Use duct construction, octave sound power, and receiver geometry to estimate breakout contributions and budget margins.
Concepts behind this calculation
Estimate octave sound power radiated through a documented duct wall and its near-line receiver contribution, including along-duct attenuation and allocated noise-budget margins.
Breakout is sound leaving through the duct wall along a run. The model combines inlet octave sound power, normalized wall transmission loss and attenuation along the duct. Its effective radiating length accounts for source energy decreasing downstream: when attenuation is zero it equals actual length, while positive attenuation reduces the effective contribution of the farther wall. Cross-sectional area and wall perimeter supply the geometric reference for radiated power.
Receiver sound pressure is a further step using a documented near-line arrangement. That relation uses actual radiating length, even when effective length is shorter. Confinement and separate ceiling or path losses must describe distinct effects. Wall transmission data are specific to size, gauge, seams and lining; a missing or bounded table cell cannot support a complete spectrum merely because adjacent frequencies are available.
How to use this calculator
Complete Construction, coefficient and receiver references and establish both Construction and octave data matched and Near-line receiver model applicable.
Select Duct-wall construction and TL source. Enter the matching custom dimensions and Custom TL rows when using a custom profile.
Enter Actual sound-radiating duct length, Duct-to-receiver distance and Documented tight-plenum correction, keeping separate path attenuation in the Octave rows.
Supply all eight Octave rows with inlet sound power, attenuation rate and allocated receiver budget; inspect support and energy consistency before interpreting margins.
Calculation method and applicability
k = alpha × ln(10) / 10; effective length = [1 − exp(−kL)] / k, with L at alpha=0. Effective area = 12 × perimeter(in) × effective length(ft). Lw,out = Lw,in + 10 × log10(S*/A) − TLout. Lp = Lw,out − 10 × log10(pi × r × L) + 10 + confinement − separate path loss (native I-P).
alpha = along-duct attenuation (dB/ft); L = radiating length (ft); r = receiver distance (ft); TLout = breakout transmission loss (dB). Lw is sound power level and Lp is receiver sound pressure level; their references and area normalization must stay distinct.
Input definitions and units
Construction, coefficient and receiver references
Required: exact material/gage/seam/lining/size, source editions and band rows, actual along-duct attenuation, source sound-power plane, path losses, allocated budgets and near-line receiver applicability. These are separate source/project inputs, not a full room-noise certificate.
Construction and octave data matched?
Confirm actual applicability of the normalized breakout TL and attenuation rate. Published specimen lengths, liners and gages are not universal material coefficients. Choices: Not established — withhold calculation; Matched for this limited comparison.
Near-line receiver model applicable?
This historical approximation is for a listener close to a duct in a wide-open ceiling plenum. Do not use it as a general reverberant-room solver. Document any confinement and separate path-loss basis . Choices: Not established — choose a suitable acoustic model; Documented near-line breakout contribution.
Duct-wall construction and TL source
The exact source dimensions/gage/seam/lining apply to table choices. There is no size, thickness or material interpolation. Missing, greater-than and parenthesized table cells remain unsupported; round/oval tables lack a complete eight-band set. Custom profiles require actual matching TL at all eight bands. Choices: Custom rectangular — actual matched TL; Custom round — actual matched TL; Custom flat oval — actual matched TL; Table 29: 12×12 in rectangular, 24 ga; Table 29: 24×12 in rectangular, 24 ga; Table 29: 48×12 in rectangular, 22 ga; Table 29: 24×24 in rectangular, 22 ga; Table 29: 48×24 in rectangular, 20 ga; Table 29: 48×48 in rectangular, 18 ga; Table 29: 96×48 in rectangular, 18 ga; Table 30: 8 in long seam, 26 ga, 15 ft specimen; Table 30: 14 in long seam, 24 ga, 15 ft specimen; Table 30: 22 in long seam, 22 ga, 15 ft specimen; Table 30: 32 in long seam, 22 ga, 15 ft specimen; Table 30: 12 in spiral, 26 ga, 12 ft specimen, 1 in liner/perforated inner wall; Table 30: 24 in spiral, 24 ga, 24 ft specimen,unlined; Table 30: 24 in spiral, 24 ga, 24 ft specimen, 1 in liner/perforated inner wall; Table 30: 24 in spiral, 16 ga, 10 ft specimen,unlined; Table 30: 36 in spiral, 20 ga, 24 ft specimen,unlined; Table 31: 12×6 in flat oval, 24 ga; Table 31: 24×6 in flat oval, 24 ga; Table 31: 24×12 in flat oval, 24 ga; Table 31: 48×12 in flat oval, 22 ga; Table 31: 48×24 in flat oval, 22 ga; Table 31: 96×24 in flat oval, 20 ga; Table 31: 96×48 in flat oval, 18 ga.
Custom larger rectangular side / oval major axis (in)
Actual source-reference cross-section dimension. For oval ducts this includes both semicircular ends; no automatic liner-thickness deduction is made. Applies to Duct-wall construction and TL source: Custom rectangular — actual matched TL or Custom flat oval — actual matched TL.
Custom smaller rectangular side / oval minor axis (in)
Positive actual source-reference dimension; oval minor axis must not exceed the major axis . Applies to Duct-wall construction and TL source: Custom rectangular — actual matched TL or Custom flat oval — actual matched TL.
Custom round diameter (in)
Actual diameter defining the source normalized TL and internal cross-section. Do not transfer coefficients between seam/liner/gage constructions. Applies to Duct-wall construction and TL source: Custom round — actual matched TL.
