Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Effect of Absorption Mechanism

Abstract

In the third of a series of papers on the measurement of acoustical impedance of absorbing surfaces using the two-microphone transfer function technique, the influence of surface absorption mechanism upon the measured impedance is described. The results from measurements of the impedance of bulk reactors are compared with values obtained from theoretical models. Materials investigated are an inhomogeneous polyurethane foam, a distributed resonance absorber, and a twin layer foam. This paper also investigates how the measurement technique behaves with samples which are bulk reacting and have surface roughness. A rough surfaced polyurethane foam sample is used. The results indicate that at frequencies for which the surface irregularities are small in comparison to the wavelength, the material can be accurately characterised by the acoustical impedance acting at an effective plane. For higher frequencies it is thought that the measuring technique becomes inaccurate due to scattering of sound by the surface roughness, and the consequent breakdown of the sound field prediction methods.

Get full access to this article

View all access options for this article.

References

Attenborough

, 1988, “Review of ground effects on outdoor sound propagation from continuous broadband sources”, Applied Acoustics 24(4), 249–320.

Brouard

Lafarge

Allard

J.F.

Tamura

, 1996, “Measurement and prediction of the reflection coefficient of porous layers at oblique-incidence and for inhomogeneous waves”, J. Acoust. Soc. Am., 99, 100–107.

Lauriks

Mees

Allard

J.F.

, 1992, “The acoustic transmission through layered systems”, J. Sound Vib., 155(1), 125–132.

Mechel

F.P.

, 1988, “Design Charts for Sound Absorber Layers”, J. Acoust. Soc. Am., 83, 1002–1013.

Vigran

T.E.

Hole

, 1988, “Sound absorbing layers: A simple prediction model for calculating absorption and the effect of on sound transmission”, Proceedings of the Institute of Acoustics (UK), 10(5), 31–38.

Waddington

D.C.

, 1990, “Acoustical Impedance Measurement Using a Two-Microphone Transfer Function Technique”, PhD Thesis, University of Salford.

Waddington

D.C.

Orlowski

R.J.

, 1996, “Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Theoretical Models”, J. Building Acoustics, 3(2), 81–104.

Waddington

D.C.

Orlowski

R.J.

, 1997, “Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Geometrical and Measurement Errors”, J. Building Acoustics, 3(4). 265–295.