Measurement of acoustic impedance of sound absorbing materials in a free field at low frequencies

Abstract

The two‐microphone technique allows the precise measurement of acoustic velocity and consequently the determination of acoustic intensity. Using this principle, a method has been proposed to measure, in a free field, the acoustic impedance of sound absorbing materials. Good results were obtained for frequencies ranging from 700 Hz to 4 kHz [J. F. Allard et al., J. Sound Vib. 114(2) (1987)]. Recently, it has been demonstrated theoretically [Y. Champoux and A.L'Esperance, J. Acoust. Soc. Am. Suppl. 1 80, S56 (1986)] that for low frequencies, even if the source is several meters from the sample, the spherical nature of the waves has to be taken into account. This paper presents experimental evidence that confirms that the modeling correctly describes the phenomena. Further several refinements to the experimental technique, allowing precise measurements for frequencies as low as 250 Hz, will be demonstrated.