HVAC Duct Calculators

Noise, installation and procurement

Duct Silencer Selection Calculator

Compare silencer ratings with airflow, pressure, and octave sound budgets to rank eligible configurations.

Calculation inputs

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An example is loaded automatically. Replace it with your project values; the result updates as you edit.

Required: candidate ID-to-model/size/revision mapping, actual face-area and envelope definitions, same signed flow/direction, air properties, dynamic IL and generated-noise rating planes, source bounds and any permitted area correction. State actual installation effects and source-data uncertainties.
Worked-example data and reference

McGill RSF-LV-L 31, historical 7/06 sheet in 2011 compilation p 12: actual 4 ft 2 face at +1250 FPM; 36/60/84 in lengths; Tables 1–4. No extra installation loss in the constructed ideal-flow example.

These example values are for the worked calculation. Replace them with the source and conditions for your own case.

Required: acoustic source plane, upstream/downstream losses without double-counting, independent background at the receiver, receiver-model applicability, outlet/source dimensions and project band-budget allocation. Include terminal self-noise and other paths where relevant; a budget allocation is not whole-room compliance.
Worked-example data and reference

Constructed single point-outlet scenario: 5000 CFM; source Lw before upstream losses; 3 dB upstream and 5 dB downstream losses. Schultz room 10000 ft 3, receiver 5 ft away, independent receiver background 30 dB; allocated octave budgets are illustrative.

These example values are for the worked calculation. Replace them with the source and conditions for your own case.

Confirm actual acoustic, geometric and pressure conditions. A reference or checkbox cannot authenticate current product availability, test certification or field data.
The receiver choices have different acoustic assumptions. Use a suitable independently established model if the room/source does not fit one of these historical profiles.
All methods use feet and octave sound power. A point outlet is the downstream radiating source, not automatically the full fan/silencer housing.
Actual flow must match the candidate rating direction and face velocity. The manufacturer sign convention is part of the data reference. Zero flow is outside this dynamic operating comparison; no interpolation or reverse-flow transfer is performed.
Compare against the actual body/envelope dimensions, not just the duct connection. Required service clearances must be included in the documented envelope.
Use the actual space and complete candidate envelope, including source/project clearances where applicable.
Lengths and acoustic configurations must match the actual source rating. Adding length does not imply a linear increase in attenuation.
Budget for this silencer plus separately documented installation loss after other system losses. An exceeded budget is retained; no airflow or fan-power prediction is inferred.
Distance is from the actual downstream acoustic source. Receiver models do not use the silencer face dimensions as an automatic substitute.
Schultz Eq 26 is for a single point source in a normally furnished, regular room with average-to-medium-dead acoustics and volume below 15,000 ft³. This is not the diffuse equation for a live or unfurnished room.
Background is already Lp at this same receiver and band. Disabling it removes that energy term; a numeric 0 dB is not silence. Do not include a contribution already represented in the source/path.
Only candidates with eight supported bands and passing every entered envelope, pressure and octave criterion are ranked. The other size/loss objective breaks ties, then lower ID.
One to 20 actual configurations. Area ft²; body/envelope dimensions in; pressure losses in. w.g.; self-noise adjustment dB from the actual manufacturer area/size rule. Default IDs 1/2/3 are RSF-LV-L 31 at 60/36/84 in; width 30 in and height 19.2 in give 4 ft² within the listed width availability. Self-noise is referenced to 4 ft², so adjustment 0. Extra drop 0 is an ideal-flow teaching assumption, not a field guarantee.
Eight unique frequencies 63–8000 Hz. Lw dB re 1 pW before upstream losses; losses dB; background/budget dB re 20 µPa at the receiver. R is the frequency-specific room constant in ft², used only for diffuse mode; enter a finite placeholder such as 0 otherwise. Background values are ignored when disabled. All example source/path/background/room/budget inputs are constructed, not a measured installation.
Worked-example data and reference

63,80,3,5,30,60,500 125,80,3,5,30,55,500 250,80,3,5,30,50,500 500,80,3,5,30,40,500 1000,75,3,5,30,35,500 2000,70,3,5,30,35,500 4000,65,3,5,30,35,500 8000,60,3,5,30,35,500

These example values are for the worked calculation. Replace them with the source and conditions for your own case.

