220 



AUDITION IN SONAR OPERATION 



primarily by movement of the hydrophone 

 through the water. 



In echo-ranging systems, much of the 

 information received is presented to the 

 sonar operator visually as well as aurally. 

 In the present chapter, however, we shall 

 be concerned with auditory discrimination 

 and will mention only casually problems re- 

 lated to combined visual-auditory presenta- 

 tion. Research on auditory- visual displays 

 is discussed in detail elsewhere in this book. 



Auditory Tasks of the Sonar 

 Operator 



In the preceding introductory paragraphs, 

 a very brief description of sonar systems and 

 of the sounds which they deliver to the 

 sonar operator has been given. Before dis- 

 cussing methods and results of auditory 

 research in relation to sonar, the principal 

 kinds of auditory discriminations required 

 of the sonar operator will be outlined. 



Listening Systems 



The operator of the listening type of sonar 

 gear must first of all detect a sound signal 

 which appears in a noise background. The 

 signal will usually be atonal in character and 

 will differ from the background in one or 

 more of these characteristics: loudness, 

 rhythm, and quality. Occasionally, as in 

 the case of the whining noise produced by 

 machinery or by friction of the driving shaft 

 of a ship's screws, a signal may have a tonal 

 character. 



Having detected a signal, the sonar opera- 

 tor must attempt to identify it and, in the 

 case of a moving target, he must try to de- 

 duce its maneuvers. Identification will be 

 made in terms of rate and quality of rhyth- 

 mic patterns heard. For example, a large 

 ship with relatively slow turning screws 

 may produce a pulsating sound with a slow, 

 heavily accented rhythmic beat, while a 

 small, rapidly moving craft will produce a 

 very rapid pulsing sound with less dis- 

 tinctly accented beats. Shaft whines or 

 squeaks may aid in identification. Marine 



animals, such as croakers and crackling 

 shrimp, produce distinctive sounds which 

 lack the rhythm or periodicity of the sounds 

 produced by a ship's screws. 



Moving targets can be discriminated from 

 non-moving targets by noting changes in 

 relative bearing. Target maneuvers may 

 be detected not only by plotting bearings of 

 the target but also by changes in rhythmic 

 patterns of sound produced by the target's 

 screws. Change in target speed will be 

 heard as increase or decrease in rate of 

 rhythmic beat. Approach or withdrawal of 

 the target will be heard as increase or de- 

 crease in intensity of the target-produced 

 sounds. However, as a target with bow 

 towards the listening ship turns away, pre- 

 senting its stem, the sounds from the screws 

 will usually become louder for a short time 

 until the distance has increased somewhat. 

 A change in quality may also become ap- 

 parent. These changes occur because the 

 sound waves produced by the screws are 

 partially blocked by the hull of the bow-on 

 vessel and will come with greater intensity 

 and less loss of higher frequencies when the 

 stern is towards the listening ship. 



To summarize: the operator of sonar 

 listening gear needs to make the following 

 auditory discriminations : 



1. Detection of a noise signal masked by 

 noise background, the signal differing from 

 the background in one or more of these 

 characteristics: loudness, rhythm, quality. 



2. Recognition of changes in a noise 

 signal masked by a noise background, the 

 usual changes being those of loudness, qual- 

 ity, and rate of rhj^thmic pulsation. 



Echo-ranging Systems 



Typically, the first task of the operator 

 of echo-ranging sonar is to detect a short 

 tonal pulse or "pip" which appears against 

 a background composed of noise and, more 

 predominantly, of a tone, irregularly modu- 

 lated, and gradually decreasing in amplitude. 

 The "pip" which is the echo from a target 

 will differ from the background in loudness 



