CHAPTER IB 



Phonoreception 



INTRODUCTION 



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■ HONORECEPTioN IS THE PROCESS of detecting vibratory motion, usu- 



■ ally of frequencies higher than several vibrations per second, in 

 W the surrounding medium. Stimulation occurs most commonly 



by means of sound waves in air (mammals, insects), but may also be pro- 

 duced by sound waves in water (fish, amphibia), or even by vibration of a 

 solid body against the skeletal structure supporting the phonoreceptive organ 

 (bone conduction in man, utilized in certain types of hearing aids). 



Sound Waves. Sound waves in any medium consist of rapid vibratory mo- 

 tion of the particles which compose the medium. The motion of one particle 

 tends to disturb a neighboring particle, which in turn disturbs the next one, 

 so that a wave of disturbance passes through the medium. When a tuning 

 fork vibrates in air, the forward movement of the prong compresses the ad- 

 jacent air, but the elasticity of the air prevents maintenance of this localized 

 region of compression, and expansion occurs at the expense of the adjoining 

 region, so that a wave of excess pressure emanates from the prong. In a simi- 

 lar manner, backward movement of the prong sets up a wave of diminished 

 pressure or rarefaction. 



If the tuning fork vibrates with the simplest to-and-fro movement it gen- 

 erates what we call a "pure" tone, and its motion may be expressed quanti- 

 tatively in two ways: by the frequency of its vibration, and by the ampli- 

 tude of its excursion. The resulting wave may likewise be described by its 

 frequency and intensity. 



The tip of the tuning fork and consequently any given particle in the 

 medium undergoes simple harmonic motion in the principal direction of 

 propagation of the wave, and its position if plotted as a function of time gives 

 a curve that is described as a sine wave (Fig. 153). The pressure at any 

 given point in the medium also varies as a sine wave which leads the dis- 

 placement wave by 90 degrees. If motion of the fork is not a simple to-and- 

 fro movement, then the form of the wave produced is complex, and so is the 

 tonal stimulus. A complex wave may be resolved mathematically into two 

 or more simple waves, which may then be described in terms of frequency 

 and intensity. 



If a sinusoidally varying force is applied to an object whose resistance to 

 motion does not vary linearly with the displacement, then the object will 

 not move in a simple sinusoidal manner, and if its motion is plotted as a 

 function of time the wave will appear to be a "distorted" sine wave. Such 

 a wave may also be resolved into two or more sinusoidal waves. If the ap- 

 plied frequency is considered to be the "fundamental" frequency of vibra- 



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