Use of pseudo-random sequences and a single microphone to measure surface impedance at oblique incidence

Abstract

The use of a pseudo-random sequence and a single microphone is suggested for the experimental determination of the acoustical properties (surface impedance, reflection coefficient, etc.) of sound-absorbing materials. An experimental system is developed with which the surface impedance and reflection coefficient at oblique incidence are determined from measurements of the impulse-response function sequentially at two locations close to the surface of the material using a pseudo-random sequence and a single microphone. This technique is validated using the measurement of a residual pressure-intensity index. The advantage of this technique is that it is possible to perform measurements of the surface properties of materials without phase-mismatch errors that occur with two-microphone methods. Models for estimating the surface impedance from plane-wave and spherical-wave hypotheses are reviewed and compared. Measurements of impedance at oblique incidence are carried out on a sheet of glass fiber in an anechoic room, as well as in a semi-reverberant room, and the results are compared. Surface impedances and reflection coefficients are presented as a function of angle of incidence. The proposed experimental system can be used to measure the acoustical properties of materials at oblique angles of incidence and in semi-reverberant environments.

An experimental study of the acoustic impedance characteristics of human hair

“…Again this assumes a surface reference plane that is offset 40 mm from the rigid backing. Contrary to the discussion by Li and Hodgson, 33 the results from either microphone separation yielded nearly identical results over the entire frequency range of interest. For the same surface reference plane, the alignment of the fiber also had little effect, although a diminutive resonance was observable over a small frequency range for the parallel alignment tested.…”

“…This was to minimize errors associated with the assumption of a constant source angle relative to material surface and the microphones. 33 A source separation of 0.4 m was used and incidence angles from 90°͑normal incidence͒ to 40°were tested. For each angle of incidence, impulse response measurements were obtained using maximum length sequences ͑MLS͒ produced by the Brüel & Kjaer DIRAC software.…”

A Comparison of Three Methods to Calculate the Surface Impedance and Absorption Coefficient from Measurements Under Free Field or in situ Conditions

“…The measurement of the surface impedance in free-field conditions or in situ is at opic of interest since this is an on destructive test and can be performed under realistic mounting conditions. There are manyr eferences in literature addressing the issue of impedance deduction [1,2,3,4] in approaches that calculate the surface impedance from measurements of pressure and /o rp article velocity above asurface. The accuracyofthis deduction is dependent on the description of the sound field above the surface.…”

In situ measurements of surface impedance and absorption coefficients of porous materials using two microphones and ambient noise

“…Garai [1] has indicated that the reverberation room and tube methods are essentially laboratory methods so they are unsuitable for measuring absorption characteristics in situ. Various reflection methods [11][12][13][14][15][16][17][18][19][20] have been devised and developed to carry out in situ measurement in the expectation that they can solve the problems of other methods. In particular, the methods proposed by Allard et al [11][12][13][14] or Garai [1] constitute efficient and useful reflection methods.…”