CHAPTER XIII 

 SPEECH, MUSIC AND HEARING 



13.1. Introduction. — The major portion of this book has been con- 

 cerned with the theory, design and testing of acoustical apparatus for the 

 reproduction of sound. The ultimate significant destination of all repro- 

 duced sound is the human ear. The physiological and psychological ef- 

 fects of the reproduced sound are the most important factors in any sound 

 reproducing system. Sounds heard may be classified as speech, music or 

 noise. An enormous amount of valuable data relating to speech and hear- 

 ing have been collected. This data is extremely useful in the development 

 and design of sound reproducing equipment. It is beyond the scope of 

 this book to present all the pertinent data of physiological and psychological 

 acoustics. For information beyond that given in this chapter the reader 

 may consult the references. It is the purpose of this chapter to show the 

 principal characteristics of speech, music and hearing and the relation be- 

 tween these characteristics and the objective characteristics discussed in 

 the preceding chapters. 



13.2. Hearing Mechanism^. — The hearing mechanism may be 

 divided into three parts: the outer ear, the middle ear, and the inner ear. 

 The outer ear consists of the external ear or pinna and the ear canal which 

 is terminated in the ear drum or tympanic membrane. Behind the ear 

 drum is the middle ear, a small cavity in which three small bones or ossicles 

 form the elements of a mechanical transformer for transmitting vibrations 

 mechanically from the ear drum to an aperture termed the oval window 

 in the inner ear. The casing of the inner ear (the cochlea) is a bony struc- 

 ture of a spiral form (two and three quarter turns). The cochlea is divided 

 along its length into three parts by the basilar membrane and Riessner's 

 membrane. These three parallel canals are wound into the spiral. On 

 one side of the basilar membrane is the organ of Corti, which contains the 

 nerve terminals in the form of small hairs extending into the canal of the 

 cochlea. These nerve endings are stimulated by the vibrations in the 

 cochlea. 



^^ Fletcher, " Speech and Hearing," D. Van Nostrand Co., New York. 



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