222 



AUDITION IN SONAR OPERATION 



The dependent variables which the experi- 

 menter observes and measures are dis- 

 criminatory responses of experimental 

 subjects. Where human subjects are used 

 these responses will as a rule be verbal re- 

 ports; where lower animals are used, dis- 

 criminations will be indicated by overt 

 motor reactions. Standard procedures used 

 in psycho-physical studies are in common use 

 in the psychological laboratory. See, for 

 example, Guilford (21), Stevens and Davis 

 (52), and Thurstone (56). 



Physio-physical Methods 



In studying the physiological changes 

 evoked by changes in the physical stimulus, 

 the independent variables are again attri- 

 butes of the physical stimulus, such as in- 

 tensity, frequency, and complexity; but 

 the dependent variables, in this case, are 

 directly recorded physiological changes in 

 the auditory system. The physiological 

 changes which have been most commonly 

 used in the past are the microphonic response 

 of the cochlea and the action potentials of 

 the auditory nerve or of the higher neural 

 pathways (5, 16, 52, 62). Recently Bekesy 

 (4) has developed a technique which makes 

 possible visualization of movement of coch- 

 lear structures. This method appears to 

 have much promise. Experimental sub- 

 jects in physio-physical investigations will 

 usually be lower animals. 



Psycho-physiological Methods 



Finally, it is necessary to relate physiologi- 

 cal changes and psychological phenomena. 

 The independent variables will be exper- 

 imentally produced alterations of the phys- 

 iological mechanism of hearing, i.e., of the 

 middle ear, inner ear, or neural structures; 

 and the dependent variables will be dis- 

 criminatory responses of the experimental 

 subjects. 



As in physio-physical studies, animals 

 will necessarily be used as experimental 

 subjects in most psycho-physiological ex- 

 periments. Methods have been developed 



which make possible accurate determination 

 of the ability of animals to discriminate 

 changes in auditory stimuli (6, 13, 29, 45). 

 Some information may also be derived from 

 clinical studies of man in which variation 

 of the auditory structures is produced by 

 disease or other natural causes rather than 

 by the experimenter (12, 20). 



Results of Relevant Research and 



Suggestions for Future 



Investigations 



Experiments Using Noise Stimuli 



Auditory theory has been founded, in 

 large part, upon experiments in which 

 pure-tone stimuli were used. Field investi- 

 gations, such as those in underwater acous- 

 tics, indicate the inadequacy of present 

 theory in dealing with problems relating to 

 complex sounds. An extension of basic re- 

 search to include a detailed analysis of how 

 the auditory system functions when acti- 

 vated by stimuli of complex frequency spec- 

 tra is, therefore, suggested. 



In laboratories which have done research 

 on auditory problems related to sonar, noise 

 stimuli used in experiments more often than 

 not have been designed to simulate as ex- 

 actly as possible the actual noises which oc- 

 cur during operations at sea. For example, 

 careful film or disc reproductions have been 

 made of the cavitation noise produced by a 

 ship's screws and of the ambient water 

 noise. For basic studies, a constant and 

 controllable noise source is required. There- 

 fore, thermal (or "white") noise in which 

 all frequencies are present and the spectrum 

 of sound is continuous is to be preferred. 



Determination of Psycho-physical Relation- 

 ships 



Using white noise stimuli, psycho-physical 

 experiments which have been performed with 

 pure tones can be paralleled, and changes in 

 the psychological phenomena of loudness 

 and pitch as a function of such variables as 

 band width, intensity, and region of the fre- 

 quency spectrum can be determined. Meas- 



