A New Acoustical Phenomenon

WITH regard to Dr. Erskine-Murray' observation of the behaviour of aeroplane sounds (NATURE, June 16, p. 490), attention may be directed to the fact that any combination of confused noises will behave in the same manner, such, for example, as the noise of rustling leaves, escaping steam, a shower of rain on trees or tin roofs, or of a distant train in motion. If one stoops towards the road or approaches a reflecting wall while any of these noises are going on, the pitch of the sound rises, and when one is in the act of standing up, or of withdrawing from the wall, it descends. The grating of carriage-wheels on the road, or rather the noises reflected downwards from the body of the carriage, have a like effect when the observer is standing perfectly still. In this case, however, for some reason not clear to the writer, careful listening shows that the pitch falls as the vehicle nears the observer, and rises as it recedes from him. If the sound is a single continuous note, such as that of a whistle blown by a bicyclist riding past the observer, beats are heard as the whistle advances and also as it recedes, these being due to interference between the direct sound-waves and those reflected from the road. The occurrence of beats in such circumstances is perhaps not generally recognised.