Development of a high-frequency precision anechoic test facility

Abstract

Although anechoic wedges with absorption coefficients of 0.99 as tested in typical impedance tubes meet ISO inverse square-law standards, they may not be satisfactory for anechoic rooms used for measurements of microphone response characteristics. Due to size restrictions, impedance tube tests are limited to frequencies below about 250 Hz and there is no standard method to test wedges for absorption at higher frequencies. This paper describes how pulse signal and reverberation room tests were employed to rank the high-frequency performance of three types of anechoic wedges considered for a microphone test facility: Standard patented MetadyneTM (perforated metal) wedges—conventional fiberglass wedges and an improved high performance Metadyne wedge system. The results of laboratory tests and tests conducted in anechoic rooms are presented. In all cases the preferred system, high-performance Metadyne wedges, is shown to meet the stringent acoustic absorption requirements needed for this application. The paper indicates the need for new standards for laboratory testing of anechoic wedges at high frequencies and an alternate method to inverse square-law tests for evaluating anechoic room performance such as stepped frequency sweep tests. Such a new standard will be proposed in the presentation.