Design and construction of apparatus for determining the velocity of sound in gases at high pressures and temperatures

The purpose of the present work ie to present results obtained from studies using a variable path type interferometer designed for generating sound waves above the audible limit in gases at high pressures and temperatures. Ultrasonic or high frequency sound waves are generated by means of a quarts crystal vibrating at its fundamental frequency. The crystal is placed on top of a vertical cylinder which is fitted with a moving piston or reflector. In this way a series of standing waves is produced. As the piston is moved in the cylinder, periodic reaction of voltage occurs across the crystal as the piston moves through successive half-wavelength sound intervals. The acoustic velocity or the velocity of sound in the gas is the product of the frequency of the sound source and measured wave length interval. electrical driving oscillator of the "electron coupled" type is used to drive the crystal at its natural frequency. The frequency of the driving oscillator is determined by "beating" against a crystal controlled oscillator of known frequency. Voltage change appearing on the crystal is measured by a vacuum tube voltmeter. A heating system using superheated steam is used to maintain constant temperature conditions in a steel jacket or tank in which the interferometer is housed. An automatic photographic recording apparatus is used to obtain a curve representing voltage variation vs piston displacement which is driven through suitable gearing from the piston rod. Knowing the scale of the ordinate or drum speed, the wave length can be evaluated