Estimating the airflow resistivity of porous materials in an impedance tube using an electroacoustic technique

Abstract

Airflow resistivity is an essential parameter for characterizing air-saturated porous sound-absorbing materials theoretically and selecting sound-absorbing materials in practice. Although standardized methods can determine this non-acoustic parameter in the laboratory, many indirect alternative methods have been proposed to measure it. One of them is the technique presented in the 1980s by Ingard and Dear using a standing wave tube, a loudspeaker, and two microphones. This paper suggests an electroacoustic procedure based on a modification of the Ingard and Dear setup. Equations are derived through the transfer matrix method. After a simple calibration, the airflow resistivity of a material sample is indirectly estimated from the total electric impedance measured at the loudspeaker input connection terminals. Thus, implementing the proposed method is straightforward and inexpensive, since microphones and complex instrumentation are unnecessary. The method is tested by comparing measured values of the airflow resistivity of different material samples with those obtained through the Ingar and Dear approach and the ISO standardized method. Reasonably good agreement is observed, confirming the validity of the electroacoustic method.