Some calibrations and applications of the aerodynamic fan reference sound source

Introduced by H. C. Hardy [Noise Control (May 1959)], the mechanical reference sound source (RSS) provides a broadband sound field by turbulent air shear at the perimeter of a rotating open centrifugal fan wheel. For a given wheel rpm, geometry, and air density, the emitted sound level is constant within less than 1 dB when averaged over a few seconds. The sound power is relatively uniform in octave or third-octave bands from 125 to 8000 Hz. Usable levels of sound power are emitted above and below these frequencies, with field measurements in the 63-Hz octave band feasible in large rooms. Small motor-driven units have endured for decades. Hemi-anechoic, reverberation room and intensity methods JR. Peppin et al., NOISE-CON 88, and W. D. Gallagher (private communication)] for RSS calibration will be described. Low-frequency discrepancies between free-field and reverberation room calibration methods [P. Brnel, B & K Review (1978, No. 3)] have diminished as the reverberation room method is improved (better sound diffusion, space averaging, and decay analysis) [P. K. Baade, 1989 ASHRAE Trans. (January 1989)]. The portability and reliability of this device suit it especially to worldwide use in hostile environments such as factory assembly rooms and construction sites. Some recent experiences in application of the Acculab RSS to small and large device sound power measurement and room sound absorption will be described.