SOUND LOCALISATION 617 



diffracted the more readily round the head to the ear away from the sound, 

 than will those of short wavelength (high pitch). 



(3) The sounds reaching the nearer ear will arrive earlier than those 

 stimulating the other, because of the time taken to travel round the head. 

 The nerve impulses from the two ears will not therefore arrive at the same 

 instant, and by an appreciation of the difference in time the approximate- 

 position of an external object can be gauged. 



That this factor is of great importance can be shown by experiment in 

 the following manner. A stethoscope with two earpieces is fitted in position, 

 and to its mouthpiece is applied a loud tuning-fork. The tube connecting 

 the mouthpiece to one of the earpieces has an adjustable U-piece like a 

 trombone so that the distance travelled by the sound in reaching that ear 

 can be varied. The other tube has a length which is equal to that of the 

 others, with the U-piece in its mid position. When a note is sounded and the 

 U-piece altered, the position of the sound appears to move from one side to 

 the other according to which ear has the shorter tube. 



(4) The sounds reaching the ears travel not only through the air but also 

 through the bones of the skull. This can be proved by placing a tuning-fork, 

 which has been sounded and allowed to fade until its note is inaudible, on 

 one of the teeth ; the sound will be conveyed by the root of the tooth to bone, 

 and by bone conduction to the ears. Therefore when a sound enters the 

 right ear, it will travel by bone conduction to the left, and owing to its rate 

 of travel being different to that of sound in air, it will reach the left ear at 

 a different instant to which the sound travelling round the head will reach it. 

 If one ear is facing the sound, the bone-conducted sound will probably reach 

 the other ear before the air-conducted does. On the other hand when 

 both ears are equidistant, it is clear that the air-borne sound will arrive first 

 at both ears, and the bone- conducted sound very much later. By an 

 appreciation of the difference in time of the arrival of the two sounds it is 

 probable that localisation is effected. 



(5) In animals, the ability to turn the ears in different directions and so 

 find the direction of maximum intensity, must be of the utmost possible value 

 in sound localization. 



