I : 2/ Sound and the Ear 5 



needed for hearing. Frequencies higher than the audible range are not 

 sensed until the energy becomes so great as to cause local heat and pain. 

 Between these two extremes lie the frequencies to which the ear is 

 sensitive. The exact frequency range depends on the person; it is 

 influenced by his age and by the environment. 



All vertebrates have a hearing apparatus homologous to our ear, 

 although the frequency ranges to which they respond are varied. Many 

 other animals such as insects are sensitive to vibratory energy over a wide 

 range of frequencies, but their receptors are different, and the mechan- 

 isms involved in their response may be different. Even the single-celled 

 animal, paramecium, can respond to vibratory energy in some fashion. 

 Thus, there are many different types of sensory systems excited by 

 vibratory mechanical energy. One of these, namely the human hearing 

 apparatus, has been chosen for presentation in this chapter and in its 

 sequel, Chapter 6, in Section B. 



2. Acoustics 



The physical aspects of sound transmission and the vibration of the ear 

 are a subdivision of acoustics. The latter, in turn, is a branch of 

 physical mechanics. In order to read with understanding journal 

 articles dealing with the ear and hearing, it is very helpful to be familiar 

 with the terminology of acoustics and with the electro-acoustic analogs 

 often used. The various acoustic terms useful in describing studies of 

 hearing are defined and discussed briefly in Appendix A, entitled 

 "Auditory Acoustics." In contrast, this section of this chapter contains 

 only a few of the acoustic terms used most frequently in studies of the 

 ear and hearing. 



Perhaps most familiar is the terra frequency which describes how many 

 times a second the sound pattern is repeated. The simplest possible case 

 is one in which the sound pressure, p, can be described by an equation 

 such as 



p = po sin 2-nvt (1) 



where p Q is the acoustic pressure amplitude, / is the time, and v is the 

 frequency. This is referred to as a pure tone. The latter term is applic- 

 able since tone (or pitch) is the sensation associated with frequency. 

 Most sounds consist of a mixture of frequencies which gives the sound its 

 characteristic quality and timbre. A tuning fork comes close to pro- 

 ducing a pure tone. One can come even closer by using an electronic 

 oscillator and loudspeaker. 



Any complex tone can be represented as a sum of simpler pure tones. 



