Low-frequency sound propagation in porous media: Glass spheres and pea gravel

The sound propagation properties of two air-filled granular materials: large sifted pea gravel and 10 mm diameter glass spheres have been measured in an impedance tube. The experimental method was essentially the same as reported earlier [Swenson et al. Low-frequency sound wave parameter measurement in gravels. Appl Acoust 2010; 71: 45–51] for two other kinds of gravel: crushed limestone and undifferentiated pea gravel. Additional sampling and processing steps were applied to the microphone signals such that instead of tones, band-limited random noise was used as the input signal, and spectral domain complex pressures are now offered as input to the estimation algorithm. The estimation process extracts the best-fit attenuation coefficient, phase velocity, and characteristic impedance for the material over the signal frequencies, all with better precision than we previously obtained. Quadratic approximations for the acoustical parameters are given over the frequency range 25–160 Hz. The media are both slightly attenuating and dispersive, having attenuation coefficients within 0.13–0.34 Np/m, phase velocities smaller than those in air (180–240 m/s), and characteristic impedance approximately 3–5 times that for air. Pea gravel was more attenuating, and had slightly higher characteristic impedance, but lower phase velocities than the glass spheres.