CONDENSER AND CARBON MICROPHONES 55 



on the other hand is definite, simple, and capable of being accurately 

 duplicated in different laboratories. In view of this situation it would 

 seem advisable to retain, at least for the present, the thermophone or 

 pressure method of calibration for general use. In cases where precise 

 quantitative measurements are required a field calibration of the 

 microphone should of course be secured under the conditions of actual 

 use. \^arious methods of making such a calibration have been pro- 

 posed. The Rayleigh disc has been used extensively in this work 

 thus far but there are certain very definite limitations to the extent 

 to which it can be applied. An interesting discussion of the use of 

 the Rayleigh disc may be found in papers by E. J. Barnes and \V. 

 \Vest,i« and L. J. Sivian." 



It would seem reasonable to expect that future design work would 

 be directed toward reducing transition, resonance and phase difference 

 effects to a minimum. The results of work along this line have been 

 reported by S. Ballantine ^^ and D. A. Oliver.i^ j^ both instances 

 the mechanical design is such that the resonant cavity in front of the 

 diaphragm is eliminated and the housing is spherical or streamline 

 to reduce the diffraction effect. There has as yet been little oppor- 

 tunity to determine the extent of the practical improvement effected 

 by these changes in design and the whole discussion continues to be 

 somewhat academic in character. 



Carbon Microphoni<: 



Bell's original microphone was essentially a generator and hence 

 was limited in its output to the maximum speech power available at 

 its diaphragm. The demand for telephonic communication over 

 longer distances led to the early introduction of a carbon microphone. 

 In this instrument the resistance of the carbon element is caused to 

 vary in response to the sound pressure on the diaphragm and produces 

 changes in the current supplied from an external source of electrical 

 potential, which are fairly faithful copies of the pressure changes which 

 constitute the sound wave. The carbon microphone is therefore in 

 general an amplifier in which a local source of power is controlled by 

 the acoustic power of the sound wave. 



The carbon element or "button" of the first microphones (Edison, 

 1877) was made from plumbago compressed into cylindrical form. 



^^''The Calibration and Performance of the Rayleigh Disc," E. J. Barnes and 

 W. West, Inst, of Elec. Eng. Journal, 1927, Vol. 65, pp. 871-880. 



""Rayleigh Disc Method for Measuring Sound Intensities," L. J. Sivian, 

 Philosophical Magazine, March 1928, pp. 615-620. 



»2 Contributions from the Radio Frequency Laboratories No. 18, S. Ballantuie, 

 April 15, 1930. „ ^ ^ ^,. 



13 "An Improved Microphone for Sound Pressure Measurements, D. A. Oliver, 

 Journal oj Scientific Instruments, April, pp. 113-119. 



