TABLE VI

Observer R. Showing the influence of the binaural ratio on the localization of clangs. Artificial sensitivity series. Left ear plugged. Liminal distance: right ear, 18.5 cm.; left ear, 3 cm. Ratio, Right Left=6.2. Stimulus, Galton whistle, 20,000 vibrations per second.

Average displacement toward axis of stronger ear, 34.4°.

Tables XI and XII show the results of fairly successful attempts to correct this observer's tendency to displace sounds towards the left. In Table XII with a ratio left:right = 1.5 we find an average displacement of 2.1° towards the right; in Table XI with a ratio left: right = 2.1 we find an average displacement of 1.4° towards the left. The former tables thus show over-correction; the latter, under-correction.

B. THE RELATIVE IMPORTANCE OF INTENSITY AND TIMBRE AS FACTORS IN LOCALIZATION

In these experiments, tuning forks were used as the source of sound. The object was to find out to what extent the con

but will be displaced toward the axis on the side of the weaker ear, because sounds of equal intensity have always had that connotation in the observer's past experience. Likewise, when the ears have been made equally sensitive by plugging the stronger, sounds which come from positions to either side of the median plane will always be displaced toward the naturally weaker ear, because now they are heard by the two ears with a relative loudness which, in the observer's past experience, has always connoted a position relatively nearer the weaker ear.

TABLE VII

Observer S. Showing the influence of the binaural ratio on the localization of clangs. Artificial sensitivity series. Right ear plugged. Liminal distance: right ear, 18 cm.; left ear, 70 cm. Ratio, Left: Right = 3.8. Stimulus, Galton whistle, 20,000 vibrations per second.

Average displacement toward axis of stronger ear, 34.6°.

ditions obtaining in the former experiments influence the localization of simple tones. Three cases are possible. (a) These conditions may exert no influence at all. We should then have to conclude that, in the former experiments, the binaural ratio produces its effect wholly as a difference in the timbre of the sound as heard by the two ears. That is, since timbre depends upon the number and the proportionate strength of the overtones in the clang, in case one ear is more sensitive than the other, the timbre of the sound heard by one ear will differ from that heard by the other ear because of the different number of overtones present in the two cases.1 (b) The conditions may exert some influence, but not

"This view of the way the binaural ratio serves as a localizing clue was first advanced by Rayleigh in 1876, and later by Sylvanus Thompson (Op. cit., p. 415) in 1882. Thompson says: "Judgments as to the direction of sound are based, in general, upon the sensations of different intensity in the two ears, but the perceived difference of intensity upon which a judgment is based is not usually the difference in intensity in the lowest or fundamental tone of the compound (or 'clang'), but upon the difference in intensity of the individual tone or tones of the clang for which the intensity difference has the greatest effective result in the quality of the sound . . . It is completely open to doubt whether a pure simple tone heard in one ear could suggest any direction at all."

FERREE AND COLLINS

as much as was exerted upon the sound of the Galton whistle. In this case, we should have to conclude that differences of intensity both in the fundamental and in the overtones of the clang served as a localizing clue in our experiments. (c) They may exert an equal influence upon the sound of the tuning fork and upon the clang. This would indicate that differences in the intensity of the fundamental tone alone were operative as local signature.

TABLE VIII.

Observer F. Showing the influence of the binaural ratio on the localization of clangs. Artificial sensitivity series. Left ear plugged. Liminal distance: right ear, 39 cm.; left ear, 8 cm. Ratio, Right Left=4.9. Stimulus, Galton whistle, 20,000 vibrations per second.

Average displacement toward axis of stronger ear, 17°.

The stimulus was given as follows in these experiments. The observer, blindfolded, with head firmly clamped and ears tightly closed, sat in position in the sound-cage. A heavy unmounted tuning fork of 480 vibrations per second and a cylindrical resonator were used as the source of sound. These were substituted for the telephone receiver of the soundcage. The fork was plucked by the fingers covered by a chamois glove, and was allowed to vibrate for a few seconds to allow possible high overtones, harmonic or inharmonic, to die out. It was then held over the mouth of the resonator.

As soon as the tone became audible, the observer's ears were uncovered and the sound was listened to for about one second, at the end of which time the fork was removed from the mouth of the resonator, and the direction in which the sound was heard was indicated by the observer. In no case were any of the noises attendant upon the stimulation of the fork heard; and a tone as simple as a tuning fork is capable of giving was obtained. The duration of the stimulus was roughly the same

TABLE IX

Observer F. Showing the influence of the binaural ratio on the localization of clangs. Artificial sensitivity series. Right ear plugged. Liminal distance: right ear, 10 cm.; left ear, 41 cm. Ratio, Left: Right=4.1. Stimulus, Galton whistle, 20,000 vibrations per second.

Average displacement toward axis of stronger ear, 18.2°.

as that of the Galton whistle used in the earlier experiments, and care was taken to give the stimulus as nearly as possible the same intensity each time. The stimuli were all given at the level of the ears, and no account of vertical displacements was taken in recording the results, since these have no direct bearing upon the purpose of the experiment.

Tables XIII, XIV, and XV give the results of this investigation. These results, on the average, show that the ratio of sensitivity of the two ears affects the localization of simple tones almost, if not quite, as much as it does the localization of clangs of the degree of complexity of the Galton whistle.

TABLE X

Observer R. Showing the influence of the binaural ratio on the localization of clangs. Sensitivity of two ears equated. Left ear plugged. Liminal distance: right ear, 18.5; left ear, 18.2 cm.; Stimulus, Galton whistle, 20,000 vibrations per second.

Average displacement toward axis of right ear, 31.4°.

TABLE XI

Observer R. Showing the influence of the binaural ratio on the localization of clangs. Attempt to correct localizing error. Left ear plugged. Liminal distance: right ear, 19 cm.; left ear, 40 cm. Ratio, Left: Right= 2.1. Stimulus, Galton whistle, 20,000 vibrations per second.

Average displacement from median plane, 1.4° (left).