Manfred schroeder and acoustical impedance

Abstract

Manfred Schroeder was a man with many interests. Between the years 1966 and 1967, he invented devices for measuring the surface impedance of materials as well as the vocal tract impedance. At the time, he would only consider normal incidence; however, he already anticipated that the behavior of materials at grazing incidence would have relevance to room acoustics, a field of research he is well known for. To achieve this aim, an experimental set-up to determine the characteristic impedance of absorbers at grazing incidence has been developed. The sample is placed on the bottom of a rectangular impedance tube and the horizontal wave number above the sample is calculated from the pressure transfer function between two microphones. From this wave number, the absorber properties can be deduced. While the method works reasonably well for highly absorbing samples, the non-ideal sound field in the tube—as confirmed by finite element simulations—limits its usefulness in case of less absorbent materials. Improvements of the method are suggested.