10 HAYES— DETECTION OF SUBMARINES. 



to the orientation of the receiver than from other directions by 

 means of screening, etc. By rotating such a receiver about a ver- 

 tical axis, the direction of a sound source can be roughly determined 

 by judging the position of the receiver for maximum or minimum 

 sound intensity. Such a receiver is shown in principle in Fig. i, 

 Plate II. The receiver, represented by the small circle, is placed 

 within a heavy lead cone. The English have utilized this principle 

 in all of their listening devices. 



Binaural Principle. — Experiment proves that the direction of a 

 sound can not be judged with any degree of accuracy by one ear 

 alone, unless the pitch of the sound is fairly high (above 800 or 

 1000), but by using both ears the listener can locate the direction 

 with considerable accuracy for any pitch within the range of the 

 audible and the accuracy proves to be greatest when the direction 

 of the sound is about normal to a line joining the two ears. 



Suppose the sound source is to the right of the listener. The 

 sound received by the left ear will differ in two respects from that 

 received by the right ear. First, the left ear receives the sound later 

 than does the right ear and secondly, the intensity of the sound is 

 somewhat less in the left ear because of the sound shadow cast by 

 the head. The difference of intensity in the two ears is very slight 

 for sounds of low frequency but becomes greater as the pitch is 

 raised, due to the fact that the dimensions of the head are such that 

 it only serves as an efficient screen for sounds of short wave-length. 

 A single ear therefore becomes a screened receiver for high pitch 

 sounds. The determination of direction, when both ears are used, 

 depends largely on the difference in the time between reception at 

 the two ears. This is especially true for sounds of low frequency, 

 although the fact that intensity is slightly different at the two ears 

 may also be of some help. Whenever a sound reaches the two ears 

 at the same time, it appears to come from a direction perpendicular 

 to the line joining the ears and the listener judges the sound to be 

 somewhere in the plane which is the perpendicular bisector of this 

 line. If the sound source is to the right of this plane, the sound 

 reaches the right ear first and the listener judges the sound to come 

 from this direction. Sound is judged as coming from the right or 

 left, depending whether it reaches the right or left ear first re- 

 spectively. 



