REFERENCES 



227 



design of equipment, and (2) by use of tech- 

 niques which lower the discriminatory 

 thresholds. The first of these has been a 

 matter of major consideration on the part of 

 the physicist and engineer. The second 

 has received little direct attention, although 

 a number of studies, aimed as a rule towards 

 the solution of other problems, have pro- 

 duced results which deserve further con- 

 sideration. 



In a recent experiment, for example, Shaw, 

 Newman and Hirsh (49) have found that: 



The absolute threshold for pure tones is 

 lower when both ears are stimulated than when 

 either ear is stimulated alone. The normal 

 summation at threshold is a function of the 

 relative sensitivity of the two ears. For a 

 group of listeners with substantially normal 

 hearing in both ears, the binaural threshold is 

 from one to two decibels lower than the best 

 monaural threshold. 



Similarly, it has been found that smaller 

 differences in frequency and in intensity can 

 be detected when the stimuli are presented 

 binaurally rather than monaurally (52, pp. 

 87 and 141-142). These results are in part 

 supported by evidence from physiological 

 experiments which indicates that the abso- 

 lute threshold for intensity is raised by the 

 destruction of one cochlea (7) . 



Several investigators (15, 25, 11) have 

 reported improvement in auditory dis- 

 crimination resulting from coincident visual 

 stimulation (see 47 for a review of these ex- 

 periments.) This phenomenon of inter- 

 sensory facilitation is not well substantiated 

 and is open to various interpretations. The 

 possibility remains, nevertheless, that some 

 real gains may be made by combined visual 

 and auditory presentation of near liminal 

 signals. 



It has been a common observation on the 

 part of researchers in audition that the 

 ability to detect minimal intensities, to 

 discriminate changes in frequency and in- 

 tensity, and to recognize particular patterns 

 of sound increases with practice. Studies 

 such as that of Wyatt (65) on improvability 



of pitch discrimination are important addi- 

 tions to knowledge derived from more casual 

 observations. Many additional investiga- 

 tions must be conducted before we shall 

 know with any degree of certainty the limits 

 of improvement, individual differences in 

 improvability, the degree of transfer of 

 training from one particular type of dis- 

 crimination to another, the most effective 

 training procedures, and the factors which 

 account for improvement. 



Significant improvement in auditory dis- 

 crimmation may sometimes be brought 

 about by new and unexpected discoveries. 

 Potentially promising is the recent observa- 

 tion that interaural phase relations affect 

 the detectability of a tone heard against a 

 background of noise (27, 28, 33, 36). 



When a tone is presented to both ears 

 against a background of noise, the tone is 

 more easily heard if it is in phase at the two 

 ears while the noise is out of phase (or vice 

 versa) than when both the tone and the noise 

 are either in phase or out of phase. For 

 example, when a tone of 200 cps is led to the 

 two ears along with white noise, the de- 

 tectability of the tone is improved by as 

 much as 13 db merely by reversing the 

 interaural phase of the tone and leaving the 

 noise alone. 



It is even possible to improve the de- 

 tectability of a tone simply by reversing the 

 connections to one earphone, thereby making 

 the interaural phase of both the tone and 

 the noise 180°. 



These laboratory findings may or may 

 not prove applicable to sonar listening. 

 The important point is that through the 

 prosecution of vigorous research in university 

 laboratories new and unsuspected facts and 

 principles will continue to emerge in a steady 

 stream. Some of these discoveries will have 

 a revolutionary impact on the mihtary art. 



References 



1. Ades, H. W., & Feldeb, R. The acoustic 

 area of the monkey (Macaca mulatta). 

 /. NeurophysioL, 1942, 5, 49-54. 