One unique row per candidate/frequency at the exact entered signed face velocity and configuration. Flags 0=reported point;1=IL lower bound or generated-noise upper bound, only when the source explicitly establishes that direction. Missing bands remain absent; do not put 0 for unknown sound power. Use all eight actual matched bands for selection. McGill p 12 positive 1250 FPM values are reported points; its parenthesis convention for other rows has distinct IL/noise bound directions. Conservative bounds are not exact field measurements.
Worked-example data and reference

1,63,6,0,53,0 1,125,10,0,39,0 1,250,15,0,39,0 1,500,33,0,38,0 1,1000,43,0,41,0 1,2000,35,0,46,0 1,4000,26,0,41,0 1,8000,14,0,36,0 2,63,4,0,53,0 2,125,6,0,39,0 2,250,9,0,39,0 2,500,22,0,38,0 2,1000,27,0,41,0 2,2000,21,0,46,0 2,4000,17,0,41,0 2,8000,11,0,36,0 3,63,7,0,53,0 3,125,17,0,39,0 3,250,21,0,39,0 3,500,41,0,38,0 3,1000,54,0,41,0 3,2000,44,0,46,0 3,4000,37,0,41,0 3,8000,21,0,36,0

These example values are for the worked calculation. Replace them with the source and conditions for your own case.

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Formula and source basis

Concepts behind this calculation

Compare actual silencer configurations against octave receiver budgets, including transmitted sound, regenerated noise, background, pressure loss and installation size.

A silencer both attenuates incoming sound and generates flow noise. Dynamic insertion loss reduces the source contribution, but the regenerated sound enters on the receiving side and must be combined in acoustic energy. Upstream and downstream losses belong to their respective parts of that path. Independent receiver background is already sound pressure at the listener, so it joins the comparison only after the source-power contributions have been transferred to that receiver.

More insertion loss cannot solve a band whose fixed generated-noise and background floor already fills the budget. Product comparison also requires exact signed flow and face velocity, sufficient installation envelope and acceptable installed pressure loss. The receiver model must fit the actual space and downstream radiating source. Missing acoustic bands or unmatched operating points prevent selection rather than being replaced with quiet assumptions.

How to use this calculator

  1. Document Product, installation and octave-data references and Source/path, receiver and allocated-budget references; establish both data and receiver assumptions.
  2. Choose Point-receiver calculation and enter its distance, room volume or directivity and source dimension as applicable, together with Independent receiver background included status.
  3. Enter Signed actual volume flow, available installation dimensions and Remaining installed-loss budget. Add matching Candidate rows and select Selection objective.
  4. Supply Receiver-band rows and Acoustic rows for every required band, preserving source bound flags. Review fixed floors and unsupported states before candidate ranking.

Calculation method and applicability

Transmitted Lw = source Lw − upstream loss − dynamic IL. Combine transmitted and area-corrected generated Lw with 10 × log10(sum of 10^(L/10)); subtract downstream loss and apply the selected point-receiver transfer. Add independent receiver background in energy space. Required IL subtracts fixed self-noise/background energy from the band budget; withhold it when the fixed floor fills that budget.

Lw = band sound power level (dB re 1 pW); IL = dynamic insertion loss (dB). Receiver Lp is sound pressure (dB re 20 µPa). Convert level contributions to linear energy before combining them.

