THE SYMBALLOPHOxNE: 



A DOUBLE STETHOSCOPE FOR THE 



COMPARISON AND 



LATERALIZATION OF SOUND 



PAIRED ORGANS of hearing are a convenience and almost a necessity in the 

 forms of animal life in which they are found. In the primitive sense, these 

 animals are able to locate sources of sound which may indicate danger or a 

 supply of food. The eyes are then directed to the point of origin of the sound 

 and the identity of the agent may be determined. The hunter, whether animal 

 or man, is greatly aided by the sounds produced by game in motion. Con- 

 versely, the hunted instinctively recognizes the safety value of a fixed position 

 with respect to both sound and sight. In our highly mechanistic age the man 

 on the street is at a great disadvantage if he cannot locate an onrushing vehicle. 



The localizing functions of the paired organs of hearing are extremely sensi- 

 tive. Psychologically, sounds arising from a point in the midline or in the 

 plane of projection of the long axis of the head reach the tympanic membranes 

 of both ears simultaneously and are of equal intensity. Unless the subject 

 moves his head, he is unable to locate the origin of the sound in this plane 

 which is perpendicvdar to the axis between the two ears. If, however, the sound 

 arises from one side of the midline, the point of its origin may be lateralized 

 with remarkable accuracy chiefly by means of the difference in the time of 

 arrival and in intensity of the sound waves that reach the tympanic mem- 

 branes of the two ears. The difference in the time of arrival of the sound waves 

 is determined by the difference in distance between the source of the sound 

 and the two ears; the difference in intensity is determined by the reduction in 

 intensity caused by deflection of the sound waves which pass around the head 

 to reach the ear on the side more remote from the source of the sound (fig. i). 



The simple device shown in figiue 2 readily demonstrates that a normal per- 

 son's ears are capable of detecting differences in time of 0.000003 sec. This 

 function may be tested by connecting a rigid hollow metal or glass tube of con- 

 venient bore with flexible rubber tubing of equal bore and length, to the head- 

 and earpieces of an ordinary binaural stethoscope. Small holes are drilled into 

 the rigid tube, one exactly in the center, one on each side 5 mm. from the 

 central aperture, and others equidistant from the central aperture but at 

 variable distances from the center. The subject inserts the earpieces into his 

 external auditory canals in the manner ordinarily employed by the physician 

 and closes his eyes. Sounds are produced by blowing or by vibrating a tuning 

 fork over the apertures. In normal subjects the sounds that enter the central 

 aperture are not lateralized but those that enter either aperture 5 mm. from 

 the center are readily lateralized to the side on which they originated. Sounds 



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