MEASUREMENT OF NOISE 



273 



The phase angle d in radians is given by 



2x^ 



d = 



X 



11.16 



where d = distance between the first position of the pointer without the 

 branch to the second position with the branch in place, in 

 centimeters. The direction towards the loud speaker units 

 is positive and 

 X = wavelength of sound in air, in centimeters. 

 11.7. Measurement of Noise. — Due to the complexity of the human 

 hearing mechanism and to the various types of sounds and noises it is 

 impossible, at the present time, to build a noise meter which will show the 

 true loudness level. The discrepancies can be determined by actual use 

 and suitable weighting factors applied to the results. Objective mea- 

 surements are almost indispensable in any scientific investigation. The 

 noise meter or sound level meter provides a system for measuring the 

 sound level of a sound. 



A schematic diagram of a sound level or noise meter is shown in 

 Fig. 11.17. The microphone should be calibrated in terms of a free wave. 



^ 



ATTENUATOR 



40 DB 

 70 DB 

 SWITCH 



MICROPHONE 



SQUARE o-/^"^ 



LAW (meter) 





Fig. 11.17. Schematic arrangement of the components of a noise meter. 



The directional characteristics of the microphone should be independent 

 of the frequency. The attenuator and meter should be calibrated in 

 decibels. A sound meter reading 60 db means a sound level of 60 db above 

 the reference level. The reference point of the decibel scale incorporated 

 in a sound meter shall be the reference sound intensity at 1000 cycles in a 

 free progressive wave, namely: 10~^^ watts per square centimeter. The 

 response frequency characteristic of the human ear shows less sensitivity 

 for frequencies above and below 3000 cycles, Fig. 13.1. The overall 

 frequency ^^ response of an ideal noise meter should be the reciprocal of 

 the ear response frequency characteristics. This would make the noise 

 meter unduly complicated. The response frequency characteristics rec- 

 ommended for the noise meters by the American Standards Association are 



^^ Amer. Tent. Standards for Sound Level Meters Z 24.3. American Standards 

 Association, New York City, 1936, or Jour. Acous. Soc. Amer., Vol. 8, No. 2, 1936. 



