Acoustic impedance of small, circular orifices in thin plates

Measurements of acoustic impedance have been made on a series of small, circular orifices (0.1–0.3 mm radius) in thin plates (0.038 and 0.38 mm thickness). Both real and imaginary components, over the frequency range 0.6–3 kHz, were obtained using an impedance tube technique. The zero frequency limit of resistance was measured with a flow resistance device. Only small amplitude acoustic signals were considered. For the range of orifice radii and frequencies selected the boundary layer thickness is comparable to the radius, and the usual high-frequency expressions for orifice impedance do not apply. Within the limits of experimental error (approximately 12% for resistance and 6% for reactance) the measured values are in reasonable agreement with Thurston’s general but approximate theory [G. B. Thurston, J. Acoust. Soc. Am. 24, 653–656 (1952)]. The measurements support the simple idea that both real and imaginary components of orifice impedance can be calculated assuming an effective total end correction 16a/3π, where a is the orifice radius.