RESULTS OF RELEVANT RESEARCH 



225 



Modulation 



Problems encountered in studying the 

 detection of echoes against a background of 

 reverberations (70) suggest the need for 

 experiments to determine the effects of am- 

 phtude, frequency, and phase modulation of 

 pure tones upon the physiological processes 

 and psychological phenomena of hearing. 



The problem of modulation in relation 

 to hearing has been discussed by Stevens 

 and Davis (52) and a number of experiments 

 cited (42, 48, 54, 61, 64, 66). These must 

 be considered as preliminary only in view of 

 the many variables yet to be explored. In- 

 vestigations should be extended to the meas- 

 urement of effects of modulation of complex 

 sounds as well as of pure tones. 



Hearing of Short Pulses of Sound 



In certain types of modern echo-ranging 

 gear the use of extremely short pulses of 

 sound and the consequent short echoes re- 

 turned by targets calls attention to a num- 

 ber of auditory discrimination problems 

 which have not been intensively investi- 

 gated. 



Stevens and Davis (52, pp. 100-106 and 

 pp. 154-159) have reviewed experiments 

 (prior to 1938) in which short impulses of 

 sound were used in studying auditory func- 

 tion. In their review are included psycho- 

 physical studies on pitch as a function of 

 duration (8, 53), the effect of duration upon 

 the difference limen (DL) for frequency dis- 

 crimination (3), the threshold of successive- 

 ness — i.e., the minimum time of separation 

 of two sounds in order that they appear as 

 two rather than as one (9, 55) loudness as 

 a function of duration (2, 37, 51), loudness 

 as a function of the form of the pressure 

 wave of the sound impulse (10, 51), and 

 loudness as a fimction of the frequency of 

 repeated impulses (51). 



In a recent experiment TumbuU (58) has 

 examined the effect of stimulus duration 

 upon pitch discrimination for pure tone sti- 

 muli of 128, 1024, and 8192 cps. He has 

 found that decreasing the duration of a tone 



to be compared with a standard tone of fixed 

 duration has slight effect upon the relative 



difference limen ( -^ ) until the length of the 



comparison tone has been reduced to 0.1 to 

 0.5 seconds, depending upon the frequency. 

 At this point the difference limens begin to 

 increase markedly. In the region of .01 to 

 .03 seconds, accuracy reaches a virtual zero. 



Turnbull also found that for a given dura- 

 tion of the comparison tone, decrease in 

 intensity reduced the accuracy of pitch 

 discrimination, and that this reduction was 

 greater for tones of relatively short dura- 

 tion. 



Garner and Miller (19) have investigated 

 the differential sensitivity to intensity as a 

 function of the duration of the comparison 

 tone. For tones of 500 and 1000 cps, the 

 former at sensation levels of both 40 and 70 

 db and the latter at 40 db only, they found 



that the relative difference limen 



(tO 



was 



approximately constant for durations above 

 3(X) milliseconds. Below 300 milliseconds 

 the reduction in accuracy of discrimination 

 was much less rapid for the 500 cycle tone at 

 a sensation level of 70 db than at 40 db. 

 However, additional data should be obtained 

 before any broad generalization about the 

 effects of intensity level is made. 



To further our knowledge of auditory 

 function, numerous psychophysical studies 

 in which duration of stimulus is a variable 

 need to be performed. The results of 

 Turnbull and of Gamer and Miller should 

 be extended to include tests with a wider 

 range of frequencies and intensities. Func- 

 tions paralleling those for tonal stimuli 

 should be determined for noise stimuli. It 

 would also be of interest to compare the 

 loudness-duration functions for different in- 

 tensity levels, intensity being measured as 

 rms, peak and average pressures. 



No attempt has been made to measure 

 systematically the effects of short impulses, 

 tonal or non-tonal, upon physiological mech- 

 anisms of the inner ear or auditory nervous 



