Low-frequency absorption of sound in air

Abstract

An extensive series of sound absorption measurements were taken in air over a range of frequencies from 10–2500 Hz, of temperature from 10°–50 °C, and of relative humidity from 0.3%–92%, all at a pressure of 1 atm. The N2 and O2 vibrational absorption components were extracted from a relatively large background absorption by means of a differential technique using a background gas, 89.5% N2–10.5% Ar, which matches the sound speed of air but has no molecular absorption over the range of experimental frequencies. The measurements reveal: (1) The humidity dependence of the relaxation frequency of N2 in air exceeds that in binary N2–H2O mixtures, possibly because the vibrational modes of CO2 provide a competing relaxation path for V–V exchange between N2 and H2O molecules; (2) the temperature dependence of the relaxation frequency of humid N2 is nearly the same in air as in binary mixtures; and (3) at very low humidities the relaxation frequency of O2 appears to approach a limiting value much lower than that determined from a prior study. The present experimental results, together with those of an earlier study at high frequencies, provide a substantial data base leading to revised formulas for standard values of the relaxation frequencies of N2 and O2.

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