250 



MEASUREMENTS 



m = mass of the disk, in grams, 

 a = radius of the disk, in centimeters, and 

 7 = damping factor, the ratio of two successive swings. 

 From equations 11.4, 11.5 and 11.6 it is possible to determine the 

 particle velocity u in the sound wave. 



The arrangement of a Rayleigh disk for field calibrations of microphones 

 is shown in Fig. 11.2. The source of sound is a small direct radiator loud 

 speaker, with the back completely enclosed, placed halfway between the 

 disk and the microphone. See Sec. 11.5. A small loud speaker is used 

 so that a spherical wave will be emitted. If a velocity microphone is used 

 no correction need be made for the spherical wave because the Rayleigh 

 disk also measures the particle velocity. If a pressure microphone is used 



MICROPHONE 



Fig. 11.2. Arrangement of apparatus for obtaining the field response of a microphone by 



means of a Rayleigh disk. 



the appropriate correction for the accentuation in velocity in a spherical 

 wave must be made. See Sec. 1.5D and Fig. 1.3. From the geometry of 

 the system of Fig. 11.2 the deflection of the disk can be determined from 

 the deflection of the light beam on the scale. 



A high quality microphone calibrated by any of the above systems may 

 be used as a secondary standard for the calibrations of other microphones. 



B. Directional Characteristic. ■ — The directional characteristic of a 

 microphone is an expression of the variation of the behavior of the micro- 

 phone with respect to direction. A polar diagram showing the output 

 variation of the microphone with direction is usually employed. 



The directional characteristics should be obtained at representative 

 frequencies. In order to obviate any errors due to reflections the direc- 

 tional measurements should be made outdoors. Obviously, very slight 

 reflections will introduce considerable error for the angles in which the 

 response is very low. 



A cathode ray tube with a long persistence screen may be used to obtain 



