Behavior of rigid-porous layers at high levels of continuous acoustic excitation: theory and experiment

Effect of acoustically-induced elastic softening on sound absorption coefficient of hollow glass beads with inner closed cavities

This study investigated the effects of sound wave magnitude on the sound absorption characteristics of granular materials. The normal incidence sound absorption coefficients of the hollow glass bead packings were measured with different incident sound pressure levels. The measurement results exhibited spectral peaks of sound absorption coefficients at several frequencies, implying that the first peak was caused by the resonance of the hollow glass beads as a layer. Although the first peak in the sound absorption coefficients did not vary when an incident sound pressure was low, the first peak shifted to a lower frequency when the sound pressure exceeded a certain magnitude. These results indicate that the hollow glass beads are softened at high incident sound pressures. Furthermore, by modelling the velocity-dependent elasticity of the hollow glass beads, the velocity and elasticity in the hollow glass beads were simulated in the direction of the sound propagation. The simulation results indicate that the velocity and elasticity profiles depend on the sound wave magnitude. Finally, the velocities of the hollow glass beads under acoustic excitation were measured by inserting an accelerometer into the beads. The measurement results demonstrated that the velocity profile depends on the sound wave magnitude, which parallels the simulation results.

Influence of Forchheimer's nonlinearity and transient effects on pulse propagation in air saturated rigid granular materials

It has been shown in the earlier work of Umnova et al. [Noise Control Eng. 50, 204-210 (2002)] that interaction of a relatively long, high amplitude acoustic pulse with a rigid porous material can be accurately described accounting for the Forchheimer nonlinearity. In the present study, the goal is to determine whether the accuracy of the modeling can be improved in the case of a lower amplitude and a shorter pulse. A model that accounts for the Forchheimer's nonlinearity and the transient effects is developed. It is assumed that all the contributions to the viscous force are additive in the time domain. The governing equations are solved numerically using finite difference time domain scheme. The results are compared with the data for two granular materials. The latter are obtained in an impedance tube and in a shock tube experiments, where acoustic pulses with various amplitudes and durations are generated.

Numerical and analytical solutions for sound propagation and absorption in porous media at high sound pressure levels

On the basis of the work of Wilson et al. [J. Acoust. Soc. Am. 84, 350-359 (1988)], a more exact numerical approach was constructed for predicting the nonlinear sound propagation and absorption properties of rigid porous media at high sound pressure levels. The numerical solution was validated by the experimental results for sintered fibrous porous steel samples and its predictions were compared with the numerical solution of Wilson et al. An approximate analytical solution was further put forward for the normalized surface acoustic admittance of rigid air-saturated porous materials with infinite thickness, based on the wave perturbation method developed by Lambert and McIntosh [J. Acoust. Soc. Am. 88, 1950-1959 (1990)]. Comparisons were made with the numerical results.

On the variations of acoustic absorption peak with particle velocity in micro-perforated panels at high level of excitation

The acoustic behavior of micro-perforated panels (MPP) is studied theoretically and experimentally at high level of pressure excitation. A model based on Forchheimer's regime of flow velocity in the perforations is proposed. This model is valid at relatively high Reynolds numbers and low Mach numbers. The experimental method consists in measuring the acoustical pressure at three different positions in an impedance tube, the two measurement positions usually considered in an impedance tube and one measurement in the vicinity of the rear surface of the MPP. The impedance tube is equipped with a pressure driver instead of the usual loudspeaker and capable of delivering a high sound pressure level up to 160 dB. MPP specimens made out of steel, dural and polypropylene were tested. Measurements using random noise or sinusoidal excitation in a frequency range between 200 and 1600 Hz were carried out on MPPs backed by air cavities. It was observed that the maximum of absorption can be a positive or a negative function of the flow velocity in the perforations. This suggests the existence of a maximum of absorption as a function of flow velocity. This behavior was predicted by the model and confirmed experimentally.

Sound absorption of porous metals at high sound pressure levels

This paper is a study about sound absorption properties of porous metals at high sound pressure levels. A method of deriving the nonlinear static flow resistance for highly porous fibrous metals is proposed by solving Oseen's equation to take account of the inertia effect, validated by experiments of airflow measurement. In order to predict nonlinear sound absorbing performance of a finite thickness porous metal layer, a numerical method is employed, verified by sound absorption measurement in an impedance tube. Accordingly, the effects of the nonlinear coefficient on the porous metal sound absorption are investigated.