Acoustic impedance of large orifices in thin plates

Abstract

Acoustic impedance of large orifices (0.5–0.75 in. diameter) in thin plates (0.062 in. thickness) was investigated. This work extended the scope previously studied by Stinson and Shaw [Stinson and Shaw, Acoust. Soc. Am. 77, 2039 (1985)] to orifice diameters that were 32 to 584 times greater than the boundary layer thickness. For a frequency range of 0.3–2.5 kHz, the resistive and reactive components were determined from an impedance tube with six fixed microphones. Sound pressure levels (SPL) were varied from 115 to 145 dB. The transition regime from constant to increasing resistances occurred at higher frequencies for larger diameters. Resistance measurements after the transition regime were in good agreement with Thurston's theory [Thurston, J. Acoust. Soc. Am. 24, 653–656 (1952)] coupled with Morse and Ingard's resistance factor [Morse and Ingard, Theoretical Acoustics (McGraw-Hill, New York, 1969)]. Measured reactances remained constant at magnitudes predicted by Thurston's theory.