LOUDNESS 379 



Definitions 



The subject matter which follows necessitates the use of a number of 

 terms which have often been applied in very inexact ways in the past. 

 Because of the increase in interest and activity in this field, it became 

 desirable to obtain a general agreement concerning the meaning of the 

 terms which are most frequently used. The following definitions are 

 taken from recent proposals of the sectional committee on Acoustical 

 Measurements and Terminology of the American Standards Associ- 

 ation and the terms have been used with these meanings throughout 



the paper. 



Sou fid Intensity 



The sound intensity of a sound field in a specified direction at a 

 point is the sound energy transmitted per unit of time in the specified 

 direction through a unit area normal to this direction at the point. 



In the case of a plane or spherical free progressive wave having the 

 effective sound pressure P (bars), the velocity of propagation c (cm. 

 per sec.) in a medium of density p (grams per cubic cm.), the intensity 

 in the direction of propagation is given by 



/ = P'^jpc (ergs per sec. per sq. cm.). (1) 



This same relation can often be used in practice with sufficient accuracy 

 to calculate the intensity at a point near the source with only a pressure 

 measurement. In more complicated sound fields the results given by 

 this relation may differ greatly from the actual intensity. 



When dealing with a plane or a spherical progressive wave it will 

 be understood that the intensity is taken in the direction of propagation 



of the wave. 



Reference Intensity 



The reference intensity for intensity level comparisons shall be IQ-^^ 

 watts per square centimeter. In a plane or spherical progressive 

 sound wave in air, this intensity corresponds to a root-mean-square 

 pressure p given by the formula 



p = 0.000207[(///76)(273/r)^]^ (2) 



where p is expressed in bars, // is the height of the barometer in 

 centimeters, and T is the absolute temperature. At a temperature of 

 20° C. and a pressure of 76 cm. of Hg, p = 0.000204 bar. 



Intensity Level 

 The intensity level of a sound is the number of db above the reference 

 intensity. 



