Dissipative Acoustic Reflection Coefficients in Gases by Ultrasonic Interferometry

Reflection coefficients for ultrasonic waves in gases, much lower than allowed by the dynamical theory of sound, first evaluated for a single frequency by J. C. Hubbard, and found by R. W. Curtis to have a frequency dependence, are satisfactorily explained by the new heat conduction theory of acoustic reflection developed by K. F. Herzfeld. This paper presents a modification of the theory of the acoustic resonator interferometer so as to include dissipative losses by emission and reflection at the source as well as losses by reflection at the reflector. The analysis of previously obtained and new data for C${\mathrm{O}}_2$, air, and He shows excellent agreement with Herzfeld's theory in all cases showing minimum losses. The requirements for securing such data, including the plane parallelism of source and reflector and modes of vibration of the source have been studied by optical, electrical and acoustical methods. The parallelism requirement is most rigorous and of a higher order of magnitude than for velocity determinations. Slight departures result in apparent losses of great magnitude. This requirement is best fulfilled by adjustment for maximum acoustic reaction, or peak height, while the interferometer is in actual operation. The sensitivity of the interferometer is greatly increased by blocking out unused radiation by reflecting plates at a quarter wave-length distance from the source.