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Angle Dependence of the Impedance of a Porous Layer

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A free field method is described to measure the specific acoustic impedance of polyurethane foam layers as a function of the angle of incidence of the sound wave. An approximate spherical wave theory, which is believed to be correct up to angles of incidence of about 60°, is used for the calculations. In a plane wave approximation, the characteristic impedance and the propagation constant of the material are calculated.

These values are then used in a plane wave theory which calculates the specific acoustic impedance as a function of the angle of incidence. The agreement with the measured values is seen to be very good for all layer thicknesses used, up to an angle of incidence of about 60°.


Es wird eine Freifeldmethode zur Messung der spezifischen Wandimpedanz von Polyurethanschaum-Schichten als Funktion des Schalleinfallswinkels beschrieben. Für die Berechnung wird eine näherungsweise Kugelwellentheorie verwendet, die bis zu Einfallswinkeln von 60° korrekt sein dürfte. In einer Ebenenwellen-Näherung werden dann Wellenwiderstand und Ausbreitungskonstante des Materials berechnet.

Diese Werte werden dann in einer Ebenenwellen-Näherung zur Berechnung der Wandimpedanz als Funktion des Einfallswinkels benutzt. Die Übereinstimmung mit den Meßwerten ist für alle untersuchten Schichtdicken bis zu Winkeln von etwa 60° sehr gut.


On présente une méthode qui permet, par des mesures en champ libre, de déterminer l'impédance acoustique spécifique de couches de mousse de polyuréthane en fonction de l'angle d'incidence de l'onde sonore. Pour les calculs, on a utilisé d'abord une théorie reposant sur l'approximation des ondes sphériques, supposée demeurer correcte jusqu'à des angles d'incidence de 60°. Au delà on a calculé l'impédance caractéristique et la constante de propagation dans le matériau au moyen d'une approximation d'ondes planes. Ces valeurs sont ensuite utilisées, dans une théorie d'ondes planes, pour calculer l'impédance spécifique en fonction de l'angle d'incidence. La concordance avec les valeurs mesurées se révèle très bonne pour toutes les épaisseurs de couche utilisées, jusqu'à des angles d'incidence de l'ordre de 60°.
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Document Type: Research Article

Publication date: April 1, 1980

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  • Acta Acustica united with Acustica, published together with the European Acoustics Association (EAA), is an international, peer-reviewed journal on acoustics. It publishes original articles on all subjects in the field of acoustics, such as general linear acoustics, nonlinear acoustics, macrosonics, flow acoustics, atmospheric sound, underwater sound, ultrasonics, physical acoustics, structural acoustics, noise control, active control, environmental noise, building acoustics, room acoustics, acoustic materials, acoustic signal processing, computational and numerical acoustics, hearing, audiology and psychoacoustics, speech, musical acoustics, electroacoustics, auditory quality of systems. It reports on original scientific research in acoustics and on engineering applications. The journal considers scientific papers, technical and applied papers, book reviews, short communications, doctoral thesis abstracts, etc. In irregular intervals also special issues and review articles are published.
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