The Localization of Sound

THE word "localization" as applied to sound is of psychological rather than physical origin. It deals with a sensation more than with a physical measurement. It differs from location in that it does not fix the position of a source of sound. Stereoscopic vision does not find its counterpart in hearing, although Hartley, in America, has attempted to show that we are partly endowed with a faculty of sound-location, or what we may call " stereotic audition. " It may be stated, however, that with our unaided listening equipment we can only obtain a sense of direction. Although we may not be conscious of it, we derive our sense of direction from an " image " which is subjective, and whose direeption, relative to the sagittal axis of the head, enables us to point to the source with a greater or less degree of accuracy. This image is located along the arc of a circle having the aural axis as a diameter. Sometimes the circle is larger than the head, i.e., there is external localization, and sometimes it is apparently inside the head. These characteristics are peculiar to the individual. The head, as normally placed, is best adapted for listening in a horizontal plane. Starch has shown that localization is most accurate in front, nearly as accurate for the back, and least accurate at the side. For all-round listening, accuracy in localization is least for the overhead position. Thus, if the head be maintained erect, an aeroplane may pass over an angle of 20 degrees of arc, 10 on each side of the vertical, without giving the listener any clue as to its change in position. It has always been recognized that, in normal cases, localization occurs by virtue of some difference 'in stimulus received by the two ears. There are three ways in which such variation in stimulus can arise, viz., by: (1) Difference in sound intensity; (2) difference in phase of the incoming sound waves; (3) difference in time of arrival of some corresponding portions of the sound waves received by the two ears. Dealing with these theories in order (1) The theory of localization by difference of sound intensity in the two ears was the first one to hold the field. Its principal advocates were Silvanus Thompson, Kessel and Fechner. These authorities found that localization occurred in the ear receiving sound of greater intensity. The most authoritative pronouncement against the theory is that of Rayleigb, who showed mathematically that by treating the head as a spberical obstruction, a maximum difference of intensity of only 10% occurs round the head for a tone of frequency 256 vibrations per second, while for a tone of 128 vibrations this is reduced to about 1%. It has been shown by Harvey Fletcher that the ear is incapable of appreciating intensity changes of less than 10%; hence no localization can be possibly obtained for notes of frequency less than 256 vibrations per second. For higher-pitched notes the intensity variation is, of course, greater, and when it becomes large, the intensity theory of localization will hold. It will be seen later that for high frequencies the phase theory breaks-down. We conclude, therefore, with Rayleigh, that localization of sounds of high pitch is chiefly due to variations of intensity. In general, unless special precautions are taken, there are high-pitched constituents in the sound, so that localization, if not obtained primarily by intensity difference, is certainly helped by it.