226 



AUDITION IN SONAR OPERATION 



system. A few observations have been 

 made incidentally in the course of experi- 

 ments aimed at the solution of other prob- 

 lems (52, pp. 327-332 and pp. 386-111). 

 Some very suggestive results have more 

 recently been reported by Galambos and 

 Davis in their studies of the activity of 

 single fibers of the auditory nerve (16, 17).'* 

 They have noted that "equilibration," the 

 decrease in voltage output of the auditory 

 nerve following onset of sound stimulation, 

 occurs in the single fiber preparation and is 

 essentially complete within a few tenths of 

 a second (16). They have also made ob- 

 servations on the "inhibitory" or "masking" 

 effect of a noise which consisted of a series 

 of sharp clicks (17). Brief atonal and tonal 

 impulses have also been used in investigating 

 the response of the higher auditory centers, 

 especially the auditory cortex (1, 5, 35, 54, 

 57). Further experiments with controlled 

 variation of duration and wave form of im- 

 pulses should contribute to our under- 

 standing of the function of the auditory 

 nervous system. Such experiments should 

 include not only the recording of electrical 

 activity of the nervous pathways and cen- 

 ters but also the measurement by behavioral 

 methods of discriminatory ability before and 

 after ablation of parts of the system. 



Auditory Fatigue 



By auditory fatigue is meant temporary 

 impairment of hearing sensitivity resulting 

 from stimulation of the ear by sound. ^ 



^ See footnote 3. 



^ Ravvdon-Smith (44) has suggested that the 

 term 'experimental deafness' rather than fatigue 

 be used to describe the temporary hearing losses 

 which he found to occur in human subjects fol- 

 lowing brief exposures to tonal stimuli in the 

 range 100-110 db. above threshold. He also 

 suggests the term 'auditory inhibition' to refer 

 to the part of this deafness which appears to be 

 due to a central nervous system phenomenon. 

 Wever (62), in a review of experiments on electrical 

 response of the cochlea and auditory nervous 

 system, suggests that "in view of the extreme 

 intensities (of sound) required (to produce de- 

 pression in the electrical responses), and the long 



Experiments which have reported fatigue 

 effects have usually used intensities of sound 

 some 1(X) db re 10"^^ watts/sq. cm. (31, 34, 

 40, 41, 43, 44, 46, 50, 67, 69). The sonar 

 operator, during his ordinary duties, is 

 probably seldom subjected to sounds of this 

 intensity, and auditory fatigue may not be 

 a factor of great importance in determining 

 his proficiency. However, in view of such 

 experiments as those of Rawdon-Smith (43, 

 44), in which he found threshold shifts as 

 high as 56 db resulting from stimulation by 

 pure tones at 100-110 db above threshold 

 and changes in differential sensitivity for 

 frequency following stimulation by pure 

 tones 70-100 db above threshold, it appears 

 within the realm of possibility that repeated 

 exposure of the ear to fairly loud sounds such 

 as encountered by the sonar operator in 

 echo-ranging may affect the sensitivity to 

 minimal intensity or differential sensitivity 

 to frequency and intensity. Further in- 

 formation on the 'fatigue' or 'inhibitory* 

 effects of pure tones and noises, including 

 intensities in the range 60-100 db, needs to 

 be acquired. Brief impulses as well as 

 steady sounds should be used and the effect 

 of repeated exposures without opportunity 

 for recovery from any losses which appear 

 should be examined.^ 



Possible Techniques of Improving Auditory 

 Discrimination 



The ability of the sonar operator to detect 

 the signal representing a target can be im- 

 proved in two waj^s: (1) by advance in the 



period taken for recovery, it would seem more 

 appropriate to regard the effect as one of injury 

 rather than as fatigue in the usual sense," and 

 that "it is probable that stimulation deafness is a 

 later and irreversible stage of the same injury 

 process." 



^ See Davis, Morgan, et al. (67). These in- 

 vestigators found no cumulative effects when ears 

 of human subjects were repeatedly exposed to 

 intense tones and noise 110-130 db re 10"^^ watts/- 

 cm^. However, subjects were not re-exposed to 

 the intense stimuli until recovery (as measured by 

 the audiogram) from the previous exposure was 

 complete. 



