Handbook of air conditioning and refrigeration / Shan K

TABLE 19.1 Approximate Natural Attenuation of Sound in
Unlined Rectangular Sheet-Metal Ducts
Attenuation, dB/ft
Duct size, P/A,
Octave band center frequency, Hz
in. � in. 1/ft 63 125 250 � 250
6 � 6 8.0 0.30 0.20 0.10 0.10
12 � 12 4.0 0.35 0.20 0.10 0.06
12 � 24 3.0 0.40 0.20 0.10 0.05
24 � 24 2.0 0.25 0.20 0.10 0.03
48 � 48 1.0 0.15 0.10 0.07 0.02
72 � 72 0.7 0.10 0.10 0.05 0.02
P/A ratio indicates perimeter/area ratio.
Source: ASHRAE Handbook 1995, HVAC Applications. Reprinted with
permission.
Ducts can be unlined or inner-lined with foil-coated encapsulated fiberglass or other soundabsorptive
materials, usually at a thickness of 1 in. (25 mm), sometimes 2 in. (50 mm), to absorb
fan noise. Inner-lined ducts have better sound attenuation than unlined ducts, and the inner-lined
sound-absorbing layer can also serve as thermal insulation.
In unlined round ducts, the sound attenuation, called natural attenuation, is only about one-tenth
that in unlined rectangular ducts as follows:
Round duct attenuation, dB/ft
Octave band center frequency, Hz
Diameter, in. � 125 250 500 � 1000
D � 7 0.03 0.05 0.05 0.10
7 � D � 15 0.03 0.03 0.05 0.07
15 � D � 30 0.02 0.02 0.03 0.05
30 � D � 60 0.01 0.01 0.02 0.02
Source: ASHRAE Handbook 1995, HVAC Applications. Reprinted
with permission.
SOUND CONTROL 19.7
Natural attenuation of unlined rectangular ducts is listed in Table 19.1.
For inner-lined ducts, the sound attenuation, or insertion loss, of a given length of duct section
depends on the cross-sectional area and the properties of the absorptive material. Sound attenuation
in inner-lined round duct with a 1-in. (25-mm) thickness of duct liner is listed in Table 19.2, and for
1-in-. (25-mm-) thick inner-lined straight rectangular ducts it is listed in Table 19.3.
TABLE 19.2 Round Duct Attenuation (Insertion Loss)
Octave band center frequency, Hz
Duct
63 125 250 500 1000 2000 4000 8000
diameter, in. Approximate attenuation for 1-in. duct liner, dB/ft
6 0.38 0.59 0.93 1.53 2.17 2.31 2.04 1.26
12 0.23 0.46 0.81 1.45 2.18 1.91 1.48 1.05
24 0.07 0.25 0.57 1.28 1.71 1.24 0.85 0.80
48 0 0 0.18 0.63 0.26 0.34 0.45 0.44
Source: Abridged with permission from ASHRAE Handbook 1995, HVAC Applications.