Measuring oblique incidence sound absorption using a local plane wave assumption (original) (raw)
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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.
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.
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.
On the local plane wave methods for in situ measurement of acoustic absorption
2015
In this paper we address a series of so-called local plane wave methods (LPW) to measure acoustic absorption. As opposed to other methods, these methods do not rely on assumptions of the global sound field, like e.g. a plane wave or diffuse field, but are based on a local plane wave assumption. Therefore, the LPW methods can be used for any given surface/absorbing material and any arbitrary sound field. The absorption coefficient can be calculated based on a measurement of the acoustic pressure in a number of points in the vicinity of the absorbing surface. The local plane wave methods are illustrated by some numerical and experimental examples.
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.
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.
A novel way to determine sound absorption, sound transmission and sound power
2016
Recently, the University of Twente and Soundinsight have developed a range of methods and a new probe which, without relying on any assumption of the global sound field, can be used to measure the incident and reflected sound intensity in any sound field. We can thus separate the incoming from the reflected waves in the actual sound field, based on which we can determine in-situ absorption/emission coefficients, in-situ transmission loss and in-situ radiated power. In this paper, we will explain the fundamental ideas behind these methods and focus on a method to measure sound absorption.
Currently, the most common type of noise path control mechanism is passive control through the use of porous polyurethane foam absorbers, dense rubber barriers, or multi-layered absorber-barrier systems. These acoustic treatments can be designed to dissipate sound energy within their structures, and/or transfer the incident sound energy through their structures. Sound energy dissipation within a porous structure, known as sound absorption, is caused because sound is converted into small amounts of heat when it is forced to travel from pore to pore. Sound absorption is a very important factor when a designer is setting noise specifications on systems that incorporate porous or multi-layered acoustic treatments. It is necessary to quantify the sound absorption effectiveness of acoustic treatments. The sound absorption coefficient, valued between 0 and 1 and a function of frequency, is typically used for sound absorption quantification. It is defined as the ratio of the amount of sound energy absorbed within and transmitted through a treatment material to the amount of sound energy incident on that material.
In Situ Sound Absorption Coefficient Measurement of Various Surfaces
2002
The measurement of various surfaces sound absorption, requires an in situ test method. After having tested several impulsive techniques, we selected the method currently included in the ISO standard 13472-1. This paper presents this method based on the use of a sequence of repeatable impulses and, compares the results with other techniques. This method allows the acquisition of the impulse response of the surface under test, even in presence of a high level of non-stationary background noise. Results obtained for various road pavements, for natural and industrial absorbing surfaces are presented and compared to the last theoretical predicting models.