Change of Pitch in certain Sounds with Distance

Abstract

SEVERAL years ago the late Prebendary Simpson, of Fittleworth, Sussex, told me of an interesting observation he had made, which some of your readers may be able to explain. While walking up and down the platform of a railway station, he noticed a peculiarity in the sound of a steam jet from an engine standing on the lines. The pitch of the sound appeared to rise as he retreated from the engine and to fall as he drew near to it. Some time after, Mr. Simpson observed the same thing again, but in this instance the noise was made by a gas flare in the open air, about which some men were at work. Since then I have found that this alteration of pitch with distance occurs with any fizzing noise of the kind, such as that of air jets, burning logs, frying fat, pouring rice or coffee beans, waterfalls, or even the rustling leaves of a single tree; with all those noises, in fact, whose sources are sufficiently localised to admit of observations of the kind being made. I found, also, on withdrawing from such a source that a point is reached after which the pitch ceases to rise, and remains practically stationary as far onward as the sound continues audible. This point is sometimes pretty definitely marked, and varies in distance from the source with different sounds, and the pitch of the stationary portion also varies in the same way. I do not think, however, that the pitch of the whole volume of sound changes, though it often appears to do so, for a similar impression is created by moving a fizzing air jet to and fro close to a wall. As it nears the wall, the whole sound seems gradually to rise in pitch and to sink again as the jet is withdrawn. But here the effect is clearly due to successive reinforcement of one part of the noise after another in the order of their wave-lengths. It is only a shifting of the point of greatest intensity, and not an actual change of pitch at all. Assuming, then, that the effect noticed by Mr. Simpson is of the same nature, that is to say, caused by a readjustment of the relative intensity of the parts, how is it to be accounted for? Is it simply a process of sifting by distance, the weaker groups of small noises, of which the fizzing sound is composed, dropping out of earshot in succession, as the observer retires from the source, till only the largest and loudest group is left, which last continues to be heard for the remainder of the distance without sensible change of pitch? If that is so, then the deeper tones of such noises would seem to have a proportionally shorter range of audibility than the higher ones; for, so far as I have observed, the pitch always sinks on approach to the source and rises on withdrawal from it, never the reverse way, as might be expected in the case of very bass roaring sounds. Perhaps, however, others may have noticed instances of the latter sort. The behaviour of the airjet fizz at the wall illustrates a kind of reciprocal action, which no doubt plays an important part in the adjustment of the pitch. The tone which is loudest for the moment appears to dominate and obscure the rest, so that, near the source, where the deeper tones are most powerful, these latter, to some extent, subdue and lower the principal one, while further off, where they become enfeebled by distance, they are in their turn still more diminished by the presence of the principal far-reaching tone.