Acoustic parameter estimates and confidence intervals for gravel at low frequencies

Rigid grain materials such as gravel offer the possibility of achieving low impedances and moderate attenuation for low-frequency acoustic waves. Using a standing wave impedance tube, we have estimated acoustic properties (wavelength, absorption coefficient, acoustic resistance, and reactance) for bulk compressional waves within and above gravel layers of several depths. In this paper we develop confidence intervals for the parameter estimates obtained by fitting measured data to a plane wave model using complex Fourier pressures. Comparisons to the three microphone method and microstructural impedance models are also made.