Page 437 51 



CHAPTER 5. ECHO SOUNDING 



51. GENERAL STATEMENT 



511. Principles of Echo Sounding 



Echo sounding is a method of measuring the depth of water by determining the time 

 required for sound waves to travel, at a known velocity, from a point near the surface 

 of the water to the bottom and return. Everyone is familiar with echoes in air and 

 realizes that they arc caused by reflections of sound from some distant object back to the 

 listener. If the time is measured between the production of a sound and the reception 

 of its echo, the distance to the reflecting surface may be determined by nmltiplying 

 one-half the time interval by the velocity of sound in air. Since the time interval is a 

 measure of the entire journey, only half the interval is used to find the distance to the 

 reflecting object. 



The same principles are applied in measuring water depths by echo sounding. A 

 sound is produced in the water near the surface; it travels to the bottom, from which it is 

 reflected back to the surface as an echo. Echo- sounding equipment is designed to 

 produce the sound, receive and amplify the echo, measure the intervening time interval, 

 and convert this interval automatically into units of depth measurement, such as feet, 

 fathoms, or meters. 



It is common knowledge that echoes in air can be heard only in certain localities, 

 and can be heard better under certain favorable conditions. This is because a surface, 

 well shaped to reflect the sound, is required at not too great a distance and conditions 

 must be favorable for transmission of the sound, so that it will not be attenuated to such 

 a degree that it cannot be heard. It is fortunate for echo sounding that echoes are much 

 less difficult to obtain from the ocean bottom than they are from objects in air. There 

 are several reasons for this: first, there is always a refecting surface, the sea bottom, that 

 generally acts as a good reflector of sound; second, sound in water is not subject to as 

 much absorption or to such large variations of attenuation as sound in air and, because 

 of less absorption, sound in water will travel many times as far Mdth reference to the 

 point of origin as sound of the same initial intensity will in air; and last, water possesses 

 characteristics favorable to echo sounding, such as a reasonably constant velocity of 

 sound. 



Continuing the comparison of echo soundmgs with the more familiar case of echoes 

 in air, it is well known that an echo follows immediately after the original sound if the 

 reflecting surface is near, and in order for the echo to be heard, the original sound 

 must be of very short duration. In the case of echo sounding this condition is aggra- 

 vated, for soimd in water travels more than four times as fast as it does in air. Because 

 of this higher velocity the sound signals used for echo sounding in shoal water must be 

 of extremely short duration. 



Sound is a disturbance that is propagated through an elastic medium by means of the 

 condensation and rarefaction of the medium, the velocity of propagation depending on 

 certain physical characteristics of the medium. Sounds in water are produced with 

 approximately the same ease and by the same methods as sounds in air are produced. 

 Most modern echo-sounding instruments generate sound in the water by means of a 



