224 



AUDITION IN SONAR OPERATION 



masking to the effective level of the masking 

 noise. 2 



Related studies have been reported by Harris 

 (24) and Flynn, Truscott, and Newman 

 (14). Detectability as a function of signal 

 duration has been investigated by the Sonar 

 Data Division of the University of California 

 Division of War Research, U. S. Navy Elec- 

 tronics Laboratory (70) in a series of experi- 

 ments in which the signal was a short tonal 

 pulse and the masking background was 

 thermal noise. 



Further psycho-physical investigations of 

 this sort are indicated. In addition, physio- 

 physical and psycho-physiological experi- 

 ments are required. As an example of the 

 kind of information that may be obtained 

 from such experiments, an experiment by 

 Galambos and Davis (17) may be cited. 

 These investigators found that the spon- 

 taneous discharge, which may be recorded 

 from a single nerve fibre of the auditory- 

 nerve,^ can be stopped by certain tones and 

 noises. Furthermore, the discharge elicited 

 by certain tones can be reduced or abolished 

 by the simultaneous presentation of another 

 tone or noise. This neural "inhibition" 

 may play an important role in masking and 

 its discovery must lead us to question the 

 adequacy of the commonly accepted theory 

 that masking occurs because the masking 

 tone or noise activates the region of the 

 cochlea and the nerve fibers ordinarily ac- 

 tivated by the masked tone. It appears that 

 masking is a more complex phenomenon 

 physiologically than has usually been as- 

 sumed, and that extensive experimentation 

 using the most refined recording techniques 

 must be done in order that we may under- 

 stand the interaction which apparently 



2 Quoted from author's summary. 



^ In a recent note in Science (18) , Galambos and 

 Davis present evidence which indicates that the 

 potentials recorded in these experiments were 

 from the cell bodies of second-order neurons rather 

 than from primary fibers in the auditory nerve. 

 This new finding does not affect the discussion 

 presented here. 



occurs peripherally in the auditory nervous 

 system and, undoubtedly, in the higher 

 auditory centers as well. 



On the psycho-physiological level further 

 data as to the analytical function of the 

 auditory system can be obtained by experi- 

 ments in which the masking effects of bands 

 of noise on pure tones is determined in 

 animals experimentally deprived of part of 

 the sensory or neural elements of hearing. 

 For example, will a band of frequencies, 

 whose individual components are not heard 

 because of experimental damage to the 

 cochlear hair cells or auditory nerve, mask 

 the hearing of frequencies unaffected by the 

 lesion? Studies which have been done 

 using pure tone stimuli (32, 39, 63) may be 

 paralleled with the added variable of a 

 masking noise background. 



Masking of Noise Signals by Noise Back- 

 ground 



Experiments similar to those which have 

 been described or suggested under the head- 

 ing of "masking of pure tones by noise" 

 should also be done using noise signals in- 

 stead of pure tones. The degree of masking 

 as dependent upon such variables as band 

 width and modulation may be investigated. 

 Only a few studies of this nature have as yet 

 been undertaken. Experiments on the 

 masking of a noise signal, simulating the 

 underwater sounds produced by a ship's 

 screws, have been reported by the University 

 of California Division of War Research (68) 

 by Harris (22), and by Snow and Neff (50). 

 Miller (38) has suggested that the results ob- 

 tained in his experiments on sensitivity to 

 changes in white noise may be regarded as 

 measures of the masking of noise by noise. 

 He points out that the "functions which 

 describe intensity discrimination also de- 

 scribe the masking by white noise of pure 

 tones and of speech," and that the deter- 

 mination of differential sensitivity to in- 

 tensity may be considered as a special case 

 of the more general masking experiment. 



