PHYSICAL CHARACTERISTICS OF SPEECH AND MUSIC 



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8000 c.p.s. and 90 c.p.s. Similarly, if the distance is increased to 

 100 meters, the limits will be found to be + 1.85 and — 1.55 octaves. 

 These relations are true only when no other sounds are present. 

 Similar limits are easily determined when the listener is in the presence 

 of any other sound whose noise audiogram is known. In that case, 

 the ordinates in the audibility curve are reduced by an amount equal 

 to the corresponding ordinate in the noise audiogram. 



These values are such that any half octave by itself within the 

 pitch limits will transmit audible sounds. This does not necessarily 

 imply that, when the undistorted speech is acting upon the ear, such 

 a half octave will transmit sounds whose presence can be detected. 

 To test this point several observers listened to speech reproduced by 

 a high quality loud speaker system which would reproduce all fre- 

 quencies from 40 to 15,000 uniformly and into which filters could be 

 introduced. These filters limited at desired cut-off positions the upper 

 and lower frequencies which were reproduced. 



A large group of observers then listened to this reproduced speech 

 and they were asked to judge which was filtered and which was 

 unfiltered. The results of such tests are shown in Fig. 18. The 





 PITCH 



+2 



Fig. 18 — Audible pitch limits for conversational speech. 



ordinates give the per cent of correct observations and the abscissae 

 the cut-off frequency of the filter. Taking a 60 per cent correct 

 judgment as a criterion for determining the detectable pitch limits, 

 then it will be seen that the lower limit is — 3.5 octaves and the upper 

 limit 3.25 octaves for male speech which agrees with the results taken 

 from the audibility curve established directly from power measure- 

 ments upon speech and the threshold of hearing as described above. 

 For female speech the limits are — 2.9 and -\- 3.4 octaves. Sum- 

 marizing, then, it is seen that the most powerful components carrying 

 conversational speech, which are of any practical importance, are 

 about 4000 or 5000 microwatts while the principal components in 



