HAYES— DETECTION OF SUBMARINES. 5 



rors or lenses for altering the direction of sound must be very large, 

 so large. indeed that their use is impractical. 



Methods for Collecting Submarine Sound Energy. 



There are two methods by which submarine sound energy can 

 be efficiently brought to the ears. The first method makes use of 

 the principle of resonance, the second method makes use of the dif- 

 ference in hydrostatic pressure between the dense and rare portions 

 of a sound wave. 



A tuned diaphragm in water can be thrown into violent agitation 

 by a comparatively faint sound source if the frequency of the sound 

 wave is the same as the natural period of the diaphragm. Calcula- 

 tion shows that in this way the diaphragm can be given an amplitude 

 of vibration about i,ooo times the natural amplitude of the sound 

 waves. And since the intensity of the sound from the diaphragm is 

 proportional to the square of the amplitude this would result in mul- 

 tiplying the sound intensity given out by the diaphragm by some- 

 thing like one million* The Germans have made use of this prin- 

 ciple in the listening gear installed on U-boats as also have the 

 British in much of their earlier work. 



A sound receiver operating on this principle can detect a sub- 

 marine at a great distance providing the submarine gives out sound 

 of the same frequency to which the receiver is tuned and also pro- 

 viding there are no other sound sources in the neighborhood giving 

 out this same pitch. 



An analysis of the sound emitted by a submarine shows a contin- 

 uous sound spectrum throughout the range of the audible. No char- 

 acteristic frequency is emitted. There is every reason to believe this 

 is also true for all surface craft having a metallic hull and it follows 

 that no distinct advantage is to be gained by using highly sensitive 

 resonant receivers since the undesirable sounds, which are always 

 more or less present, are intensified in the same proportion as the 

 sound which it is desired to locate. Sensitivity alone, beyond a cer- 

 tain point ,is of no advantage and may prove to be a disadvantage. 



The resonant receiver has two serious weaknesses. First, it only 

 responds to sounds of one frequency, the natural frequency of its 

 diaphragm. As a result all boats sound alike. The quality of their 



