204 



AUDITORY SIGNALS 



Radio Range Signals 



Radio range signals provide aircraft pilots 

 with directional flight information. The 

 signals are produced by two directional radio 

 beams with their axes crossed at right angles. 

 The indicated direction for the pilot then 

 lies along the path where the field strengths 

 of the two beams are equal. One of the 

 two beams transmits the letter A in tele- 

 graphic code, and the other the letter N. 

 These two signals are interlocked in such a 



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JNTENSITY IN OB RE I MILLIVOLT 



Fig. 1. The effect of intensity and signal-to 

 noise ratio on the discrimination between A and 

 N signals in the radio range . (After Flynn elal.,h) 



way that the pilot hears a continuous tone 

 if the two signals have equal intensity, but 

 an A or an N if he flies too far to one side or 

 the other. 



From the point of view of the psycho- 

 logical problem involved, the radio range 

 is a very simple type of auditory signal. 

 It provides directional information of only 

 one kind, and the psychological problem 

 is one of simple discrimination. If the pilot 

 can make the auditory discrimination, he 

 can fly in the correct path; if he cannot make 

 the discrimination, his flight will be incorrect 

 or erratic. 



A series of investigations was reported 

 by Flynn et al. (5) shortly after the war. 

 These experiments were concerned with the 

 factors which affect the pilot's ability to 



discriminate the radio range signals. We 

 shall discuss some of this research as an 

 illustration of the kinds of problems which 

 must be met. 



The Effect of Intensity 



One experiment was designed to determine 

 the effect of signal intensity on the discrimin- 

 ation, when the discrimination is made with 

 a fairly high noise background. Fig. 1 

 shows some of the results obtained. Each 

 curve is for a different signal-to-noise ratio, 

 expressed in decibels. It can be seen that 

 there is an optimal intensity for each signal- 

 to-noise ratio, but that the S/N ratio itself 

 has much more effect on the discrimination 

 than overall intensity. 



This kind of result is not unlike that found 

 with many more academic experiments on 

 the intensity discrimination of tones. When 

 research is specifically related to the use or 

 operation of some instrument, however, the 

 results frequently seem more practical. In 

 this case, for example, the fact that there is 

 an optimal intensity for each signal-to-noise 

 ratio provides immediately useful informa- 

 tion about the operation of radio receivers. 



Frequency Characteristics of the System 



Band-pass Filters. Another experiment 

 was concerned with the effect of noise filters 

 on the discrimination of the radio range sig- 

 nals. Here discrimination was compared for 

 the normal case (no filtering) with the use 

 of a narrow-band-pass filter — a filter which 

 rejected all noise frequencies above and 

 below a narrow band around the audio fre- 

 quency of the signal. Fig. 2 shows that 

 such filtering improves discrimination be- 

 tween the A and the N signals considerably. 

 This experiment again was done with a prac- 

 tical point in mind, but the results and the 

 experiment itself have general applicabihty. 



Type of Earphones. Still another experi- 

 ment was performed to test the practical 

 conclusion derived from the experiment just 

 cited. Some earphones have an essentially 

 flat response over a wide range of audio 



