The sensitivity and frequency response characteristics of type‐L standard laboratory microphones were measured with an electrostatic actuator at surface and at 12 different levels to 31 atm absolute (ATA) in a hyperbaric chamber. The microphones were measured in compressed air and a helium‐air environment, with and without the capillary pin. In addition, one microphone remained at a depth of 31 ATA for 24 h to measure its stability under pressure. Results indicated a loss in sensitivity in compressed air and helium‐air as a function of pressure. In a compressed‐air environment a resonant peak occurred at 4000 Hz, which shifted to 8000 Hz in the helium‐air environment. The microphones showed extremely close agreement between compression and decompression, between different pressurizations, and between different microphones. Also, there was no change in sensitivity over a 24‐h period of pressurization at 31 ATA. Calibration data at several pressures are given.