Chapter 9 

 AUDITORY DISCRIMINATION IN SONAR OPERATION 



W. D. NEFF AND W. R. THURLOW 



University of Chicago and University of Virginia 



Introduction 



The surface craft or submarine obtains 

 information about its environment through 

 numerous channels: direct vision, radar, 

 radio, and sonar. Of these sonar is of par- 

 ticular importance to the completely sub- 

 merged submarine, to the surface craft in 

 detecting submarines, and to either sub- 

 marine or surface craft when it wishes to 

 detect objects, such as mines, which are 

 hidden under the water. Sonar makes use 

 of sound waves which are transmitted readily- 

 through the water; it picks up the sound 

 signals from the water, amplifies them, and 

 analyzes them in various ways so as to 

 present to the operator of sonar equipment 

 useful items of information about objects 

 and their movements in the sea around him. 



Sonar systems may be divided into two 

 basic types. Listening systems make use 

 of the sounds emitted by objects in the 

 water, e.g., the cavitation noise produced by 

 the screws of a ship. Echo-ranging systems 

 transmit pulses of sound into the water and 

 make use of the reflected sound energy, or 

 echoes, received from objects in the path of 

 the transmitted pulses. In general, listen- 

 ing systems use sound frequencies in the 

 sonic range, 20,000 cycles per second or less, 

 the range in which transmission of sound 

 through water is most efficient. However, 

 some listening systems operate in the super- 

 sonic range, above 20,000 cps. Echo-rang- 

 ing systems, in order to control the direction 

 of transmission of sound pulses, usually use 

 frequencies in the super-sonic range. 



With both Ustening and echo-ranging 

 systems, the sound energy picked up by a 

 hydrophone (underwater microphone) is 



changed into electrical energy, which is 

 amplified and treated in one way or 

 another — e.g. passed through selective fil- 

 ters — , then is changed back to sound energy, 

 through a speaker or headphones, or to light 

 energy, through a cathode-ray oscilloscope, 

 and finally is presented to the sound 

 operator. 



In sonic listening systems, the operator 

 receives most of his information^^ aurally 

 and the sounds he hears have a*^"real" 



as 



quality, i.e., the turning of a ship's screws 

 in the water sounds like the churning of 

 water. In super-sonic listening systems, the 

 sounds as presented to the operator after 

 heterodyning have a more "artificial" char- 

 acter, are more like bursts of static or pulses 

 of noise lacking the subtle quality differences 

 of the sonic system. 



The sounds heard in the typical echo- 

 ranging system are primarily tonal but with 

 a noise background. Echoes from the 

 transmitted pulse are heterodyned so that 

 the operator hears them as a somewhat ir- 

 regularly modulated tone with a frequency 

 of 800 cps (approx.). As a pulse of sound is 

 transmitted, reverberations (echoes which 

 come principally from the water surface and 

 any small particles in the water) are first 

 heard. The reverberations gradually de- 

 crease in intensity and echoes from objects 

 at a distance are heard as more or less abrupt 

 pulses of sound of slightly greater intensity 

 and, in the case of moving objects, of differ- 

 ent frequency from the background rever- 

 berations. The echoes and reverberations, 

 both of which have a tonal character, are 

 superimposed on a background of system 

 noise and water noise, the latter produced 



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