Manuel Melon

Boundary element method for the acoustic characterization of a machine in bounded noisy environment

In this article, a boundary element method is used to recover free field conditions from noisy bounded space situations. The proposed approach is based on the Helmholtz integral formulation. The method requires the knowledge of double layer pressure fields on two parallel closed surfaces surrounding the source. First, the outgoing and ingoing pressure field are separated using Helmholtz integral. Then, the incident field scattered by the tested source is subtracted from the outgoing field to recover the pressure field which would have been radiated in free space. To simplify the process, rigid body approximation for the source is used. The method is numerically tested in the following conditions: the chosen sound source is the upper spherical cap of a rigid sphere, the source is located at the center of a rigid spherical cavity, and a monopole secondary source is added to blur the primary pressure field. Simulations give good results for ka up to 5 when the discretization of the surfaces is sufficient.

Measurement of confined acoustic sources using near-field acoustic holography

Due to excessive reverberation or to the presence of secondary noise sources, characterization of sound sources in enclosed space is rather difficult to perform. In this paper a process layer is used to recover the pressure field that the studied source would have radiated in free space. This technique requires the knowledge of both acoustic pressure and velocity fields on a closed surface surrounding the source. The calculation makes use of boundary element method and is performed in two steps. First, the outgoing pressure field is extracted from the measured data using a separation technique. Second, the incoming field then scattered by the tested source body is subtracted from the outgoing field to recover free field conditions. The studied source is a rectangular parallelepiped with seven mid-range loudspeakers mounted on it. It stands at 40 cm from the rigid ground of a semi-anechoic chamber which strongly modifies the radiated pressure field, especially on the underside. After the measured data have been processed, the loudspeaker positions are recovered with a fairly good accuracy. The acoustic inverse problem is also solved to calculate the velocity field on the source surface.

Time domain holography: Forward projection of simulated and measured sound pressure fields

In this article, the fundamental principles of forward projecting time domain acoustic pressure fields are summarized. Four different numerical approaches are presented and compared both with simulated and measured signals. The approaches differ in their definition domain: Frequency/time and space/wave vector domains. The simulated source is a planar baffled piston excited with a Gaussian pulsed velocity. The pressure radiated by two different real sources has been measured: The first source is made up of two baffled loudspeakers (a Gaussian white noise can be radiated by a third loudspeaker). The second one is a baffled aluminum plate excited by a short impact at its center. The influence of parameters such as the sound source radius, the array size, the number of microphones and the propagation distance is studied. Finally, results concerning the optimization of the sampling of the sound field are presented.

Ultrasonic characterization of the anisotropic behavior of air-saturated porous materials

Abstract This paper provides a comprehensive review of the propagation of ultrasonic waves in anisotropic porous materials. The equivalent fluid model (or Allard-Johnson theory) which is relevant for air-saturated porous media is described. It takes into account viscous and thermal losses occurring during the movement of the fluid within the motionless solid frame. When the skeleton is moving as well, the coupled Biot theory should instead be used. This theory becomes intricate when anisotropy is considered due to a very large number of physical parameters to be determined. A strong formal correspondence between the anisotropic Biot wave and the thermal wave of dynamic thermoelasticity in non-porous media is outlined. Standard ultrasonic methods, generally used at low frequency (i.e. 20–500 kHz) are very effective in order to characterize anisotropy in porous media. Both reflection and transmission configurations have been used. Special attention has been devoted to the measurements of the anisotropic tortuosity, but also to the viscous and thermal characteristics lengths. Finally, some inverse problems related to these measurements are solved and others, which are still open, are presented.

Evaluation of a method for the measurement of subwoofers in usual rooms

This paper evaluates the potential of the field separation method (FSM) for performing subwoofer measurements in a small test room with poor absorbing properties, as is commonly available. The FSM requires the knowledge of both acoustic pressure and velocity fields on a closed surface surrounding the tested source. Pressures and velocities, measured using a p-p probe on a half-sphere mesh, are collected under various conditions: in a room with variable reverberation time (6.4-0.6 s) and with four measurement half-sphere radii. The measured data are expanded on spherical harmonics, separating outward and inward propagation. The pressure field reflected by walls of the surrounding room is then subtracted from the measured field to estimate the pressure field that would have been radiated under free-field conditions. Theoretical frequency response of the subwoofer is computed using an analytical formulation derived from an extended Thiele and Small model of the membrane motion, coupled to a boundary element model for computing the radiated pressure while taking into account the actual subwoofer geometry. Measurement and simulation results show a good agreement. The effects of the measurement distance, the measurement point number, and the room reverberation time on the separation process are then discussed.

