Basic Documentation

Chapter 4–HVAC Sound Attenuation
Rectangular Acoustically Lined Sheet Metal Ducts
Rectangular ducts internally lined with at least one-inch thick fiberglass or similar material are
very effective at attenuating sound, particularly in the mid frequencies. Recall from Figure 14
on plenum lining, that fiberglass dramatically increases sound attenuation in the upper octave
bands. Also 2-inch thick fiberglass is significantly superior to the 1-inch thick material
particularly in the lower octave bands.
Figure 15. FW Values vs. dB Attenuation For Rectangular and Round Duct Elbows.
Equations have been developed over the last several years to calculate the specific dB
attenuation levels for various lining thickness; however, these are complex and would
consume extensive time. Table 17 and Table 18 provide values for rectangular ducts with 1-
inch and 2-inch internal acoustical linings based upon the Perimeter to Area (P/A) ratio of the
duct. The P/A ratio is calculated by dividing the duct perimeter (P) in feet by the area (A) in
square feet. Also, due to other factors involved in duct sound transmission, no more than 40
dB should be used when determining the attenuation values of a specific length of lined duct.
Table 17. One-Inch Thick Fiberglass Lined Rectangular Duct Attenuation Per Foot of Length.
P/A 63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
12.00 2.00 2.06 2.18 3.66 10.10 10.36 4.80 2.87
11.00 1.75 1.86 2.05 3.51 9.51 9.66 4.61 2.81
10.00 1.49 1.63 1.89 3.33 8.90 8.95 4.41 2.74
54 Siemens Building Technologies, Inc.