Low‐frequency sound absorption in air

An extensive set of sound absorption measurements was taken in air over a range of frequency from 20–2500 Hz, of temperature from 20°–50 °C, and of relative humidity from 0.3%–100%. Over the lower portion of this frequency range, where relaxation in $N2$ is prominent (except in very dry air), prior measurements are scanty. This study yielded the following conclusions: (1) The humidity dependence of the relaxation frequency of $N2$ differs in air from that in binary $N2‐H2O$ gas mixtures (246‐Hz/atm. mole % in air vs 184‐Hz/atm. mole % in binary mixtures at 20 °C). (2) The temperature dependence of the relaxation frequency of $N2$ is the same in air as in binary mixtures. (3) At low humidities (∼0.01% mole ratio), where relaxation in $O2$ dominates sound absorption in air, the measured relaxation frequencies of $O2$ agree with those reported by Harris and Tempest [J. Acoust. Soc. Am. 36, 2390–2394 (1964)] and lie substantially lower than specified by ANSI Standard S1.26‐1978.