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AUDITORY SIGNALS 



a fully developed system of auditory signals. 

 During the late war more extensive research 

 was undertaken, and the results are pre- 

 sented in a report by Forbes et al. (7). A 

 shorter report of certain of the studies was 

 later made by Forbes (6). The account 

 given here concerns these latter experiments. 



The Problem 



The chief reason for wanting auditory 

 signals in flying is to relieve the load on the 

 pilot's eyes. Present-day aircraft have a 

 multitude of dials and instruments which 

 the pilot must watch all the time, and it was 

 felt that it would be a great advantage if 

 some of this load could be shifted to his ears. 



A pilot can keep his plane flying along a 

 predetermined path if he has three kinds 

 of information — information about the three 

 dimensions in which the plane is flying. He 

 needs information about the turn of the 

 plane, its bank, and its tilt. In actual prac- 

 tice, airspeed is equivalent to the dive-climb 

 (tilt) indication, since airspeed increases if 

 the nose goes down, and decreases if the 

 nose goes up. The problem, then, was to 

 develop auditory signals which would trans- 

 mit information about the three basic indi- 

 cations. In the actual signals, airspeed was 

 almost always used instead of a dive-climb 

 indication. 



An auditory signaling system of this kind 

 is extremely complex compared to the radio 

 range. Here information of a quantitative 

 nature must be provided about three differ- 

 ent kinds of action simultaneously. To pro- 

 vide simple yes-no or directional information 

 in this situation would not be nearly precise 

 enough to enable the pilot to fly with cer- 

 tainty. 



Experimental Developments 



Preliminary Experiments. Before any 

 full-scale signaling systems were set up, pre- 

 liminary experiments were undertaken to 

 explore various possibilities for signals. 

 These showed that intensity differences be- 

 tween the .two ears gave a very inexact 



indication of direction, and that phase differ- 

 ences were likewise poor. More will be said 

 of these problems later. As a result of the 

 preliminary tests, however, several complete 

 systems were later tried in an airplane pur- 

 suitmeter task. 



Three-Tone Signals. One set of signals 

 made use of three different tones in each 

 ear, plus an additional tone to indicate bank. 

 One of the three tones had a low frequency, 

 and another had a high frequency. The 

 third tone varied in frequency between the 

 other two, and the pitch of this middle tone 

 indicated airspeed. When the middle tone 

 approached either the higher or the lower 

 frequency, beats could be heard, which set 

 airspeed tolerance limits. To indicate a 

 turn, the tones were interrupted in the ear 

 toward which the turn was occurring, and 

 the faster the rate of interruption, the faster 

 the rate of turn. Bank information was 

 produced by the insertion of a low-frequency 

 tone on the side of the low wing. Three 

 different frequencies indicated three degrees 

 of bank. 



These signals were not very successful, 

 primarily because the subjects in the experi- 

 ment tended to listen to one or another of 

 the three indications to the exclusion of the 

 other two signals. As a result, one indica- 

 would frequently go far out of bounds, while 

 one or two of the others would remain well 

 centered. 



Sweeping-Tone Signals. Other combina- 

 tions of signals were also tried out, but the 

 one which was most successful combined 

 all three indications into a single tone, which 

 was made to increase in intensity in one ear 

 while it decreased in the other ear. An 

 illusion of movement was thus produced 

 to indicate turn. The direction of move- 

 ment indicated the direction of turn, and the 

 rate at which the movement was repeated 

 indicated the amount of turn. 



While the tone was changing in intensity, 

 it was also changing in frequency. The 

 bank of the plane was indicated by the rela- 

 tive frequency of the tone as it appeared to 