Measurement of tortuosity of anisotropic acoustic materials

Tortuosity, i.e., the high‐frequency limit of the squared propagation index, has been evaluated with narrow‐band piezoelectric transducers on reticulated plastic foams. The basic procedure was recently reported by J. F. Allard, B. Castagnede, M. Henry and W. Lauriks [Rev. Sci. Instrum. 65, 754 (1994)]. Further experimental work related to the anisotropic nature of these materials has been done by probing the propagation index versus angle. A simple numerical routine has been implemented in order to recover the propagation index along principal directions. These predicted values compare well with measurements taken directly along the principal geometric axes of the samples. Slight angular deviations of the principal axes themselves have been observed. This is the very first account of the anisotropy of tortuosity measured by ultrasonic methods in air‐saturated porous materials.

Hemispherical double-layer time reversal imaging in reverberant and noisy environments at audible frequencies

Time reversal is a widely used technique in wave physics, for both imaging purposes and experimental focusing. In this paper, a complete double-layer time reversal imaging process is proposed for in situ acoustic characterization of non-stationary sources, with perturbative noise sources and reverberation. The proposed method involves the use of a hemispherical array composed of pressure-pressure probes. The complete set of underlying optimizations to sonic time reversal imaging is detailed, with regard to space and time reconstruction accuracy, imaging resolution and sensitivity to reverberation, and perturbative noise. The proposed technique is tested and compared to more conventional time reversal techniques through numerical simulations and experiments. Results demonstrate the ability of the proposed method to back-propagate acoustic waves radiated from non-stationary sources in the volume delimited by the measurement array with a high precision both in time and space domains. Analysis of the results also shows that the process can successfully be applied in strongly reverberant environments, even with poor signal-to-noise ratio.

Measurement of low‐frequency sources in non‐anechoic room using near‐field acoustic holography

to perform because free field conditions can not be easily achieved properly. Moreover, some industrial sources have to be measured in situ. In such a case, a Field Separation Method (FSM) can be used to subtract the pressure field reflected by walls of the testing room from the measured data. This approach required the knowledge of both acoustic pressure and velocity on a closed surface surrounding the source. In this paper, a spherical harmonic expansion of measured data is used to solve the problem. The proposed method is applied to the measurement of the frequency response of a closed box subwoofer tested under various conditions: in a room with variable reverberation time (6.4 s to 0.6 s). Theoretical frequency response of the subwoofer is also calculated using the Thiele and Small model. Results show a good agreement between separated data and simulations. The influences of the measurement distance and of the measurement point number required on the separation process are discussed.

A boundary element method for near‐field acoustical holography in bounded noisy environment

This paper presents a boundary element method to recover free field conditions from noisy bounded space situations. The proposed approach is based on the Helmholtz integral formulation and requires the knowledge of double layer pressure fields on two parallel closed surfaces surrounding the source. First, the outgoing and ingoing pressure fields are separated. Then, the incident field scattered by the tested source is subtracted from the outgoing field to estimate the pressure field which would have been radiated in free field. The method had been numerically tested and an experimental example is given here. The source is a rectangular box with seven loudspeakers mounted on it driven by bandwith limited white noise. The source is put at 0.4 m from the ground of a semi‐anechoic room. The ground plays a disturbant role because it produces secondary sources. The results show the effectiveness of the method particularly at frequencies where stationary waves between the ground and the underside of the box exist.

Ultrasonic measurements in air or water saturated porous media

Low‐frequency (20 kHz–180 kHz) ultrasonic measurements have been performed in numerous air‐saturated acoustic materials, i.e., various foams and felts. By using a standard pulse‐echo technique, very precise wave‐speed measurements were achieved with either a cross‐correlation procedure or by using the Hilbert transform of the transfer function between a reference signal (with no sample) and the signal transmitted through the sample. The amplitude measurements were done in the Fourier domain via the cross power. The signal processing routines were implemented in the LabVIEW 2.0 environment. Some significant physical properties (e.g., tortuosity) related to the propagation of acoustic waves in porous media [e.g., J.‐F. Allard, Propagation of Sound in Porous Media (Elsevier, New York, (1993)] were obtained. Preliminary results made at 500 kHz, with the aim of measuring the viscous as well as thermal characteristic lengths, on water‐saturated sandstone specimens will also be reported.