Basic Documentation

Duct Attenuation
P/A 63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
1.75 .223 .150 .100 .067 .045 .030 .020 .013
1.70 .219 .147 .098 .066 .044 .029 .020 .013
1.60 .209 .140 .094 .063 .042 .028 .019 .012
1.50 .199 .134 .090 .060 .040 .027 .018 .012
1.40 .190 .127 .085 .057 .038 .026 .017 .011
1.30 .180 .121 .081 .054 .036 .024 .016 .011
1.25 .175 .117 .078 .053 .035 .024 .016 .010
1.20 .169 .114 .076 .051 .034 .023 .015 .010
1.15 .164 .110 .074 .049 .033 .022 .015 .010
1.10 .159 .107 .071 .048 .032 .021 .014 .010
1.05 .154 .103 .069 .046 .031 .021 .014 .009
1.00 .148 .100 .067 .045 .030 .020 .013 .009
0.95 .143 .096 .064 .043 .029 .019 .013 .009
0.90 .137 .093 .062 .041 .028 .019 .012 .008
0.85 .132 .089 .059 .040 .026 .018 .012 .008
0.80 .126 .085 .057 .038 .025 .017 .011 .008
0.75 .120 .081 .054 .036 .024 .016 .011 .007
0.70 .114 .077 .051 .034 .023 .015 .010 .007
0.65 .108 .073 .049 .033 .022 .015 .010 .007
0.60 .102 .069 .046 .031 .021 .014 .009 .006
0.55 .096 .064 .043 .029 .019 .013 .009 .006
0.50 .089 .060 .040 .027 .018 .012 .008 .005
Rectangular Unlined, Externally Insulated, Sheet Metal Ducts
Sheet metal ducts that have an unlined interior but are thermally insulated with about one
inch of fiberglass on the exterior, will attenuate sound at approximately twice the rate of
uninsulated, unlined rectangular ducts. (Be careful not to confuse external insulation with
ducts that are internally lined with sound absorbing material. Internally lined ducts have a
much higher rate of sound absorption than externally insulated ducts.)
To determine the attenuation of externally insulated rectangular ducts, multiply the final
attenuation figures obtained by the procedure for unlined (non-insulated) ducts, by 2.0.
In the previous example, the sound attenuation of the 80 foot run of 48-inch by 30-inch duct
would thus be approximately twice the values listed if the exterior of the duct were thermally
insulated.
Siemens Building Technologies, Inc. 53