1 Scope

NOTE 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. (1.1)

NOTE Equipment covered includes straight absorptive (dissipative) silencers in rectangular and round/circular configurations, elbow silencers, splitter inserts, and pod-type cylindrical attenuators. (1.2)

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. (1.3)

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. (1.3.1)

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. (1.4)

NOTE The following are outside the scope of this standard. (1.5)

Duct liner materials and flexible duct wrap, which are part of the duct assembly and are covered by HVAC DuctworkHVAC DuctworkResolves to the current edition.sync/hvac-ductwork.
Duct fittings, access doors, turning vanes, and other general duct accessories, covered by Duct AccessoriesHVAC Duct AccessoriesResolves to the current edition.sync/duct-accessories.
Air terminal devices such as grilles, registers, diffusers, and fan-powered mixing boxes, covered by HVAC Air Distribution DevicesHVAC Air Distribution DevicesResolves to the current edition.sync/hvac-air-distribution-devices.
Variable air volume terminal units and their integral sound packages, covered by Variable Air Volume TerminalsVariable Air Volume Terminal UnitsResolves to the current edition.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 Makeup Air UnitsMakeup Air UnitsResolves to the current edition.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.

NOTE 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. (1.6)

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 E477-20	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

NOTE 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. (2.3)

3 Submittals

3.1 The Contractor shall submit the following Action Submittals for review prior to fabrication:

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.

Action Submittalscheckbox

☐ 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

3.2 The Contractor shall submit the following Closeout Submittals prior to Substantial Completion:

Operation and maintenance data describing inspection, cleaning, and fill-media replacement intervals.
Record documentation reflecting as-installed silencer marks, locations, and orientations.

Closeout Submittalscheckbox

☑ Operation and maintenance data

☑ As-installed record documentation

3.3 The Contractor shall submit the following Informational Submittals for projects where independent verification is required:

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.

Informational Submittalscheckbox

☐ NVLAP accreditation statement (number and scope)

☑ NFPA 90A flame-spread / smoke-developed certification

4 Quality Assurance

4.1 Acoustical performance data submitted for each silencer model shall originate from testing performed in accordance with ASTM E477.

4.2 Test data shall be produced by a laboratory accredited under the National Voluntary Laboratory Accreditation Program (NVLAP) for acoustical silencer testing.

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. (4.3)

4.4 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.

Acceptance Basis for Performance Dataradio

○ 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

4.5 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.

4.6 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.

5 Environmental and Service Conditions

NOTE 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. (5.1)

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. (5.1.1)

5.2 The silencer shall withstand the static pressure of the adjoining duct without panel deflection, leakage, or distortion at the system design pressure.

5.3 Casing material shall be selected from the following based on the service environment.

Casing Materialradio

● 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

5.4 The silencer casing pressure class shall match the static-pressure class of the adjoining duct run.

Casing Pressure Classselect

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.

5.5 The operating temperature range of the silencer and its fill media shall accommodate the design airstream temperature of the system it serves.

Airstream Temperature Rangerange

°F

40105

6 Acoustic Performance

NOTE 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. (6.1)

6.2 Dynamic Insertion Loss

NOTE 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. (6.2.1)

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. (6.2.2)

6.2.3 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.

6.2.4 For return and exhaust silencers, the silencer shall provide the scheduled dynamic insertion loss under reverse-flow conditions as well as forward-flow conditions.

Required Dynamic Insertion Loss — 63 Hzrange

dB

612

Required Dynamic Insertion Loss — 125 Hzrange

dB

1015

Required Dynamic Insertion Loss — 250 Hzrange

dB

1525

Required Dynamic Insertion Loss — 500 Hzrange

dB

2540

Required Dynamic Insertion Loss — 1000 Hzrange

dB

3045

Required Dynamic Insertion Loss — 2000 Hzrange

dB

3045

Required Dynamic Insertion Loss — 4000 Hzrange

dB

2035

Required Dynamic Insertion Loss — 8000 Hzrange

dB

1225

6.2.5 The flow direction for which the scheduled DIL applies shall be identified.

Rated Flow Directionradio

● Forward flow only (supply)

○ Reverse flow only (return/exhaust)

○ Both forward and reverse flow

6.3 Generated Noise Level

NOTE 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. (6.3.1)

NOTE 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. (6.3.2)

6.3.3 The generated noise level of the silencer at the design face velocity shall not exceed the scheduled sound power level in any octave band.

6.3.4 For return and exhaust silencers, the generated noise level under reverse-flow conditions shall not exceed the scheduled limit.

Maximum Generated Noise Level Targetradio

○ NC-25 equivalent (noise-critical spaces)

○ NC-30 equivalent

● NC-35 equivalent (general occupied space)

○ NC-40 equivalent (back-of-house / industrial)

6.4 Pressure Drop

NOTE 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. (6.4.1)

NOTE 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. (6.4.2)

6.4.3 The total pressure loss across the silencer at the design airflow and face velocity shall not exceed the scheduled maximum.

