A NUN-DIRECTIONAL MICROPHONE 



423 



range where the disturbance caused by the test micro])hone is negli- 

 gible. This was acc()nii)lished by setting the niicr()})hon(' flush into the 

 face of the obstacle when obtaining the disturbed sound pressure. 

 Then by the simple law given, this diffraction effect was applied to the 

 micrt)phone itself and is shown in Fig. 11. The resultant normal 

 incidence field calibration for the small condenser microphone is shown 

 in the same figure. 



500 1000 5000 10,000 20,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 11 — Response and diffraction of miniature condenser microphone. 



References 



1. W. C. Jones and L. W. Giles: "A Moving Coil Microphone for High QuaHty 



Sound Reproduction," Jour. Soc. Motion Picture Engineers, Vol. 17, Decemi)er 

 1931, pp. 977-993. 



2. Rayleigh: "The Theory of Sound," Vol. 2, 2nd ed., Macmillan & Co., Ltd., 1896, 



pp. 272-284. 



3. Stuart Ballantine: "Note on the Effect of Reflection by the Microphone in Sound 



Measurements," Proc. I. R. E., Vol. 16, December 1928, pp. 1639-1644. 



4. H. C. Harrison and P. B. Flanders: "An Efficient Miniature Condenser Micro- 



phone System," Bell Sys. Tech. Jour., Vol. 11, July 1932, pp. 451-461. 



5. L. J. Sivian and H. T. O'Neil: "Sound Diffraction by Rigid Circular Plate, Square 



Plate and SemiTnfinite Screen, Jour. Acous. Soc. Amer., Vol. 3, April 1932, 

 pp. 483-510. 



6. P. B. Flanders: "A Method of Measuring Acoustic Impedance," Bell Sys. Tech. 



Jour., Vol. 11, July 1932, pp. 402-410. 



7. Drysdale: "Mechanical Properties of Fluids," D. Van Nostrand Co., Inc., 1924, 



pp. 113-116. 

 S. I. B. Crandall: "Theory of Vibrating Systems and .Sound," 2nd ed., D. Van 



Nostrand Co., Inc., 1927, p. 55, pp. 143-149. 

 9. Dwight: " Bessel Functions for A. C. Problems," Trcins. A. I. E. E., \'oI. 48, No. 3, 



July 1929. pp. 812-820. 



