Basic Documentation

Duct Attenuation
Table 15. Approximate dB Attenuation for Each Octave Band.
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
14.4 dB 14.4 dB 16.7 dB 17.2 dB 17.2 dB 17.2 dB 17.2 dB 17.2 dB
Note that when the sound absorption coefficients are the same (as they are for 500 Hz and
higher in this example), the attenuation level is the same.
Duct Attenuation
The most efficient approach to sound attenuation is to attain the necessary level of
attenuation by means of the duct runs. While this may not always be practical, or even
attainable, it is the optimum solution from a first cost and operating cost standpoint.
The attenuation or insertion loss of sheet metal ducts is highly dependent upon the size and
shape of the duct. Smaller ducts attenuate more dB per foot than larger ducts, and for a
given effective area, ducts that are more rectangular attenuate more dB than those that are
closer to being square or round. Also, ducts that have an internal lining attenuate sound
much more than unlined ducts.
Rectangular Unlined Sheet Metal Ducts
To determine the attenuation of this type of duct, first calculate the Perimeter to Area ratio:
Perimeter to Area ratio = P ÷ A
Where:
P = the duct perimeter in feet.
A = the duct cross sectional area in square feet.
Next, use the appropriate formula below to determine the dB attenuation per foot of duct
length or refer to Table 16.
• If P/A is equal to or greater than 3:
Attenuation per foot = 17.0 x (P ÷ A)
-0.25
x Hz
-0.85
• If P/A is less than 3:
Attenuation per foot = 1.64 x (P ÷ A)
0.73
x Hz
-0.58
Example Rectangular Duct Attenuation Calculation
An unlined duct is 48 inches wide by 30 inches high. Determine the attenuation for each
octave band for an 80 foot run.
Siemens Building Technologies, Inc. 51