Basic Documentation

Chapter 4–HVAC Sound Attenuation
First calculate the P/A ratio:
P = (48 in. + 36 in. + 48 in. + 36 in. ) ÷ 12 in./ft
= 168 ÷ 12
= 14 feet
A = (48 in. x 36 in.) ÷ 144 sq in./sq ft
= 1728 ÷ 144
= 12 sq ft
P/A = 14 ÷ 12 = 1.17
With reference to Table 16, the following dB attenuation factors per foot can be interpolated:
P/A 63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
1.17 .166 .112 .075 .050 .033 .022 .015 .010
Multiplying the values obtained by 80 feet yields the following attenuation in each octave
band:
— 63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
— 13.3 dB 9.0 dB 6.0 dB 4.0 dB 2.6 dB 1.8 dB 1.2 dB 0.8 dB
Table 16. Unlined 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
5.00 .336 .188 .104 .058 .032 .018 .010 .006
4.00 .355 .198 .110 .061 .034 .019 .010 .006
3.75 .361 .202 .112 .062 .034 .019 .010 .006
3.50 .367 .205 .114 .063 .035 .019 .011 .006
3.25 .374 .209 .116 .064 .036 .020 .011 .006
3.00 .382 .213 .118 .066 .036 .020 .011 .006
2.75 .310 .209 .140 .093 .062 .042 .028 .018
2.50 .290 .195 .130 .087 .058 .039 .026 .017
2.25 .268 .180 .120 .081 .054 .036 .024 .016
2.00 .246 .165 .111 .074 .049 .033 .022 .015
1.90 .237 .159 .107 .071 .048 .032 .021 .014
1.80 .228 .153 .102 .069 .046 .031 .021 .014
52 Siemens Building Technologies, Inc.