transition shall be provided at each face.

9.4.3 A transition upstream of the attenuator shall not exceed 15° per side.

9.4.4 A transition downstream of the attenuator shall not exceed 30° per side.

NOTE 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. (9.4.5)

9.5 Outdoor and Wet-Service Construction

9.5.1 The construction required for outdoor exposure shall be as indicated in the datasheet.

Outdoor Exposure Constructionradio

● 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

9.5.2 The provisions required for condensate and wash-down service shall be as indicated in the datasheet.

Condensate and Wash-Down Provisionsradio

● None

○ Sloped baffle closures with a casing drain connection

○ Wash-down construction with an impervious media barrier and a casing drain connection

9.5.3 A casing drain connection provided under this article shall be piped to a drainage point under Condensate Drainage PipingHVAC Condensate Drainage PipingResolves to the current edition.sync/condensate-drainage-piping and shall not discharge into the space or onto the structure.

NOTE 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. (9.5.4)

9.6 External Insulation of the Casing

9.6.1 Responsibility for external thermal insulation and vapor retarder on the attenuator casing shall be as indicated in the datasheet.

Casing External Insulation Responsibilityradio

● Field-applied continuously with the adjoining duct insulation

○ Factory-applied external insulation and vapor retarder

○ Not required

9.6.2 Where external insulation is field-applied, it shall be continuous across the attenuator casing and its connections without interruption at the transverse joints.

NOTE 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. (9.6.3)

10 Acoustic Media and Baffles

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. (10.1)

10.2 Acoustic Media Class

10.2.1 The acoustic media class shall be as indicated in the datasheet.

Acoustic Media Classradio

● 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

NOTE The four classes differ in what stands between the media and the air: (10.2.2)

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

NOTE 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. (10.2.3)

10.3 Fibrous Acoustic Media

10.3.1 Requirements in this article apply where a fibrous acoustic media class is indicated in the datasheet.

10.3.2 The fibrous media material shall be as indicated in the datasheet.

Fibrous Media Materialselect

Glass fiber

Mineral wool

Not applicable, packless construction indicated

10.3.3 Glass fiber media shall conform to ASTM C1071.

10.3.4 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.

10.3.5 Fibrous media shall have a density not less than the value indicated in the datasheet.

Minimum Fibrous Media Densityrange

lb/ft³

1.523468

10.3.6 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.

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. (10.3.7)

10.3.8 Fibrous media shall be tested for fungal resistance in accordance with ASTM C1338 and shall show no growth.

10.3.9 Fibrous media shall be retained without erosion, migration, or fiber release at a passage velocity not less than the value indicated in the datasheet.

Media Erosion Resistance Ratingrange

FPM

200030004000500060008000

NOTE 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. (10.3.10)

10.3.11 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.

NOTE 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. (10.3.12)

10.4 Packless Acoustic Media

10.4.1 Requirements in this article apply where packless construction is indicated in the datasheet.

10.4.2 Packless attenuators shall contain no fibrous media in contact with or exposed to the airstream.

10.4.3 Packless attenuators shall carry measured performance data from the same test method and the same accreditation basis required of fibrous attenuators by this standard.

10.4.4 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.

NOTE 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. (10.4.5)

10.5 Perforated Facing

10.5.1 The perforated facing material shall be as indicated in the datasheet.

Perforated Facing Materialselect

Galvanized steel

Aluminized steel

Aluminum

Stainless steel Type 304

Stainless steel Type 316

Derived — the casing material indicated for the attenuator (by default)

10.5.2 The perforation pattern and open area shall be those used in the specimen from which the submitted performance data were measured.

10.5.3 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.

NOTE 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. (10.5.4)

10.6 Surface Burning Characteristics

10.6.1 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.

10.6.2 The attenuator assembly shall comply with NFPA 90A, or with NFPA 90B where the system it serves falls within that standard's scope.

NOTE 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. (10.6.3)

11 Transfer-Duct Attenuators

NOTE 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. (11.1)

11.2 Requirements in this article apply to attenuators serving a room-to-room transfer path.

11.3 Transfer-duct attenuators shall provide the scheduled insertion loss in the 250 Hz through 4000 Hz octave bands with no airflow through the attenuator.

11.4 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.

11.5 Where a transfer-duct attenuator passes through a fire-rated or smoke-rated assembly, the damper required for that penetration shall be provided under Fire And Smoke DampersFire and Smoke DampersResolves to the current edition.sync/fire-and-smoke-dampers.

NOTE 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. (11.6)

12 Identification

12.1 Each attenuator shall carry a permanently attached label stating the tag, the manufacturer, the model, the face dimensions, and the overall length.