Maximum Total Pressure Droprange

in. w.g.

0.10.15

6.5 Face Velocity

NOTE 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. (6.5.1)

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. (6.5.2)

6.5.3 The silencer shall be sized so that the face velocity at design airflow does not exceed the scheduled maximum.

Maximum Face Velocityrange

FPM

10001500

7 Silencer Type and Configuration

NOTE 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. (7.1)

7.2 The silencer type shall be as scheduled and shall match the duct shape and orientation it serves.

Silencer Typeradio

● 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

NOTE 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. (7.3)

Performance Configurationradio

● Standard (balanced DIL and pressure drop)

○ Low-pressure-drop (wide baffle spacing, bell-mouth noses)

○ High-insertion-loss (narrow baffle spacing, extended length)

7.4 Active Length

NOTE 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. (7.4.1)

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. (7.4.2)

7.4.3 The silencer active length shall be as scheduled.

Active Lengthselect

3 ft (900 mm)

5 ft (1500 mm)

7 ft (2100 mm)

10 ft (3000 mm)

7.5 Round Silencer Diameter

7.5.1 For round and pod-type silencers, the casing diameter shall match the adjoining round duct.

Round Silencer Diameterrange

in.

660

Per drawings — round duct schedule (deferred by default)

8 Construction

NOTE 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. (8.1)

8.2 Casing

8.2.1 The casing shall be constructed of the scheduled material and gauge, with airtight seams suitable for the scheduled pressure class.

8.2.2 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.

8.2.3 Casing seams and joints shall be sealed to achieve the duct leakage class of the adjoining duct system.

8.2.4 Connection flanges or slip-and-drive edges shall be compatible with the connection method of the adjoining duct.

Connection Typeradio

● Flanged (companion angle / TDC)

○ Slip-and-drive

○ Raw edge for field connection

8.3 Baffles and Fill Media

8.3.1 The acoustic baffles shall contain the scheduled fill media, retained against erosion into the airstream at the design face velocity.

8.3.2 Fill media shall be glass fiber or mineral wool at a minimum density of 1.5 lb/ft³ (24 kg/m³).

8.3.3 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.

NOTE 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. (8.3.4)

NOTE 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. (8.3.5)

8.3.6 The fill-media encapsulation shall be as scheduled.

Fill Media Encapsulationradio

● Perforated-face liner (standard, fill protected)

○ Fully encapsulated fill (no exposed media)

○ Filmless / no-fill acoustic baffle (cleanroom)

8.3.7 The fill media density shall be as scheduled.

Fill Media Densityrange

lb/ft³

1.53

9 Testing

NOTE 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. (9.1)

9.2 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.

9.3 Testing shall be performed at a minimum of three airflow rates spanning the silencer's rated operating range.

9.4 Silencers serving return or exhaust ducts shall be tested in both forward-flow and reverse-flow directions.

9.5 Test reports shall state the NVLAP accreditation number and scope of the testing laboratory.

Test Standardradio

● ASTM E477-20 (North American)

○ ISO 7235:2003 (international)

○ ASTM E477 or ISO 7235 (either accepted)

10 Installation

NOTE 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. (10.1)

10.2 Inlet and Outlet Conditions

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. (10.2.1)

10.2.2 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.

10.2.3 Inlet and outlet transitions shall be gradual and shall not exceed the divergence angles recommended by the manufacturer.

Minimum Upstream Straight Ductrange

duct diameters

12

10.3 Flanking and Bypass Paths

NOTE 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. (10.3.1)

10.3.2 Plenum transition drawings shall require flanking paths to be sealed and lined.

10.3.3 Plenum transitions, offsets, and connections immediately adjacent to the silencer shall be acoustically lined where shown to prevent flanking transmission around the silencer.

10.3.4 Gaps between the silencer casing and surrounding construction shall be sealed airtight and acoustically.

10.3.5 The silencer shall be installed in the orientation and location shown, with the rated flow direction matching the system airflow. silencer location and orientation

10.4 Support

10.4.1 Silencers shall be independently supported from the structure and shall not impose load on the adjoining duct.

10.4.2 Support spacing and method shall accommodate the silencer weight, including fill media, without casing deflection.

11 Delivery, Storage, and Handling

NOTE 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. (11.1)

11.2 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.

11.3 Silencers shall be handled and rigged at designated lift points so that the casing is not racked or the baffles displaced.

11.4 Silencers showing wet, contaminated, or damaged fill media shall be rejected and replaced.

12 Warranty

12.1 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.

Warranty Periodselect

1 year

2 years

5 years

12.2 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.

13 Spare Parts

NOTE 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. (13.1)

13.2 Where required, the Contractor shall furnish replacement fill panels or baffle sections for silencers serving erosive or high-velocity airstreams.

Spare Fill Panels Furnishedradio

● None

○ One replacement baffle set per silencer type

○ Replacement fill panels for erosive/corrosive systems only