BASIC PSYCHOLOGICAL PROBLEMS 



209 



serves as his reference or base line. What 

 are some of the factors that affect the dis- 

 crimination between tones? 



Intensity Discrimination 



The classic research on the discrimination 

 between intensities of tones was done by 

 Riesz (27) in 1928. His experiments ex- 

 plored the effects of both intensity and fre- 

 quency on intensity discrimination over 

 fairly wide ranges. His work was compre- 

 hensive, and yet does not provide enough 

 information. He used a technique of beats, 

 in which the observer's task was to detect 

 the presence of beats between two tones. It 

 is quite likely that another method would 

 not yield the same functions as he obtained. 



Futhermore, many things besides intensity 

 and frequency affect intensity discrimina- 

 tion. Montgomery (24) listed quite a few 

 of the factors which affect the discrimination, 

 although it is unlikely that one list would 

 cover all possible effects. Garner and Miller 

 (14) showed that the duration of a compari- 

 son tone affects intensity discrimination, but 

 here again the technique was so limited that 

 the complete story is not available. 



Later experiments by Garner (13) showed 

 that the effect of duration on intensity dis- 

 crimination depends on how the tones are 

 presented. If a short comparison tone is 

 contrasted with a longer standard or refer- 

 ence tone, discrimination is much poorer 

 than if both standard and comparison tones 

 have a short duration. (See Fig. 3.) Un- 

 fortunately, the story does not quite end 

 there, because in this same experiment it was 

 shown that this statement is true only when 

 the silent interval between the standard and 

 the comparison tones was on the order of 

 half a second. With very short silent pe- 

 riods, discrimination was as good with one 

 method of presentation as with the other. 



The problem of duration, incidentally, is 

 one of the more important ones needing 

 research for auditory signaling applications. 

 In auditory signaling, the total amount of 

 information presented in a given period of 



time depends directly on the speed with 

 which any single item of information can be 

 given. Thus it is necessary to present in- 

 formation as rapidly as possible, although it 

 cannot be presented so fast that the discrim- 

 ination process breaks down. We need to 

 know, then, what the limits of discrimination 

 are for short tones. 



Other factors which undoubtedly have an 

 effect on intensity discrimination, but about 

 which we know very little, are the repetition 

 rate of tones (how many discriminations per 

 unit time), the transition between tones 



TONE DURATION IN MILLISECONDS 



Fig. 3. The effect of tone duration on intensity 

 discrimination 

 For the upper curve, the standard (first) tone 

 was always 500 milliseconds, and the comparison 

 tone had the duration indicated on the abscissa. 

 For the lower curve, both tones had the duration 

 indicated on the abscissa. With shorter silent 

 intervals between tones, both curves resemble the 

 upper one shown here. (After Garner, 13) 



(whether abrupt or gradual), and knowledge 

 about when tones are going to occur. 



Frequency Discrimination 



In 1931 Shower and Biddulph (28) re- 

 ported an extensive series of experiments on 

 frequency discrimination. Their results 

 show the effect of both intensity and fre- 

 quency on frequency discrimination. They 

 used a warble technique in which the tone 

 was subjected to a small amount of fre- 

 quency modulation. The observer's task 

 was to detect the modulation. Here again, 

 functions obtained with other procedures are 



