In-situ Estimation of Impedance in Enclosed Space using Circular Microphone

Abstract

Impedance of wall enclosing a listening room is one of the major factors that determine the sound quality. Therefore, to improve the acoustic environment of a listening room, it is important to accurately identify the impedance of the material forming the wall. The impedance tube technique is a well-known standard method for measuring impedance. This technique, however, requires a specimen of a wall installed in the impedance tube, and only the impedance corresponding to normal incidence can be measured. Since sound can impinge from various directions inside a room, the impedance estimated by the tube technique cannot represent the actual impedance in a practical situation. In order to circumvent this problem, various in-situ measurements have been proposed, but most of conventional techniques have problems such as source singularity or inaccurate separation of the incident wave and reflected wave. In this research, the impedance is directly estimated in an enclosed space using a circular equivalent source method, which can reconstruct a local sound field from the measurement by a microphone array. The sound pressure and velocity fields on the wall surface are reconstructed by the equivalent source method, and the surface impedance is estimated from the reconstructed sound field. The feasibility of the proposed method is validating using 2D sound field simulation, and the conditions for accurate impedance prediction, such as microphone and equivalent source configurations, are investigated to provide a guideline for successive acoustic impedance estimation an enclosed space.