An improved two-microphone transfer function method for measuring oblique absorption coefficient in the free field using numerical modeling

Abstract

The use of the two-microphone free field transfer function technique has remained unchanged since its development. In practice, finite sample sizes limit measurement fidelity due to edge effects. The sound field contribution from edge diffraction has generally restricted the technique from use at low frequencies (<300 Hz) and required test panels greater than several square meters in area. This effort intends to characterize the diffraction contribution to the sound field for relatively small panels. Using numerical modeling, samples of like acoustic properties were excited by a point source at normal incidence to quantify the diffraction term. Following validation via comparison with impedance tube data, the diffraction term was incorporated in an updated derivation of the complex reflection coefficient equation and validated experimentally for a known reference material at both normal and oblique incidence. The goal of this work is to validate measurements of oblique absorption coefficient in an anechoic chamber for panels smaller than one square meter at frequencies down to 100 Hz.