R Lanoye

Measuring the free field acoustic impedance and absorption coefficient of sound absorbing materials with a combined particle velocity-pressure sensor

Acoustic surface impedance of sound absorbing materials can be measured by several techniques such as the impedance tube for normal impedance or the Tamura method for normal and oblique surface impedance. In situ, the acoustic impedance is mostly measured by use of impulse methods or by applying two-microphone techniques. All these techniques are based on the determination of the sound pressure at specific locations. In this paper, the authors use a method which is based on the combined measurement of the instantaneous sound pressure and sound particle velocity. A brief description of the measurement technique and a detailed analysis of the influence of the calibration, the source type, the source height, the sound incidence angle, and the sample size are included.

Prediction of the sound field above a patchwork of absorbing materials

The aim of this paper is to investigate the acoustic performance of sound absorbing materials through a numerical wave based prediction technique. The final goal of this work is to get insight into the acoustic behavior of a combination of sound absorbing patches. In order to address a wide frequency range, a model based on the Trefftz approach is adopted. In this approach, the dynamic field variables are expressed in terms of global wave function expansions that satisfy the governing dynamic equations exactly. Therefore, approximation errors are associated only with the boundary conditions of the considered problem. This results in a computationally efficient technique. The main advantage of this method is the fact that the sound absorbing patches do not have to be locally reacting. In this article, the wave based method is described and experimentally validated for the case of normal incidence sound absorption identification in a standing wave tube. Afterwards, the method is applied to simulate some interesting setups of absorbing materials.

In Situ, Broad Band Method to Determine the Normal and Oblique Reflection Coefficient of Acoustic Materials

It has been shown that the combination of a particle velocity (u) sensor and a sound pressure (p) microphone can successfully be used to determine the reflection coefficient of acoustic materials [1-4]. In this paper, a practical measurement technique and a so called pu-probe impedance meter are described that require no Kundt’s tube or anechoic room. The procedure allows fast (less than 60 seconds) sound absorption measurements under both normal and oblique angles of incidence on small test samples (less than 30*30cm). The total size of the pu-probe impedance meter is 60 centimeters and weights less than 1kg. The method can be used in combination with a sound card based software package. At first, the pu-probe is calibrated in approximately 20 seconds. The frequency response of the impedance sensor is transformed to an impulse response. This response is time windowed to cancel out the room reflections. After the calibration, the reflection coefficient (and the real and imaginary part of the impedance) of the acoustic materials are determined by putting the puprobe in close proximity of the acoustic sample.