ACOUSTIC ISOLATION VESSEL FOR MEASUREMENT OF THE BACKGROUND-NOISE IN MICROPHONES

An acoustic isolation vessel has been developed to measure the backgro und noise in microphones. The test microphone is installed in an inner vessel, which is suspended within an outer vessel, and the intervenin g air space is evacuated to a high vacuum. An analytical expression fo r the transmission coefficient is derived, based on a five-media model , and compared to experiment. At an isolation vacuum of 5 X 10(-6) Tor r the experimental transmission coefficient was found to be lower than - 155 dB at frequencies ranging from 40 to 1200 Hz. Measurements of t he A-weighted noise levels of commercial condenser microphones of four different sizes show good agreement with published values.