Low-frequency sound wave parameter measurement in gravels

Experiments were performed to measure the various sound-wave propagation parameters of two kinds of gravel, a crushed limestone and a stream-formed "pea gravel", at frequencies between 25 and 200 Hz. The apparatus was a very large standing-wave tube with an array of microphones along its entire length. Measurements were made with the tube in the vertical position, filled to various depths, with a pressure-doubling barrier at the bottom. Values were determined for wavelength and attenuation in the gravel and for input impedance at the boundary between gravel and air. The basic analysis method was to fit the parameters of a presumed Green's function representing the standing wave in the tube to the measured sound pressure amplitudes measured by the microphones. This worked well for some frequencies and some depths of gravel. In other cases, various modifications of the method gave more consistent results. For crushed limestone, the estimates of phase velocity and attenuation increased monotonically with frequency, ranging between 150 and 250 m/s and 0.1 and 0.8 Np/m, respectively. For pea gravel, the estimates increased monotonically with frequency, with values between 160 and 205 m/s and 0.4 and 1.1 Np/m, respectively.