J. G. Ryan E. A. G. Shaw A. J. Brammer G. Zhang
Inst. for Microstructural Sciences, Natl. Res. Council, Ottawa, ON K1A 0R6, Canada
An enclosure has been constructed to evaluate the performance of active noise reduction (ANR), circumaural head sets, and hearing protectors, at frequencies below 800 Hz. It consists of a massive (5-kg), flat base plate against which one cup cushion is sealed by spring pressure. The sound pressure within the cup is recorded by a microphone flush-mounted in the base plate. A 20-cm-diam aluminum tube, 20 cm in length, is attached by annular flanges to the base plate at one end and, at the other, to a 15-cm-diam loudspeaker, which drives the enclosed volume so formed. The excitation of cross modes within the tube is suppressed by a 2.0-cm cylindrical annular layer of felt, in contact with the inner wall of the tube. The performance of the measuring system will be described and results reported for the attenuation of an analog ANR head set. [Work done in collaboration with the Defence and Civil Institute of Environmental Medicine.]