An Electro‐Mechanical Frequency Analyzer, previously reported (Physical Review, August 1, 1929) has been adapted to the determination of the sound spectra of aircraft in flight. The complex source of sound is converted into potential fluctuations by a microphone and high quality amplifier. The output of the amplifier is applied to a modified quadrant electrometer, the needle of which forms part of a sharply resonant mechanical circuit. The natural period of the mechanical circuit is made continuously variable by a tri‐filar suspension and can be determined from a single micrometer reading. An analysis of a complex source is made by observing the amplitude of vibration and natural period of the mechanical circuit for each component. By a previous calibration of the instrument these observations are sufficient to determine the amplitude and frequency components of the complex sound. The instrument has been used to measure the sound spectra of typical passenger and military planes in flight.