26o 



The Symballophone 



that enter the apertures more remote from the center appear to arise at a 

 greater distance from the midline. But when the difference in distance becomes 

 as great as 15 to 18 cm., the sounds appear to arise almost opposite one ear. 



Sound travels in air at the rate of 330 m. per sec. If the ears are able to 

 lateralize the sound that arises only 5 mm. from the midline (which under 



Ridht experimental conditions would 

 ~ mean a difference in distance 



of 10 mm., or 1 cm., between 

 the two ears) they must be ca- 

 pable of detecting differences 

 in time of only 0.000003 see- 

 the time required for sound to 

 travel 1 cm. in air. This time 

 factor may be shown by the fol- 

 lowing analysis: sound travels 

 in air at the rate of 330 m. per 

 sec, or 0.330 m. (330 cm.) in 

 0.001 sec, 0.0003 m. (0.3 cm.) 

 in 0.000001 sec, and 1 cm. in 

 0.000003 sec. Some years ago, 

 while we were studying the 

 lateralizing and differentiating 

 functions of the ears, one of my 

 associates (Dr. A. M. Bassett) 

 and I were paying the usual 

 tribute to a Middle Western 

 city between trains at a mov- 

 ing-picture theater. During the 

 interlude a trumpeter with 

 three trumpets of different keys 

 appeared on the stage. The 

 trumpets were arranged as 

 shown in figure 3. We were 

 sitting in the balcony approxi- 

 mately 30 m. from the trump- 

 eter. When the trumpets were 

 blown separately or in unison, 

 we could differentiate accu- 

 rately the sound produced by 

 each instrument. Anyone with normal hearing can readily lateralize the 

 sounds produced by instruments in an orchestra or a band or by voices of 

 certain pitch or quality in a chorus. Furthermore, sounds such as voices may 

 be differentiated and located accurately, for instance around the dinner 

 table, at a tea or at a cocktail party. The organ of Corti is remarkably selective 



Fig. 1. When sounds arise from a point C which 

 lies in the longitudinal plane of the head and is 

 equidistant from the two ears, the sound waves reach 

 the auditory nerves simultaneously. When sounds 

 arise from any points to either side of point C, they 

 arrive at the two ears after different intervals of 

 time and always later in the ear more distant from 

 the source of the sound, as shown by the broken 

 lines. Likewise, the sound waves which travel 

 through the greater distance are deflected in their 

 course around the head and the intensity of the de- 

 flected sound is reduced. The differences in timing 

 and intensity of the sounds that reach the two ears 

 are the two most important conditions in determin- 

 ing the perception of lateralization. Sounds of differ- 

 ent qualities from different external sources may be 

 located simultaneously because of the highly selec- 

 tive functions of the organ of Corti. 



