X. r L T H A S O N 1 (,' V I U H A I I () \ S 30.3 



that, if the first particle in a given medium is disturbed, this disturb- 

 ance is transmitted through the medium by virtue of the coupUng 

 between the individual particles. In addition to this, the form or 

 shape of the disturbance is also transmitted as long as the medium is 

 linear (i.e., obeys Hooke's law). If the disturbance at the one end 

 is sinusoidal and if the motion of the particles is sinusoidal, it is easily 

 seen that, for a finite velocity of propagation of this disturbance, 

 there will be for any given particle a series of particles all with the 

 same displacement in the same direction at regularly spaced intervals 

 throughout the medium. The distance between two particles with 

 the same magnitude and direction of displacement is known as the 

 wavelength. Note here that wavelength has significance only for 

 repeating phenomena while frequency is defined only for sinusoidal 

 motion. If V is the velocity of propagation, X the wavelength, and 

 / the frequency, it follows then that V = f\. For ultrasonic fre- 

 quencies of 20 to 100,000 kilocycles per second, the corresponding 



TABLE I 



Sound Velocity, Acoustic Resistance (pV), and Density (p) of Various Materials 



Temp., Density, Velocity, pV X 10"' 



Material °C. g./cc. m./sec. g./cm.Vsec. 



Acetone 20 



Benzene 20 



Carbon tetrachloride . . 25 



Chloroform 20 



Ethyl alcohol 20 



Ethyl ether 20 



Glycerol 20 



Mercury 20 



Methyl alcohol . . 20 



Oil, transformer. . . 25 



Oil, castor 25 



Water 25 



Heavy water 25 



Xylol 22 



Brass 20 



Glass 20 



Nickel 20 



Steel 20 



Quartz 20 



Aluminum 20 



Rubber 



