A novel way to determine sound absorption, sound transmission and sound power (original) (raw)

An alternative coefficient for sound absorption

2013

The acoustic absorption coefficient is a number that indicates which fraction of the incident acoustic power impinging on a surface is being absorbed. The incident acoustic power is obtained by spatial integration of the incident intensity, which is (classically) defined as the time-averaged intensity associated with the incident sound field. The measurement of the effective, in situ, sound absorption coefficient is problematic as the determination thus requires a decomposition of the sound field in an incident and reflected field which, generally, is virtually impossible to do. This paper introduces an alternative coefficient with which the effective acoustic absorption can be expressed. This coefficient is based on an alternative definition of the incident intensity; the time average of the positive values of the instantaneous intensity. The alternative coefficient is much easier to use in a sense that it follows directly from an in situ, instantaneous intensity measurement. The coefficient does not rely on any assumptions other than the assumption that the linearized wave equation is satisfied (and thus the acoustic energy corollary). As a result, one does not need to decompose the sound field in incident and reflected waves. Hence, one does not need to have prior information about the incident sound field. Accordingly, one does not need to have prior information about the source. The coefficient can be determined in any sound field, either transient or stationary, free field and diffuse/(semi-)reverberant sound fields. The alternative coefficient is illustrated by means of several numerical examples.

Measuring Sound Absorption: Considerations on the Measurement of the Active Acoustic Power

Acta Acustica United With Acustica, 2014

Using alocal plane wave assumption, one can determine the normal incidence sound absorption coefficient of a surface by measuring the acoustic pressure and the particle velocity normal to that surface. As the measurement surface lies in front of the material surface, the measured active and incident acoustic power will generally deviate from those at the material surface, leading to ap ossibly inaccurate sound absorption coefficient. This phenomenon is particularly pronounced for poorly absorbing surfaces if sound is not normally incident overthe whole material surface. Based on an analytical model, it is shown that the accuracycan be improvedbyextending the measurement surface upon which the active acoustic power is measured. Experimental results demonstrate the usefulness of this approach, in particular for poorly absorbing surfaces.

Comparative analysis of measurement techniques of the sound absorption coefficient of a material

2014

Las tendencias actuales apuntan al desarrollo de nuevos materiales economicos y ecologicos con optimas propiedades mecanicas, acusticas y termicas. En la caracterizacion acustica del material es habitual medir su coeficiente de absorcion sonora. Las dos tecnicas usuales de medida de este parametro son en camara reverberante y en tubo de Kundt. No obstante, existen tecnicas de medida “in situ” del coeficiente de absorcion que permiten una comprobacion del comportamiento real en la forma definitiva de colocacion del material. En este trabajo se presenta un estudio comparativo del coeficiente de absorcion sonora medido en un material usando distintas tecnicas de medida.

A numerical study of a method for measuring the effective in situ sound absorption coefficient

Journal of the Acoustical Society of America, 2012

The accuracy of a method [Wijnant et al., Proc. of ISMA 31, Leuven, Belgium (2010), Vol. 31] for measurement of the effective areaaveraged in situ sound absorption coefficient is investigated. Based on a local plane wave assumption, this method can be applied to sound fields for which a model is not available. Investigations were carried out by means of finite element simulations for a typical case. The results show that the method is a promising method for determining the effective areaaveraged in situ sound absorption coefficient in complex sound fields.

In situ sound absorption measurement: investigations on oblique incidence

A novel method for the measurement of sound absorption has been developed. By assuming that, in a single point, the sound field consists of an incident- and a reflected plane wave, the locally incident- and reflected intensities can be determined. To this purpose, the active intensity and the sum of the potential-and the kinetic energy density (only the particle velocity component in the direction of interest is used) are measured. For normal incidence, the method was validated. Here, the method is applied to cases with oblique sound incidence. The influence of the angle of incidence and the non-planarity of the incident and reflected waves on the accuracy of the sound absorption coefficient are investigated. Analytical models as well as FE-simulations were applied to describe a few typical cases. Finally, parameter bounds for some applications will be given.

A numerical study of a method for measuring the effective in situ sound absorption coefficient

The Journal of the Acoustical Society of America, 2012

The accuracy of a method [Wijnant et al., Proc. of ISMA 31, Leuven, Belgium (2010), Vol. 31] for measurement of the effective areaaveraged in situ sound absorption coefficient is investigated. Based on a local plane wave assumption, this method can be applied to sound fields for which a model is not available. Investigations were carried out by means of finite element simulations for a typical case. The results show that the method is a promising method for determining the effective areaaveraged in situ sound absorption coefficient in complex sound fields.

Measuring oblique incidence sound absorption using a local plane wave assumption

Acta Acustica united with Acustica, 2014

In this paper a method for the measurement of the oblique incidence sound absorption coefficient is presented. It is based on a local field assumption, in which the acoustic field is locally approximated by one incident-and one specularly reflected plane wave. The amplitudes of these waves can be determined with an unidirectional sound intensity probe. The local active-and incident acoustic intensity are straightforwardly obtained. The area-averaged sound absorption coefficient is calculated after spatial integration of these quantities over the surface area of interest. Alternatively, one may use a three-dimensional intensity probe. In that case, the determination of the amplitudes of the plane waves can be formulated as a least-squares problem. Measurements performed for a sound absorbing foam demonstrate that accurate results can be obtained, even under non-ideal acoustic conditions. Measurements carried out for a periodic absorber show that the method is accurate below the cuton frequency of scattering as long as the amplitude of the evanescent surface waves is significantly smaller than that of the specularly reflected wave.