Basic Documentation

Chapter 4–HVAC Sound Attenuation
Frequency is in Hz Width is in inches.
When the FW values are determined, refer to the appropriate curve in Figure 15 to determine
the dB attenuation at each octave band.
Example Rectangular Duct Elbow Attenuation Calculation
An unlined duct is 36 inches wide by 28 inches high. Determine the attenuation at each
octave band provided by a square elbow that has turning vanes.
First calculate the FW factor for each octave band:
FW = (Frequency x Width) ÷ 1,000
FW = (Frequency x 36 inches) ÷ 1,000
FW = Frequency x 0.036
Table 22 lists the calculated FW factors and the corresponding attenuation dB values using
the “RECTANGULAR, SQUARE, UNLINED, WITH VANES” curve.
Table 22. Calculated FW Factors and Attenuation dB Values.
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
FW
2.3 4.5 9.0 18.0 36.0 72.0 144 288
ATTN:
2 dB 5 dB 6 dB 3 dB 3 dB 2 dB 2 dB 2 dB
Since round ducts are more efficient in conveying air than the same area of rectangular duct,
they are also more efficient at conveying sound, and will provide less attenuation whether
they are lined or unlined.
In Figure 15, the curves that apply to lined elbows, are based upon at least a 1-inch thick
lining, and the lining extending at least one to two duct diameters ahead of and after the
elbow.
Duct Takeoffs and Divisions
When a takeoff occurs on a main duct or the main duct divides, the sound energy is also
divided between the resulting duct runs after the takeoff, so that neither duct at the point
where it leaves a junction has all of the sound power level energy entering the junction. The
following formula will estimate the resulting dB attenuation that occurs with reference to the
junction and a specific duct leaving the junction:
Attenuation = 10 Log [Branch Area ÷ Total Area]
Branch Area = Branch Duct Area (Square Inches)
Total Area = Total Duct Area Leaving Junction (Square Inches)
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