214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



aviation industry appropriated the word supersonic to refer to veloci- 

 ties greater than that of sound, physicists were forced to devise the 

 word ultrasonic for frequencies higher than those of audible sound. 

 To retain its original meaning the superheterodyne receiver should 

 today be called ultraheterodyne I 



PROPERTIES 



The properties which give rise to the unusual effects of ultrasonic 

 waves follow from principles common to all wave phenomena. In a 

 given medium the wavelength of a wave is inversely proportional to 

 the frequency, so a high frequency implies a short wavelength. 

 Furthermore, the directional character of wave propagation is a 

 function of the wavelength. Suppose that a vibrating circular piston 

 is used to generate sound waves. If the frequency is low the waves 

 spread out from the source in all directions and bend around corners. 

 As the frequency is raised the waves begin to assume directional 

 characteristics, that is, more of the wave energy is propagated in 

 certain directions than in others and bending becomes less pro- 

 nounced. At high frequencies most of the wave energy is concen- 

 trated in a truncated cone. The angle of the cone is a function of the 

 ratio of the wavelength of the wave to the diameter of the piston 

 source ; the smaller the ratio, the smaller the angle of the cone. Waves 

 of high frequency, or short wavelength, will therefore be propagated 

 essentially in a given direction with negligible bending. Ultrasonic 

 waves have been generated at frequencies as high as 500 megacycles, 

 corresponding to a wavelength in air equal to that of visible red light. 

 Such ultrasonic radiation has all the directional properties of a beam 

 of light. Unfortunately, the attenuation of the radiation is also pro- 

 portional to the frequency, or rather to the square of the frequency, 

 so that sharply defined beams cannot be propagated over long dis- 

 tances. Nevertheless, even at frequencies as low as 20 kilocycles, 

 beams of ultrasonic waves are well enough defined to be used in sub- 

 marine detection. 



The intensity of radiation being defined as the energy passing 

 through a unit area per unit time, it is apparent that the concentration 

 of ultrasonic radiation into a cone makes it possible to produce beams 

 of very high intensity. During the past decade ultrasonic sources 

 have been made to generate as much as 50 watts per square centimeter, 

 and beams of radiation have been focused to yield intensities as high 

 as 5,000 watts per square centimeter. These magnitudes become im- 

 pressive when compared with the intensities of familiar audible 

 sounds. At a distance of 2 meters from a trumpeter the sound inten- 

 sity is about one-millionth of a watt per square centimeter. If all the 

 sound energy generated by a full symphony orchestra could be concen- 



