Measurement of impedance at grazing incidence with a standing-wave tube

• Published in: The Journal of the Acoustical Society of America 2011-04, Vol.129 (4_Supplement), p.2429-2429
• Main Authors: Mellert, Volker, Kruse, Roland
• Format: Article
• Language: English
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.3587945

As guest editor of Ando’s book “Concert Hall Acoustics” Schroeder wrote 1985 in his foreword “This lack of low frequencies in the first overhead reflection revealed another low-frequency deficiency that had hitherto gone unnoticed: a progressive attenuation of low frequencies in the direct sound as it grazes across the rows of seats. (This “seat effect” must exist in many other halls, but it is usually masked by the presence of low-frequency components in the early overhead reflections).” Schroeder addressed the acoustics in a famous concert hall. Early digital computing helped to suggest improvements. Even today it remains a problem to exactly determine properties of acoustic material. Determination of the effective impedance of acoustic materials at normal incidence is a standardized procedure (ISO 10534). However, at grazing incidence, the situation is more complicated. Using a modified impedance tube, in which the material under investigation is placed parallel to the direction of wave propagation, and applying Scott’s equation (1946) linking the (complex) wave number in the channel with the properties of the porous material, today’s laptop power can even provide efficient calculations of the impedance at grazing incidence. This paper will also discuss challenges on the uniqueness of the solution.