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

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.