momentary contact with dry ice, it imparts considerable 

 heat to it. This causes release of gas and considerable 

 pressure at the point of contact, and the metal object will 

 be set into vibration. Depending upon the shape and size 

 of the vibrating object, sounds of various frequencies and 

 surprisingly high levels are thus produced. Many varia- 

 tions of this method of generating sound are possible. 

 (This is a crude but effective method of testing vibrations 

 in metals at very low temperatures.) The frequencies 

 can be measured with a calibrated oscillator by means of 

 Lissajous patterns. 



THE "WOLF WHISTLE' 



In this sound source a condition which is normally 

 avoided as undesirable is intentionally put to use. It is 

 well known to players of string instruments that under 

 certain conditions beyond their control, a sympathetic 

 resonance can occur between the vibrating string and the 

 sound box. This undesired tone is call a 'wolf-tone. " 



In the "wolf whistle" sound source, the vibrating 

 string is replaced by a more rugged vibrating body such 

 as a small metal saw blade, steel clock-spring, reeds, rods, 

 or bars. The vibration is started by placing the body in 

 front of a jet of air or water. Varying the tension adjusts 

 the frequency. The vibrator is in turn placed at the open 

 end of a resonant chamber which is sympathetically 

 excited by the vibrating element. When the two frequencies 

 are identical, the resulting sound is loud. The adjustable 

 deflection plate can be used to control the intensity of the 

 sound generated. The two variations of the "wolf whistle" 

 shown in figure 37 have been operated in air but no under- 

 water measurements have been made of their performance. 



60 



