THE WORLD OF SCIENCE

Te Awamutu Courier, Volume 53, Issue 3821, 14 October 1936, Page 3

THE WORLD OF SCIENCE

.*. DIRECTION-FINDING BY SOUND. RESULTS OF LATEST RESEARCH. Before the Royal Institution of London on Ist May, Dr W. S. Tucker, Director of Acoustical Research in the British Air Defence Experimental Establishment, delivered an address on Direction-Finding by Sound, which is reported fully in "Nature” of 18th July. Much of the information contained in this article is taken from this source, though I have a personal interest in the problem going back to a date 20 years ago, when the idea of locating hostile aircraft by night in this way first occurred to me—as it did, no doubt, to many others— and I was impelled to put forward certain suggestions towards a practical outcome which, in the light of the remarkable developments revealed by Dr Tuckei- and of which a sketch will be given here now appear almost as jejune in their crudity, writes K.G. in Melbourne Age. The particulars which Dr Tucker gives of the methods and appliances which have been evolved by military engineers and armament firms for locating aircraft by sound not only serve to indicate the high importance which the British War Office, and doubtless the Defence departments of other nations—attach to this instrument of defence against attack from the air but the success achieved, illustrate* convincingly the soundness of a policy in trusting the solution of such problems to men of scientific training, and providing them with full facilities for the necessary experimentation and developmental work. Dr Tucker made a good beginning in this same field long before his appointment to his present post, for, during the Great War it was he who hit upon the happy idea of using the cooling effect of a puff of air upon a hot wire—an extremely fine one—to register the passage of the sound wave from the firing of a big gun, an idea most successfully applied to “sound ranging”—that is, the location of the emplacement of the enemy’s guns—by the British and Australian armies during the latter years of the war. Dr Tucker’s address to the Royal Institute was not, however, confined to the purely military aspect of location by sound. He gives interesting details of the comparative natural powers of man and other animals in this respect, from which it appears that the dog, the cat, the hen—and the chicken —are all superior to the human animal.

A cat, for example, is able to locate a mouse by its rustling movements well within one degree of its true diraction —a truly amazing feat of aural sense preception when we consider that this corresponds to location within one inch at a distance of six feet.

It would be interesting to know whether all night-hunting animals are similarly endowed, and whether they are superior in accuracy of location to other animals. Dr Tucker also discusses at some length the problem of the physical basis of this power of appreciating the direction from which a sound proceeds, a problem to which the late Lord Rayleigh devoted much attention, and complete examination of which requires the co-operation of the physicist, the physiologist and the psychologist. Rayleigh’s conclusion, based on numerous experiments, was that this power depends not so much on the sounds being louder at one ear—-that, one, of course, which is nearer to the source of sound—than at the other, though this difference of loudness can undoubtedly contribute, more especially in the case of high-pitched sounds, but rather on a phase difference of the two sounds due ito a slight difference in the time of arrival. Whether for the one reason or the other, it is clear that binaural preception of the sound is involved, and consequently that a person who is wholly deaf in one ear cannot locate the direction of a sound at all. .Rayleigh tested the truth of this deduction on Mr F. Galton, and found that Mr Galton’s power of localising sounds was, indeed, far inferior to that of persons having both ears effective. When addressed loudly and at length, by a little boy standing a few yards in front of him he was under the impression that the vioce was behind. This remark of Rayleigh’s seems to me, however, inconclusive, for, in the case of a sound coming from a point directly in front or directly behind the head of the listener, there would be a difference neither in phase nor intensity at the two ears. In fact, it is not early as flasy, even for a person of normal audition to distinguish in complete darkness and without extraneous aids, Whether a sound comes fro mthe front or from the rear, as it is to place it either to the light er to tho left. The same comment applies to sounds coming from directly overhead. More convincing evidence of the essential part played by the two ears are the experiments, quoted by Tucker, in which the capacity of the female cricket to fly directly to the chirping male is entirely lost if the tympanum of one ear is removed, and of a sheep dog whose remarkable power of direc-tion-finding by the sound of a whistle was baffled when one ear was tightly bandaged! Tucker accepts the more modern version of Rayleigh’s phase-difference theory, due chiefly to Professor van Hornbostel, that the most important factor in direction finding by sound is the difference in the time of its arrival at the two ear. Yet this difference is so exceedingly small that its discrimination by the brain centre to which the aural nerves convey the sensation is almost incredible. Thus, in the case of the cat cited above, the time difference can still be

distinguished When the interval is only three-millionths of a second, and, in the case of a chick responding to the cluck of her mother-hen, only half the amount! The best military sound-locaters today, Dr Tucker tells us, can give an accuracy of a quarter of a degree on a fixed sound source—a performance representing a sixteenfold improvement on the unaided sense, and about two degrees on a flying aeroplane. This advance has been achieved by extending and enlarging the ears, as it were, by means of enormous horns, or by parabolic mirrors which concentrate the sounds at their focus; and, further, by separating the openings of’ these artificial ears so that the “base” of perception instead of the normal six inches from ear’ to ear is from five to ten times this distance. On a similar principle, the distance between the eyes on which our estimates of the distance of an object are based is enlarged in optical rangefinders. Two pairs of such horns or mirrors, mounted pair and pair in perpendicular directions, enable both the azimuth and the altitude of a distant plane to be simultaneously determined; the sound is, of course, also magnified greatly in intensity, ahd thus heard from a distant source with a better approximation to its true quality—both of which circumstances aid in its better location. It appears that training of the observers can effeat very great improvement in their capacity to locate direction of the source of sound —this may explain the alleged superiority of blind people, a theme dramatically exploited by Mr H. G. Wfells—and that, “by careful training, a listening team of two locators can achieve an accuracy of one degree for a reasonable proportion of the time during which the aircraft is within hearing, and at heights above 5000 feet.”