Appendix A 



Auditory Acoustics 



The purely physical part of hearing belongs within the field commonly 

 referred to as auditory acoustics ; it is discussed in this appendix in more 

 detail than it is presented in Section 2 of Chapter 1. Acoustics is defined 

 as a study of vibration and sound. Sound refers to the propagation of 

 elastic disturbances in a continuous (3 dimensional) medium, whereas 

 vibration often is restricted to elastic disturbances in simpler systems, 

 such as springs or loudspeakers. In either case, what happens is that 

 certain particles are displaced from their equilibrium positions, thereby 

 developing potential energy. Later, these same particles are restored to 

 their equilibrium positions releasing energy. These motions can be 

 handled mathematically only if they are sufficiently small. Theories 

 using the approximation of very small vibrations are called infinitesimal 

 amplitude acoustics. They describe most of the properties of sound which 

 are important in a study of hearing. 



This appendix is a compilation of various physical terms used to 

 characterize audible sound. They are summarized in Table I on page 

 590 ; each is described briefly in the text. 



Any vibration or elastic disturbance may occur only once, or the 

 phenomenon may be repeated. If it is repetitive, one can distinguish a 

 certain frequency, that is the number of times per second that the 

 particle has the same displacement and velocity. A very simple case 

 arises if the motion of a given particle can be described by 



£ = A x sin 2-TTvt + A 2 cos 2-nvt (1) 



589 



