Absorption of Sound in Gases at Reduced Pressures

Abstract

The absorption of sound in air is being measured for pressures in the region from one to ten mm of Hg and for frequencies in the neighborhood of 75 kc. Two rochelle salt transducers are mounted in a cylindrical tank which is surrounded by a water jacket for temperature control. The air in the tank has been dried and freed of CO2 by passing it through a tube containing CaCl2, then through a tube containing Ascarite, and finally through a dry ice trap. A triangular pulse of one-millisecond duration with sharp leading edge is applied to one of the transducers. With this shape of pulse, it is possible to distinguish the initial pulse from echoes which arrive one-fifth of a millisecond later. Measurements taken at atmospheric pressure show that for distances greater than 3 cm from the source, the pressure follows a 1/r relationship to within 5 percent. The second transducer is used to measure the sound pressure at various distances from the source. Its electrical output is first fed to a tuned amplifier with 3-db half-band width of 5 kc and then viewed on a cathode-ray oscilloscope. Data on absorption coefficients in air will be presented. Present plans include extension of the investigation to other gases.