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

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.