SOUND WAVES 



3 



condensations); the points below represent negative pressures (expansions, 

 rarefactions) with respect to the normal atmospheric pressure represented 

 by the straight line. 



From the above examples a few of the properties of sound waves and 

 vibrations in general may be defined. 



BOARD 



Fig. 1.1. Production of sound waves by a vibrating piston. 



Periodic Quantity. — A periodic quantity is an oscillating quantity the 

 values of which recur for equal increments of the independent variable. 



Cycle. — One complete set of recurrent values of a periodic quantity 

 comprises a cycle. Or, in other words, any one set of variations starting 

 at one condition and returning once to the same condition is a cycle. 



Period. — The period is the time required for one cycle of a periodic 

 quantity. 



Frequency. — The number of cycles occurring per unit of time, or which 

 would occur per unit of time if all subsequent cycles were identical with 

 the cycle under consideration is the frequency. The unit is the cycle per 

 second. 



Fundamental Frequency. — A fundamental frequency is the lowest com- 

 ponent frequency of a periodic wave or quantity. 



Harmonic — A harmonic is a component of a periodic wave or quantity 

 having a frequency which is an integral multiple of the fundamental fre- 

 quency. For example, a component, if the frequency of which is twice the 

 fundamental frequency, is called the second harmonic. 



Subharmonic. — A subharmonic is a component of a complex wave having 

 a frequency which is an integral submultiple of the basic frequency. 



Wavelength. — The wavelength of a periodic wave in an isotropic medium 

 is the perpendicular distance between two wave fronts in which the dis- 

 placements have a phase difference of one complete cycle. 



