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

Abstract

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. 71, 45-51 (2010)] 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.

Sound attenuation in air–water media with rough bubbly interface at low frequencies considering bubble resonance dispersion

“…Sikora et al [4], by focusing on ultrasounds, recently developed a beam tracing with refraction (BTR) method which simulates the reflection and refraction of sound in a medium. Although the goal of the majority of these studies has been the investigation of sound in the propagation medium, sound attenuation in the spectral region from 20 to 200 Hz remains largely an unexplored area in underwater acoustics [5]. However, various factors that are involved in sound attenuation cause major difficulties in seeking an accurate solution.…”

“…In the real ocean surface, the generated sub-surface bubbles can significantly affect the incident sound to sea surface at low frequencies. Theory of sound propagation in fluid containing bubbles has been studied by different acousticians including Medwin [7], Hall [8], McDonald [9], Henyey [10], Prosperetti [11], Gauss and Fialkowski [12], Verestchagina and Fedotovsky [13], Godin [14][15][16], Kuo [17], and White et al [5]. For instance, Prosperetti [11] suggested the natural sound-producing mechanisms at frequencies between 20 and 500 Hz due to wave turbulence and oscillating bubble clouds.…”