176 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 32 



(a) In translating vague aural judgments and comparisons into 

 facts and figures ; 



(5) In elucidating the causes and characteristics of noises; 



(c) In comparing the results of different investigators; and 



(d) In setting up such arbitrary standards of noise as may be 

 desired in the light of social, technical, or legal requirements. 



The practical measurement of noise usually comprises one or more 

 of the following operations : 



(1) The physical measurement of the " over-all " power or energy 

 content of the noise, the result being ultimately expressible in abso- 

 lute units (e. g., dynes or microwatts per square centimeter). 



(2) The physical analysis of the noise into its spectrum of fre- 

 quency components (cycles per second). This is often most illumi- 



Phijsical Mccisurcmcnl' of Noise . 



Noise Yo he _ 

 measured 



Condenser 

 Microplioiie 



Valve 

 Ai)ipli{ii 



I rrcqucncij j 

 D Welcjhicr ; 

 j (i{ desired )', 



Rech{, 



o,.d .0 

 Noise Oefccfor 



I '-| CaPhotle-rdu ] 

 "I oscilioarupli J 



Figure 5 



nating in tracking down the sources of individual components, par- 

 ticularly of machinery noises. 



(3) The physical determination of the wave form of the noise, 

 though this is often difficult to interpret and to ultilize quantitatively, 

 particularly if the noise is aperiodic. 



(4) The aural measurement in some accepted unit of the loudness 

 of the noise, or in other words, the valuation of the " noisiness " as 

 perceived by the ear — the physiological arbiter of noise. 



PHYSICAL MEASUREMENT OF NOISE 



We have already discussed the measurement of sound energy by 

 means of the condenser microphone and amplifier. A schematic lay- 

 out is shown in Figure 5. The amplified current is connected either 

 to a rectifier and microammeter (graduated in decibels, if desired) 

 for measurement purposes, or alternatively to a cathode-ray oscil- 

 lograph if it is desired to examine the wave form. In view of the 

 fact that the microammeter readings are measures of physical in- 

 tensity and not of loudness (owing to the selective sensitivity of the 

 ear to pitch), a frequency- weighting network is sometimes inter- 



