Ultrasonic Absorption and Relaxation Times in Nitrogen, Oxygen, and Water Vapor

Abstract

The absorption coefficient α is measured in nitrogen, oxygen, and water vapor by a direct travelling-wave method which employs a pulse-modulated signal of about 65 kc. The apparatus used is a modification of that previously reported by C. Ener, O. Herbjørnsen, Y. Fujii, and L. Nyborg [J. Acoust. Soc. Am. 32, 929 (1960)]. Argon is used as a calibrating gas, and the experimental values of α agree with the theoretical classical values within 1%. The rotational relaxation times in nitrogen and oxygen for 22°C and 1 atm are obtained from the experimental excess absorption and other thermodynamic quantities as (7.6±0.6) × 10−10 sec for nitrogen and (5.1±0.4) × 10−10 sec for oxygen. The coefficient α in water vapor is measured for temperatures between 42° and 183°C and over the range of angular frequency/pressure from 1 to 50 Mc/atm. In this range, no absorption peak is observed. The observed excess absorption over classical is considered to be due primarily to vibrational relaxation, since the excess absorption increases with the temperature nearly proportionally to the specific heat of the vibrational state with the lowest eigenfrequency, whereas the specific heat of the rotational states remains practically constant with temperature. If the excess absorption is completely due to vibrational relaxation, this permits the calculation of a relaxation time, which at 1 atm is found to lie in the interval from 0.8×10−8 to 2.0×10−8 sec. This is of the same order of magnitude as that reported by other observers for higher temperature. (This work was supported by the Office of Naval Research.)