Input definitions and units

Product, installation and octave-data references
Required: candidate ID-to-model/size/revision mapping, actual face-area and envelope definitions, same signed flow/direction, air properties, dynamic IL and generated-noise rating planes, source bounds and any permitted area correction. State actual installation effects and source-data uncertainties.
Source/path, receiver and allocated-budget references
Required: acoustic source plane, upstream/downstream losses without double-counting, independent background at the receiver, receiver-model applicability, outlet/source dimensions and project band-budget allocation. Include terminal self-noise and other paths where relevant; a budget allocation is not whole-room compliance.
Product, direction and source/path data established?
Confirm actual acoustic, geometric and pressure conditions. A reference or checkbox cannot authenticate current product availability, test certification or field data. Choices: Not established — withhold comparison; Documented for the entered comparison.
Receiver-model assumptions established?
The receiver choices have different acoustic assumptions. Use a suitable independently established model if the room/source does not fit one of these historical profiles. Choices: Not established — choose a suitable acoustic model; Selected profile documented.
Point-receiver calculation
All methods use feet and octave sound power. A point outlet is the downstream radiating source, not automatically the full fan/silencer housing. Choices: Schultz — normally furnished room below 15,000 ft³; Classical diffuse field — reflective unfurnished room; Outdoor free field — supported point-source distance.
Signed actual volume flow (CFM)
Actual flow must match the candidate rating direction and face velocity. The manufacturer sign convention is part of the data reference. Zero flow is outside this dynamic operating comparison; no interpolation or reverse-flow transfer is performed.
Available installation width (in)
Compare against the actual body/envelope dimensions, not just the duct connection. Required service clearances must be included in the documented envelope.
Available installation height (in)
Use the actual space and complete candidate envelope, including source/project clearances where applicable.
Available installation length (in)
Lengths and acoustic configurations must match the actual source rating. Adding length does not imply a linear increase in attenuation.
Remaining installed-loss budget (in. w.g.)
Budget for this silencer plus separately documented installation loss after other system losses. An exceeded budget is retained; no airflow or fan-power prediction is inferred.
Radiating point source to receiver (ft)
Distance is from the actual downstream acoustic source. Receiver models do not use the silencer face dimensions as an automatic substitute.
Normally furnished room volume (ft³)
Schultz Eq 26 is for a single point source in a normally furnished, regular room with average-to-medium-dead acoustics and volume below 15,000 ft³. This is not the diffuse equation for a live or unfurnished room. Applies to Point-receiver calculation: Schultz — normally furnished room below 15,000 ft³.
Documented radiating-source directivity (Q)
Positive actual source directivity at the receiver. Do not infer it from the silencer body or from a universal room factor. Applies to Point-receiver calculation: Classical diffuse field — reflective unfurnished room or Outdoor free field — supported point-source distance.
Maximum radiating-source dimension (ft)
Outdoor equation excludes distances below twice this dimension. At 2–5 dimensions, the source warns of possible underprediction up to 5 dB; the calculator retains the warning without adding an arbitrary correction. Applies to Point-receiver calculation: Outdoor free field — supported point-source distance.
Independent receiver background included?
Background is already Lp at this same receiver and band. Disabling it removes that energy term; a numeric 0 dB is not silence. Do not include a contribution already represented in the source/path. Choices: Absent from this allocated-path comparison; Include documented independent receiver background.
Selection objective
Only candidates with eight supported bands and passing every entered envelope, pressure and octave criterion are ranked. The other size/loss objective breaks ties, then lower ID. Choices: Lowest total pressure loss, then shortest length; Shortest body length, then lowest pressure loss.
Candidate rows: ID, face area, width, height, length, signed rated FPM, rated drop, extra drop, noise adjustment (CSV)
One to 20 actual configurations. Area ft²; body/envelope dimensions in; pressure losses in. w.g.; self-noise adjustment dB from the actual manufacturer area/size rule. Default IDs 1/2/3 are RSF-LV-L 31 at 60/36/84 in; width 30 in and height 19.2 in give 4 ft² within the listed width availability. Self-noise is referenced to 4 ft², so adjustment 0. Extra drop 0 is an ideal-flow teaching assumption, not a field guarantee.
Receiver-band rows: Hz, source Lw, upstream loss, downstream loss, background Lp, allocated Lp budget, room R (CSV)
Eight unique frequencies 63–8000 Hz. Lw dB re 1 pW before upstream losses; losses dB; background/budget dB re 20 µPa at the receiver. R is the frequency-specific room constant in ft², used only for diffuse mode; enter a finite placeholder such as 0 otherwise. Background values are ignored when disabled. All example source/path/background/room/budget inputs are constructed, not a measured installation.
Acoustic rows: candidate ID, Hz, dynamic IL, IL lower-bound flag, generated Lw, noise upper-bound flag (CSV)
One unique row per candidate/frequency at the exact entered signed face velocity and configuration. Flags 0=reported point;1=IL lower bound or generated-noise upper bound, only when the source explicitly establishes that direction. Missing bands remain absent; do not put 0 for unknown sound power. Use all eight actual matched bands for selection. McGill p 12 positive 1250 FPM values are reported points; its parenthesis convention for other rows has distinct IL/noise bound directions. Conservative bounds are not exact field measurements.

How to interpret the result

Use the ranked eligible ID only with its eight-band support, pressure and envelope margins. A pass based on explicitly conservative source bounds remains a bound-based comparison. The path budget does not establish total-room acceptance or actual installed accuracy.

