T.E. Vigran

Total absorption peak by use of a rigid frame porous layer backed by a rigid multi-irregularities grating

The acoustic properties of a low resistivity porous layer backed by a rigid plate containing periodic rectangular irregularities, creating a multicomponent diffraction gratings, are investigated. Numerical and experimental results show that the structure possesses a total absorption peak at the frequency of the modified mode of the layer, when designed as proposed in the article. These results are explained by an analysis of the acoustic response of the whole structure and especially by the modal analysis of the configuration. When more than one irregularity per spatial period is considered, additional higher frequency peaks are observed.

The acoustic properties of panels with rectangular apertures

A model for the acoustic properties of a plate perforated with slots of rectangular shape is proposed. The model is based on known expressions for the complex density and compressibility of a pore of rectangular shape together with the radiation impedance of a rectangular shaped piston in a baffle. For the so-called end correction of a rectangular aperture in a plate, an approximate solution is shown to fit an exact solution for the imaginary part of the radiation impedance, the latter solution based on the work of Lindemann [J. Acoust. Soc. Am, 55, 708-717 (1974)]. Two different procedures are tested to calculate the mutual influence of the apertures on the end correction, the one calculating the mutual impedance of neighboring pistons in the plate, the other by calculating the end correction of a piston placed in the end of an infinitely long tube. The model is used calculating the input impedance and absorption coefficient of a Helmholtz resonator with such a plate, comparing with measurement results. The fit between predicted and measured results, using plates with narrow slits, is good, but it is believed that the model also cover a wider range of dimensions for such a slotted plate.

On spatial sampling using the scanning intensity technique

Abstract Based on a very accurate temporal and spatial sampling of the radiated intensity and pressure field over a thin vibrating aluminium plate, different scanning procedures and patterns have been simulated by averaging subsets of the basic data. In general, the different simulated scanning patterns gave almost identical results in very good agreement with the ‘ideal’ averaging i.e. averaging all data. Changing the line density, changing the starting point, excluding data at the edges and not scanning after straight lines gave far less deviations than the deviations in the ‘ideal’ averaged intensities with increasing background noise levels.

Total absorption peak by use of a rigid frame porous layer backed by a rigid grating.

The acoustic properties of a low‐resistivity porous layer backed by a rigid plate containing periodic rectangular irregularities, creating multicomponent diffraction gratings, are investigated. Numerical and experimental results show that the structure possesses a total absorption peak at the frequency of the modified mode of the layer when designed as proposed in the article. These results are explained by an analysis of the acoustic response of the whole structure and especially by the modal analysis of the configuration. When more than one irregularity per spatial period is considered, either additional higher frequency peaks or additional total lower frequency peaks are observed.

A Helmholtz resonator with elongated orifice

For panel absorbers there is an increasing trend toward a design not utilizing fibrous components. For the distributed Helmholtz type of resonator absorbers, the challenge is to increase the natural losses, making the absorbers reasonably broadbanded. In this paper a new type of distributed resonators is investigated. Using a double‐plate construction, the resonator necks have been substantially elongated in the lateral direction, while their widths have been kept small. Thus the viscous losses in the resonator necks have been increased compared to traditional resonators. Measurements are compared with a theoretical model using analytic solutions for the slit impedances, the end corrections, and the resistance of the inner and outer surfaces. For the volume between the plates, a finite‐difference approach is used. The results show a high dependency on the plate separation, and to attain high absorption over a broad frequency range, this separation should be on the order of one boundary layer.

In situ absorption measurements using a transfer function technique and MLS

The transfer function (or two‐microphone) technique is a tool for measuring materials absorption related parameters as impedance or absorption coefficients. But this method is laboratory bound in its original form due to free field assumption and practical issues. It is shown that after proper modifications the transfer function technique can be used successfully in noisy and reverberating fields and work well at low frequency even in small measurement rooms, becoming therefore a really in situ measurement technique. A wide frequency range implementation is presented, based on a two‐channel MLS measurement system, ‘‘energy ratio invariant’’ time windowing, a basic specular reflection propagation model with spherical decoupling, and a particular geometrical configuration. The measurement method is tested on a wide range of absorbers. For impedance measurements using infinite surface propagation models the uniqueness of the inversion procedure is investigated using the conformal mapping technique. In additi...