Sound Velocity Measurement in Gases: A Study of the Ultrasonic Interferometer Method

Abstract

A simple, Pierce type, acoustic interferometer has been constructed for the purpose of measuring wavelengths and determining velocities of compressional waves in gases at ultrasonic frequencies under controlled conditions of temperature, pressure and constitution. Measurements have been made in dry, CO2-free air at room temperature and atmospheric pressure. The frequencies of the piezo-electric quartz crystals were 126.8 and 676.7 Kc/s. From the measured wavelengths and frequencies velocities have been computed and corrected to standard conditions. The total experimental uncertainty in the measurements was of the order 0.1 metres/sec. Some effects observed by other workers have been confirmed, in particular, a decrease in velocity with increasing distance from the source. Possible reasons for these effects have been examined. The velocity of sound in air was considered already sufficiently well established to permit its use in assessing the performance of the apparatus. A number of modifications and precautions required to enable precise measurement of velocity and absorption by this method have become apparent.