The simplest condenser microphone in terms of construction and use is a charged, aluminized Teflon membrane placed directly over a conducting back plate. In this configuration, the low-frequency open circuit sensitivity is determined approximately by the product of the “effective” electret bias voltage and the compressibility of the fluid trapped between the membrane and the backplate, typically 1 mV/Pa in air. This talk will describe the procedures for calibration of such a transducer in a resonator and its application to the measurement of constituents in binary or pseudobinary gas mixtures. Other aplications to low-temperature physics will be described that include acoustic measurement of temperature fluctuations with a perforated electret membrane having a detection threshold of ≈ 10 nanokelvin/ √ Hz in superfluid 4He at 1 deg above absolute zero, and the measurement of the temperature dependence of the order parameter of superfluid 3He-A and 3He-B at temperatures of 11000 of a degree above absolute zero. [Work supported by the Office of Naval Research.]