Measurement of Sound Absorption Coefficient under Anechoic Conditions by Transfer Function Method

Abstract

The sound absorption coefficient is a fundamental parameter for characterizing acoustic materials, widely used in building acoustics, noise control engineering, and material design. Its accurate determination, however, remains a challenging task due to the strong dependence on measurement conditions, sound field characteristics, and experimental assumptions. Traditional methods, such as reverberation room and impedance tube techniques, provide standardized but often limited information, particularly with respect to angle-dependent behavior and intrinsic material properties. This paper provides an overview of existing approaches for measuring the sound absorption coefficient, with particular emphasis on free-field and anechoic conditions. Key challenges, including low-frequency limitations, finite sample effects, and sensitivity to measurement geometry, are discussed. A measurement methodology based on a semi-circular configuration is then introduced. The proposed system employs a swept-sine excitation and a transfer function approach using three microphones to estimate the reflection properties of the material. Two realizations of the semi-circular measurement setup are presented, enabling flexible control of incidence angles and improved characterization capabilities. The paper aims to contribute to the ongoing development of reliable and versatile measurement techniques for sound absorption under controlled acoustic conditions.