212 



AUDITORY SIGNALS 



An interesting thing about threshold changes 

 was that the threshold was lower for faster 

 repetition rates even at rates so slow that 

 no real integration could be taking place. 

 At faster rates, of course, it is reasonable to 

 expect that there would be some increase 

 in effective energy as more and more tones 

 are added in a brief period of time. 



Auditory Localization 



One area of auditory research that has 

 received considerable attention throughout 

 the years has been the problem of auditory 

 localization. Still, much of the large litera- 

 ture has proved almost useless for predicting 



REPETITION RATE (TONES/SEC ) 



Fig. 5. The effect of repetition rate of short tones 

 on loudness 

 The ordinate values indicate the intensity of 

 the repeated tones necessary for them to have a 

 loudness equal to that of a steady tone with an 

 intensity level of 80 db. These curves are equal 

 loudness contours. The duration of the repeated 

 tones is shown for each curve. (After Garner, 12) 



how well any auditory signaling system will 

 work. The main reason for this is that most 

 of the research has been concerned primarily 

 with understanding how we localize sound 

 sources in real life, rather than with the 

 problem of simulating an apparently located 

 sound source. 



Localization of Sound Sources 



Stevens and Ne\vman (32) have provided 

 data on our ability to locate a source of a 

 pure tone in open air. The error of the 

 localization is shown in Fig. 6. Localization 

 is poorest in the area of 2000 to 4000 cps, 

 but is much better at the lower and higher 



frequencies. These authors postulate that 

 this effect is due to the fact that interaural 

 intensity cues are best at the higher fre- 

 quencies, where the shading effects of the 

 head are greatest; and that interaural phase 

 cues are best at low frequencies, where the 

 wave length of a tone is long enough for 

 phase differences to have some meaning. 



The literature has shown without question 

 that both intensity differences and phase 

 differences between the ears can serve as 

 cues for the localization of a sound source. 

 In addition, of course, differences in time of 





FREQUENCY IN CPS 



Fig. 6. Localization errors for pure tones in open 

 space 

 The ordinate shows the average error made in 

 determining the direction of a pure tone source at 

 different frequencies. (After Stevens and New- 

 man, 32) 



arrival of tones can serve as cues for non- 

 continuous tones. 



Simulated Localization 



When the problem of auditory signaling is 

 mentioned, usually the first thing that comes 

 to mind is the use of some indication of 

 lateral displacement of a sound source. If 

 time, intensity, and phase differences be- 

 tween the two ears are the cues we use in 

 localizing a sound source, then it should be 

 simple to produce an apparent displacement 

 of a sound source by stimulating the two 

 ears differently in one of these respects. 

 Unfortunately, it is not as simple as that. 

 The simulated localization is never as effec- 

 tive as real localization, at least when only 

 one type of stimulus cue is used. 



Intensify Differences. Both Garner (9) 



