Some Investigations into the Velocity of Sound at Ultra-sonic Frequencies Using Quartz Oscillators

Abstract

The velocity of sound at ultra-sonic frequencies is measured over a frequency range from 40 to 216 kilocycles per second using quartz crystals as sound oscillators. The emitted sound is reflected back upon the source and as its phase changes the plate current of the quartz oscillator goes through a series of maximum values. The distance moved by the sound reflector between these maxima is a half wave-length of the emitted sound.The velocity is obtained in air free from C${\mathrm{O}}_{2}$ at three values of the relative humidity, dry, 45 percent relative humidity at 20\ifmmode^\circ\else\textdegree\fi{}C, and in air saturated with water vapor at 20\ifmmode^\circ\else\textdegree\fi{}C.The velocity is found to change with the distance from the source, becoming smaller as the distance becomes greater. This was particularly noticeable at 42 kilocycles.At a distance of 45 centimeters, all values except those below 60 kilocycles become within 0.03 percent of an asymptotic value of 331.60 meters per second.The velocity is found to depend upon the number of wave-lengths from the source.The effect of humidity upon the velocity is found to be expressed by ${V}_{H}={V}_{0}+0.14H$ where ${V}_{0}$ is the velocity in dry air at 20\ifmmode^\circ\else\textdegree\fi{}C and ${V}_{H}$ is the velocity at any relative humidity "$H$" at 20\ifmmode^\circ\else\textdegree\fi{}C.