Some properties of the sound emitted by airscrews

Abstract

A condenser-microphone amplifier system has been employed to determine the intensities and directional properties of the first six harmonics in the sound of rotation of an airscrew operating at zero rate of advance and actuated by a silenced engine. The sound-energy associated with the frequencies considered has been found to be 18 watts, the fundamental (first harmonic) being responsible for 50 per cent. of this, while the first three harmonics together contribute 90 per cent. Maximum intensity occurs from 15° to 30° behind the plane of rotation, and in this region the sound-output is particularly steady. Large intensity-fluctuations occur along the axis of the slip-stream. The intensity follows the inverse-square law of distance at points further than 200 ft. from the airscrew centre, a 2.4-power law holding approximately for nearer points. Cathode-ray oscillograms demonstrate the existence of frequencies of the order of 600 ~ which are probably not components of the sound of rotation. The hypotheses of Lynam and Webb, as modified by Paris, are briefly discussed in relation to the results of experiment.