Actual sound-radiating duct length (ft)
The attenuation integral uses this length. The receiver relation also uses actual length, not the shorter effective radiating length.
Duct-to-receiver distance (ft)
Source-defined distance from the duct to the listener for the near-line approximation. No universal near/far cutoff is invented; suitability requires an acoustic basis .
Documented tight-plenum correction (dB)
Zero is the wide-open ceiling-plenum estimate. The source notes up to 6 dB increase in a tight slab/ceiling space; enter an evidenced value 0–6, not an assumed default for every room. Keep separate ceiling/path attenuation distinct.
Exactly eight unique frequencies 63–8000 Hz. Lw is octave sound power dB re 1 pW at this duct inlet; alpha is dB/ft; separate losses and allocated receiver budget are dB. Negative Lw/budget values are valid. Do not paste dBA, room Lp, break-in TL, total lined-duct insertion loss or an NC scalar into these columns. Example alpha follows Table 16 24×24 in unlined data; the eighth source level and all budgets are constructed.
Eight exact frequencies and nonnegative actual normalized TLout values in dB. The example is a 24×24 in 22 ga rectangular source profile; it is not a generic round/oval preset. Use actual matched data for another construction. Do not replace missing or bounded data with zero. Applies to Duct-wall construction and TL source: Custom rectangular — actual matched TL or Custom round — actual matched TL or Custom flat oval — actual matched TL.
How to interpret the result
Compare supported receiver contributions with the allocated breakout budget band by band. Preserve unsupported frequencies and the receiver-model qualification; these results concern one airborne wall path and do not rate total room noise or structural duct rumble.
Worked examples
Worked example
Inputs: 2019 Example 4 geometry:24×24 in 22 ga rectangular duct,25 ft radiating length,receiver 5 ft away. First seven inlet octave levels are 90,85,80,75,70,65,60 dB. Table 29 TL values are 20,23,26,29,32,37,43 dB; Table 16 alpha is .25,.2,.1,.03,.03,.03,.03 dB/ft. Extend to 8000 Hz with constructed inlet 55 dB and Table 29 TL 45 dB; project budgets 70,60,55,50,45,40,35,30 dB are constructed.
Calculation: At 125 Hz, k = 0.2 × ln(10) / 10 and effective length = 14.8479 ft. Area = 576 in² and perimeter = 96 in. The equations give radiated sound power 76.727 dB and near-line correction−15.941 dB, so receiver contribution = 60.786 dB. Against the allocated 60 dB budget, margin = −0.786 dB.
Result: Seven supported bands meet the entered budgets;125 Hz exceeds its allocation by 0.786 dB. This is one airborne breakout contribution under the stated historical receiver model. It is not a complete room NC/RC/dBA evaluation or a certified actual installation. Printed source examples contain arithmetic/table discrepancies; the independent calculation is retained.
Assumptions and limits
This is a named 2019 ASHRAE method, not current-code approval. It estimates a single duct airborne breakout contribution at 63–8000 Hz, not very-low-frequency structural duct rumble, vibration, joints, leakage, whole-system noise or room reverberation.
Exact source wall profiles retain dimensions, historical gage, seam, lining and specimen provenance. No geometric/material/gage interpolation, heavier-wall rule, automatic liner deduction or lagging benefit is inferred. Custom coefficients must use the same normalized TL reference area as the entered geometry.
Table 30 includes lower-bound and parenthesized cells, and round/flat-oval tables lack high-frequency data. These cells remain unsupported. No complete eight-band budget result or aggregate is manufactured from partial data.
The source corrected method assumes a suitable constant attenuation rate for each band along this run. Modern lined-duct insertion loss varies with length; do not divide a total catalog loss by length without a documented valid rate/model. Source Examples 5/6 contain historical alpha and arithmetic inconsistencies.
The near-line receiver equation uses actual radiating length and I-P constant 10. It assumes the source wide-open ceiling-plenum receiver arrangement. A tight space may be up to 6 dB louder; the correction requires a basis . Do not add a point-source room correction or subtract the same ceiling loss twice.
Negative valid sound levels are preserved. Inputs implying more radiated sound power than incident power are rejected for inconsistent TL/attenuation/length, rather than clipped into a plausible-looking answer.
Allocated octave budgets concern this path only. Background, other ducts, diffusers and correlated sources require a complete compatible acoustic model before any overall room-noise claim. This page does not compute NC, RC, NR or dBA.
Daikin AG 31-010 gives a simpler short-duct method and a receiver constant 10.5, while the selected 2019 source uses 10. Those methods are not blended. Its printed breakout example also has length/area and 0 dB-floor discrepancies; it is context, not an exact full-case numeric benchmark.
FAQs
Can I divide a catalog lined-duct insertion loss by length to obtain attenuation rate?
Only if the source supports a constant rate for the actual run and band. Total insertion loss can vary nonlinearly with length; dividing it by length does not establish the attenuation model used here.
Why is there no complete budget result for my round duct table profile?
Some historical profiles lack usable values across all eight bands. Supported bands can still show their individual contribution, but missing, lower-bound or otherwise unsupported cells prevent a complete budget conclusion.
Daikin AG 31-010: HVAC Acoustic FundamentalsPages 44–47 provide independent manufacturer context for the breakout sound path and line-source receiver. Its constant 10.5/simple-length examples are distinct from the implemented 2019 model; no forced agreement.
