METHODS OF INVESTIGATION 



221 



and, if from a target moving towards or 

 away from the echo-ranging ship, in pitch. 

 The change in pitch is due to the Doppler 

 effect. Moving objects in the water may 

 be distinguished from stationary objects both 

 on the basis of relative bearing changes and 

 Doppler. 



If the target contacted is a moving ship, 

 its maneuvers may be plotted from changes 

 in relative bearing and changes in range. 

 More important for our present discussion, 

 however, are the maneuvers which can be 

 detected directly by the sonar operator from 

 the sounds which he hears. Due to the 

 Doppler effect, if the target is headed toward 

 the echo-ranging ship, the echoes from it will 

 be higher in pitch than the reverberations; 

 if it is headed away from the echo-ranging 

 ship, the echoes will be lower in pitch; if it 

 is beam-on (i.e., neither headed towards nor 

 away from the echo-ranging ship), the echoes 

 will be of the same pitch as the reverbera- 

 tions. Changes in the target ship's course 

 will, therefore, be reflected in changes in the 

 pitch differences between reverberations and 

 echoes. For example, if the sonar operator 

 is receiving echoes whose pitch is approx- 

 imately the same as that of the reverbera- 

 tions and then the echoes gradually become 

 higher in pitch, he knows that a beam-on 

 target has turned towards him and is now 

 bow-on. 



Target aspect, i.e., the heading of the 

 target with respect to the beam of sound 

 striking it, and changes in target aspect are 

 also reflected in terms of loudness, duration, 

 and quality of the echoes heard by the sonar 

 operator. A target at right angles to the 

 sound beam striking it will return an echo 

 which is louder, "sharper" in quality, and 

 briefer in duration than the echoes from a 

 target which is headed towards or away from 

 the projector sending out the sound pulses. 

 Furthermore, the echo from a target, whose 

 stern is towards the echo- ranging ship, will 

 differ from one which is bow-on because of 

 the variations in the echo produced by the 

 wake. Echoes from stem-on targets will 



usually be longer in duration and ''mushier" 

 in quality. 



These examples will serve to illustrate the 

 principal kinds of auditory discriminations 

 required of the operator of echo-ranging 

 sonar. They may be summarized as: 



1. Detection of a tonal signal masked by 

 a background composed of an irregularly 

 modulated tone and of noise, the signal 

 differing from the background principally 

 with regards to loudness or pitch. The 

 rate of change of loudness — the abruptness 

 of the echo — is also a factor to be considered. 



2. Recognition of changes in the tonal 

 signals masked by the tonal and noise back- 

 ground, the changes being those of pitch, 

 loudness, duration, and quality. 



Methods of Investigation^ 



In basic auditory research three classes 

 of methods are commonly used, namely, psy- 

 cho-physical, physio-physical, and psycho- 

 physiological. Theories of hearing have 

 been based upon the results obtained by 

 all of these methods. Immediate answers 

 to the auditory problems encountered in 

 sonar will undoubtedly come from psycho- 

 physical studies. From a long-range point 

 of view, however, it is desirable to extend 

 our knowledge of the whole process of hear- 

 ing — to know what physiological events are 

 set off by changes in the physical stimulus 

 and to know what physiological processes 

 underlie the discriminatory responses made 

 by the living organism. 



Psycho-physical Methods 



Psycho-physical methods are those in 

 which the independent variables which are 

 manipulated by the experimenter are vari- 

 ous attributes of the physical stimulus, 

 e.g., frequency, intensity, and complexity. 



1 This and the following section on Results of 

 Relevant Research, etc. are reproduced, with 

 minor changes, from a report prepared by one of 

 the authors (W. D. N.) for the Panel on Under- 

 water Acoustics, Committee on Undersea War- 

 fare, NRC. 



