MEASURED 1/F NOISE IN THE MEMBRANE MOTION OF CONDENSER MICROPHONES

An acoustic isolation vessel described previously [K. C. T. Ngo and A. J. Zuckerwar, J. Acoust. Sec. Am. 93, 2974-2980 (1993)] has been employed to investigate the background noise in condenser microphones over the frequency range from 2 Hz to 25.6 kHz. Measurements were conducted on five B&K condenser microphones ranging from 1/8 in. to 1 in. diameter and representing both pressure and free field types. The experimental evidence reveals the existence of a purely mechanical 1/f noise component in the membrane motion of condenser microphones. The level of significance for the tested microphones, based on the t statistic, ranges from 0.985 to 0.999. It is found that the membrane 1/f coefficient correlates very well with the air layer resistance RA, which is responsible for the membrane damping. The theoretical implications of the existence of 1/f noise in the damped motion of vibrating systems are discussed.