182 Prof. A. Steinhauser on the Theory 



with respect in particular to the circumstance that it is per- 

 formed with two ears. It is concerned, further, in deciding 

 what part binaural hearing plays in the various phenomena of 

 hearing in general, and the various advantages thereby gained. 



Theory of Binaural Audition. 



Preliminary Observations. 



The sound produced by any source of sound is, as is known, 

 brought to our consciousness in the following manner : — The 

 vibrations originated by the sounding body are taken up by 

 solid or fluid bodies which are adjacent to it, or more com- 

 monly are taken up by the surrounding atmospheric air, and 

 are thence propagated until they reach either directly, or by 

 reflexion, the auditory passages of our ears, and thus in- 

 fluence the drumskin, the auditory ossicles, and finally the 

 auditory nerve. 



Moreover an essential adjunct for fine hearing is the ex- 

 ternal flap or pinna, which usually acts as a funnel, to conduct 

 into the ear the vibrations which, in consequence of their di- 

 rection, reach but cannot enter it. These vibrations, travelling 

 through the air in straight lines, we may for brevity call rays 

 of sound, by analogy with the expressions " rays of light " and 

 u rays of heat." 



According to the nature of the path by which the sound- 

 rays reach the single ear, we may distinguish between direct, 

 indirect, and mixed monaural hearing. 



In direct monaural audition the rays proceeding from the 

 sonorous body reach the ear immediately, or in straight lines, 

 and enter the auditory meatus either directly or after under- 

 going reflexion on the pinna or flap. 



In indirect monaural audition the rays proceeding from the 

 source of sound do not reach the ear directly (that is to say, 

 not in straight lines), but after undergoing simple reflexion on 

 the ground or on some other surface, or even occasional mul- 

 tiple reflexion, in which case the path travelled by the rays 

 forms a zigzag line. Such a kind of reflexion is in many 

 cases absolutely necessary if the rays of the sonorous body are 

 to reach the ear at all. 



In mixed monaural audition the rays proceeding from the 

 source of sound reach the ear partly directly, and therefore in 

 straight lines, partly indirectly, or after previously undergoing 

 reflexion, and consequently in zigzag lines. 



parti. Rep. Brit. Assoc. 1878: " Phenomena of Binaural Audition/' 

 part 2. Trans. Assoc. Francaise, 1878 : " Sur des Phenomenes de lAudi- 

 tion Binauriculaire." — Tbansl.] 



