648 BELL SYSTEM TECHNICAL JOURNAL 



the alternating sound pressure and whose motion is converted (indi- 

 rectly, as a rule) into a visual record. These acoustic voltmeters 

 have gradually evolved from Scott's phonautograph and Koenig's 

 manometric capsules and indicating flames of some 70 years ago to 

 the present day technique. Their extensive use accounts for the 

 fact that sound measurements frequently are expressed in terms of 

 pressure rather than in terms of power. In many cases, when the 

 relation between pressure and velocity is not known or too complex 

 a function of frequency to be manageable, — the description of the 

 sound wave must be confined to pressure values alone. 



The first requisite for absolute measurements of speech pressures 

 is a microphone which admits of an absolute electroacoustic calibration 

 over the range of audio frequencies, and which has a substantially 

 uniform sensitivity over the most important part of that range. 

 The development of the condenser microphone and of the thermophone 

 method for calibrating it, supplied this need. This fundamental 

 contribution to the subject is due to E. C. Wente.^ Using such a 

 calibrated condenser microphone in conjunction with a vacuum tube 

 amplifier and an oscillograph which were uniformly sensitive up to 

 about 6000 p.p.s., I. B. Crandall and C. F. Sacia,'* and I. B. 

 Crandall * obtained oscillograms of the fundamental speech sounds 

 in the English language. Essentially, these oscillograms give a 

 picture of the instantaneous pressures throughout the duration of 

 the sound, impressed on the microphone diaphragm at 2.5 cm. from 

 the speaker's lips. A certain amount of similar work on German 

 speech sounds has been published by Trendelenburg.^ From the 

 standpoint of phonetics these pressure amplitude oscillograms of in- 

 dividual sounds are a most comprehensive source of information. 

 Their chief use has been in determining the frequency spectra of the 

 fundamental speech sounds. Sacia "^ and Sacia and Beck ^ have used 

 them to compute the mean and the peak powers of those sounds. 



From the standpoint of engineering applications, speech power has 

 a twofold interest: (a), there is the question of designing microphones, 

 receivers, circuits, room acoustics, of controlling noise levels, etc., 

 {b), the control of apparatus (chiefly adjustment of amplification) 

 while it is handling speech from persons who acoustically are not 

 controllable. 



^Phys. Rev., July 1917, April 1922, June 1922. 



« Bell Sys. Tech. Jour., April 1924 and Oct. 1925, resp. 



^ Wiss. Veroff. a. d. Siemens-Konzern, 1924 and 1925. 



8 Bell Sys. Tech. Jour., Oct. 1925. 



' Bell Sys. Tech. Jour., July 1926. 



