Chapter 4 — PHYSICS OF SOUND 



Figure 4-23.- 



51.7 

 Bottom bounce effect. 



Bottom Bounce 



For long-range search In water depths over 

 1000 fathom;^, newer sonar equipment may operate 

 in what is called the bottom bounce mode. The 

 transducer is tilted downward at an angle so 

 that the sound beam strikes the bottom and is 

 reflected back to the surface, as shown in 

 figure 4-23. 



Because of the depression angle (15° to 45°) 

 the sound beam is affected less by velocity 

 changes than are sound pulses transmitted in 

 the normal mode. At great depths, however 

 (2500 fathoms and greater), the sound beam 

 usually will be refracted before reaching the 

 bottom, thus producing a convergence zone effect. 



DOPPLER 



You probably are familiar with the changing 

 pitch of a train whistle as the train passes near 

 you at high speed. As the train approaches, 

 the whistle has a high frequency. As the train 

 goes by, the frequency drops abruptly and be- 

 comes a long, drawnout sound. The apparent 

 change in frequency of a signal resulting from 

 relative motion between the source and the 

 receiver is known as doppler effect. Figure 

 4-24 illustrates doppler effect. 



Each sound wave produced by the whistle is 

 given an extra "push" by the motion of the 

 train. As the train comes toward you, the re- 

 sultant effect is an increase in pitch, caused 

 by compression of the waves. As the train 

 moves away from you, the sound waves are 

 spread further apart, resulting in a lower pitch. 



Because doppler effect varies inversely with 

 the velocity of sound, the effect is much less 

 marked in the sea than It is in the air. Doppler 



can, however, be noted by the Sonar Technician 

 if he listens carefully. 



Although a stationary receiver was used for 

 illustrative purposes, the same effects can be 

 observed if the receiver is moving toward a 

 stationary source. 



DOPPLER AND REVERBERATIONS 



Earlier in this chapter you learned that 

 as a sound pulse moves through the water 

 it loses some of its energy to the water particles. 

 The particles reradiate the energy in all direc- 

 tions. Part of the r eradiated sound is returned 

 to the sonar receiver, and the effect known as 

 reverberation is heard. The water particles 

 become sound sources and, as your ship moves 

 through the water, doppler effect is noticed. 

 The following example should help clarify doppler 

 effect for you. 



Your ship is underway at a speed of 15 

 knots; sound velocity is 4800 feet per second; 

 sound frequency is 12 kHz. The sonar is keyed, 

 and 1 second later it is keyed again. During 

 the 1-second interval, the first pulse travels 

 4800 feet. When the second pulse is transmitted, 

 it is only 4775 feet from the first one because 

 the ship has traveled 25 feet. Using the formulas 

 given in connection with wavelength, you can 

 easily determine that the apparent frequency 

 of the reverberations from ahead of you is 

 approximately 12.1 kHz whereas those from 

 behind your ship have a frequency of about 

 11.9 kHz. Reverberations from the beam will 

 have the same frequency as the transmitted 

 pulse, because the ship is neither going toward 

 the particles nor away from them. The doppler 

 effect of reverberations is illustrated in figure 

 4-25. To determine echo doppler, the Sonar 

 Technician compares the tone of a target echo 

 to the tone of the reverberations. 



LOW 



HIGH 



71.37 

 Figure 4-24. — Doppler effect. 



51 