12.2 Each attenuator shall carry a permanent arrow indicating the flow direction for which it is rated.

12.3 Labels and flow direction arrows shall remain legible and visible from the access side after the attenuator is installed and insulated.

12.4 Attenuator identification shall additionally comply with Mechanical IdentificationMechanical IdentificationResolves to the current edition.sync/mechanical-identification.

NOTE 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. (12.5)

13 Installation

13.1 Approach and Discharge Conditions

NOTE 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. (13.1.1)

13.1.2 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.

13.1.3 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.

13.1.4 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.

13.2 Flanking and Bypass Paths

NOTE 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. (13.2.1)

13.2.2 Gaps between the attenuator casing and surrounding construction shall be sealed airtight over the full perimeter.

13.2.3 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.

13.2.4 Where the assembly penetrated is fire-rated, the penetration shall be firestopped under FirestoppingFirestoppingResolves to the current edition.sync/firestopping.

13.2.5 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.

NOTE 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 HVAC DuctworkHVAC DuctworkResolves to the current edition.sync/hvac-ductwork. (13.2.6)

13.3 Support and Seismic Restraint

13.3.1 Each attenuator shall be independently supported from the building structure.

13.3.2 An attenuator shall not impose any part of its weight on the adjoining duct or on its transverse joints.

13.3.3 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.

13.3.4 Supports shall be spaced so that the casing does not deflect enough to open a transverse joint or displace a baffle.

13.3.5 Seismic restraint of attenuators shall be as indicated in the datasheet.

Seismic Restraint of Attenuatorsradio

○ Not required

○ Required for attenuators exceeding the weight exempted by ASCE/SEI 7

○ Required for all attenuators regardless of weight

Derived — Seismic Design CategorySeismic Design CategoryParameterEach project supplies its own value.seismic-design-category and the component exemptions of ASCE/SEI 7 (by default)

13.3.6 Seismic restraint hardware, attachment to structure, and the associated calculations shall be provided under Vibration Isolation And Seismic RestraintVibration Isolation and Seismic RestraintResolves to the current edition.sync/vibration-isolation-and-seismic-restraint.

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. (13.3.7)

13.3.8 Attenuator supports and hangers shall additionally comply with Hangers And SupportsHangers and Supports for Mechanical Piping and EquipmentResolves to the current edition.sync/hangers-and-supports.

13.4 Orientation and Location

13.4.1 Attenuators shall be installed in the positions and orientations shown on the mechanical plans.

13.4.2 Each attenuator shall be installed with its rated flow direction matching the direction of airflow in the duct.

13.4.3 Attenuators shall be installed so that casing joints and duct connections remain accessible for disassembly after all adjacent work is complete.

14 Field Verification of Space Sound Levels

NOTE 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. (14.1)

14.2 The extent of post-installation sound level verification shall be as indicated in the datasheet.

Post-Installation Sound Level Verificationradio

○ Not required

○ Measurement in a representative sample of spaces carrying a scheduled criterion

○ Measurement in every space carrying a scheduled criterion

14.3 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.

14.4 Measurements shall be taken after the systems serving the space have been balanced and accepted under Testing Adjusting And BalancingTesting, Adjusting, and Balancing for HVACResolves to the current edition.sync/testing-adjusting-and-balancing.

14.5 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.

14.6 Where a measured level exceeds the criterion applicable to the space, the Engineer of Record shall make the initial determination of the cause.

14.7 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.

14.8 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.

15 Delivery, Storage, and Handling

15.1 Attenuators shall be delivered with the manufacturer's protective covers in place over both open faces and shall be shipped with those covers sealed.

15.2 Attenuators shall be stored indoors, off the ground, and protected from weather, standing water, and construction dust until they are installed.

15.3 Protective face covers shall remain in place until the attenuator is connected to the duct on both faces.

15.4 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.

15.5 An attenuator delivered or stored with wet, soiled, corroded, or physically damaged acoustic media shall be replaced at no cost to the Owner.

NOTE 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. (15.6)

16 Warranty

16.1 The manufacturer shall warrant each attenuator against defects in materials and workmanship for the period indicated in the datasheet.

Warranty Periodrange

years

123510

16.2 The warranty period shall begin on the date of Substantial Completion.

16.3 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.

16.4 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.

16.5 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.

16.6 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.

16.7 Warranty administration and the Owner's remedies shall additionally comply with Warranties And BondsWarranties and BondsResolves to the current edition.sync/warranties-and-bonds.

17 Spare Parts

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. (17.1)

17.2 Spare acoustic media modules shall be furnished as indicated in the datasheet.

Spare Acoustic Media Modules Furnishedradio

● None

○ 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

17.3 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.