Abstracts of Technical Articles from Bell System Sources 



Exploration of Pressure Field Around the Human Head During 

 Speech} H. K. Dunn and D. W. Farnsworth. A sin,8:le speaker in 

 a seated position repeated a fifteen-second sample of connected speech, 

 while r.m.s. pressure measurements were made in thirteen frequency 

 bands, and at seventy-six positions, in different directions and dis- 

 tances. The results are applicable to intelligibility and microphone 

 placement problems. They show, in general, the greater variation 

 with direction at higher frequencies. Directivity due to the size of 

 the mouth opening appeared to enter above 5600 cycles per second, 

 the a.xis at these frequencies being about 45° below the horizontal, 

 in front. 



Frequencies below 1000 cycles per second were found strongest 

 directly downward from the lips, or nearly so. The power radiated in 

 different directions has been calculated, and a summation gives a 

 spectrum of the total speech power emitted by the mouth. It is 

 proposed that similar spectra for other speakers may be obtained from 

 pressure measurements at a single point, using the relations discovered 

 for this speaker. The necessity for protecting a microphone used 

 close to the mouth, from the puffs of air accompanying the speech, 

 is demonstrated and explained. 



A Tubular Directional Microphone.- W. P. Mason and R. N. 

 Marshall. A tubular directional microphone is described which 

 consists of a pressure type microphone coupled to an acoustic im- 

 pedance element composed of a large number of tubes whose lengths 

 vary by equal increments. The function of this variation in length is 

 two-fold. Plrst, the multiple resonances of the individual tubes occur 

 at intervals so close together that the net effect of the bundle is that 

 of an acoustic resistance over a fairly wide frequency range and so 

 does not impair the high quality of the attached microphone. Second, 

 high directivity is secured, because for sound incidence other than 

 normal each tube introduces a different path length with phase 

 cancellation resulting in a composition chamber between the micro- 

 phone and the ends of the tubes. The theory of operation is summar- 



1 Jour. Acous. Soc. Amer., January 1939. 



2 Jour. Acous. Soc. Amer., January 1939. 



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