It has been observed that the free air pressures of a sound wave may be increased at the diaphragm of a microphone in a ratio greater than 2:1 at certain frequencies, when there is concavity in the face of the microphone. A method of calculating the magnitude of this increase is applied to a condenser transmitter of the Wente type, and close agreement is found with experimental results. The calculations involve certain simplifying approximations the validity of which is discussed.