Worked examples

Worked example

Inputs: Historical McGill RSF-LV-L 31 p 12: 4 ft² face at +1250 FPM (5000 CFM), 36/60/84 in lengths; dynamic IL and generated noise use matching source rows. Candidate 1 is 60 in, rated drop 0.27 in. w.g.; Candidate 2 is 36 in/.25 and Candidate 3 is 84 in/.30. Constructed project: 36×24×84 in envelope, remaining drop budget 0.28 in. w.g., 3 dB upstream/5 dB downstream losses, 10000 ft³ furnished room and 5 ft receiver, independent 30 dB background and illustrative band budgets.

Calculation: At 63 Hz for Candidate 1: source 80 dB − upstream 3 dB − IL 6 dB = transmitted 71 dB Lw. Generated Lw 53 dB, with zero area adjustment, combines energetically to 71.068 dB. The Schultz receiver transfer is −7.388 dB; after 5 dB downstream loss, the path gives 58.681 dB Lp. Adding independent 30 dB background gives 58.686 dB. Against 60 dB budget, margin=1.314 dB and required IL accounting for fixed floors=4.667 dB.

Result: Candidate 1 meets all entered eight-band, envelope and pressure criteria. The 36 in candidate exceeds the 63 Hz budget; the 84 in candidate exceeds the remaining pressure allowance. Product coordinates are source data; project loads, paths, receiver and budgets are constructed. Historical shaded velocity/pressure ranges carry source cautions for acoustically sensitive applications; this is not a recommendation or actual field guarantee.

Assumptions and limits

  • Only actual matched product data are compared. Candidate face velocity is signed flow divided by source face area; missing or differing flow/direction is unsupported. No IL/self-noise/pressure interpolation, cross-size transfer, current availability or automatic density correction is assumed.
  • Dynamic IL and generated noise are different source quantities. Combine the receiving-side regenerated sound with transmitted source energy before applying downstream losses. A large IL alone cannot overcome a fixed self-noise or independent-background floor.
  • Reported points and explicitly documented lower-IL/upper-noise bounds remain distinct. A bound-based passing comparison is conservative for those entered bounds; failure means it is not demonstrated at those bounds. A missing or unknown cell is never 0 dB or a silent source.
  • The selected historical point-receiver profile must fit the source and room. Schultz is for normally furnished regular rooms below 15,000 ft³; the classical diffuse equation is for reflective unfurnished rooms; outdoor free field excludes distances below 2 maximum-source dimensions and may underpredict within 2–5 dimensions. No whole-room or arbitrary room-model claim is made.
  • Source and self-noise must use compatible Lw references; background is independent receiver Lp. Correlated tones require phase-aware analysis. Do not combine dBA/NC scalars with octave power or treat source Lw as room Lp.
  • Other ducts, flanking, breakout, terminal self-noise and room sources must be represented by compatible documented contributions when judging a total criterion. This page reports the entered path/background comparison and does not calculate NC, RC, NR, dBA or current-code compliance.
  • Extra installed pressure loss is separately entered and added to rated loss. Do not double-count ordinary loss or assume available system airflow when fan/static capability is unknown. Envelope inputs must include real service/installation clearances.
  • The McGill source is a 2011 compilation with p 12 dated 7/06, not a current product certificate. It notes interlaboratory IL and generated-noise variation up to 6 dB at 63 Hz and 3 dB elsewhere, plus cautions in shaded conditions. The arithmetic does not guarantee those variations or actual field accuracy.
  • The full example combines actual historical product coordinates with constructed source/path/room/budget data. No complete actual installation, current catalog availability, ASTM certification, capture/comfort or AdSense readiness is established.

FAQs

Why is required insertion loss withheld in a band?

The fixed self-noise and independent background may already consume the band’s entire energy budget. Further attenuation of the incoming source cannot bring that fixed floor below the target; review the noise source, background or selected configuration.

Can I use a positive-flow rating for the same magnitude of reverse flow?

The signed direction is part of the product rating. Reverse flow can change dynamic attenuation, generated noise and pressure behavior, so this comparison needs matching signed data and does not transfer the forward rating automatically.

Sources

Related calculators

Fan Installation System Effect Calculator Apply documented inlet and outlet geometry to estimate system-effect pressure and supported airflow changes. VAV Terminal Airflow and Selection Calculator Compare zone loads, ventilation, and exact catalog points to check terminal flow, pressure, and capacity margins. Duct Breakout Noise Calculator Use duct construction, octave sound power, and receiver geometry to estimate breakout contributions and budget margins.

Related reading

Select a silencer using insertion loss, generated noise, and pressure drop Rank silencer candidates only when actual airflow, pressure, space, source spectrum, receiver budget, insertion loss, and regenerated noise are supported by matching data.