DECIBELS 15 



in the specified direction through a unit area normal to this direction at 

 the point. The unit is the erg per second per square centimeter. It may 

 also be expressed in watts per square centimeter. 



The intensity, in ergs per second per square centimeter, of a plane wave is 



1 ^ -- ^ pu = pcu^ 1.54 



PC 



where p = pressure, in dynes per square centimeter, 



u = particle velocity, in centimeters per second, 

 c — velocity of propagation, in centimeters per second, and 

 p = density of the medium, in grams per cubic centimeter. 

 The product pc is termed the specific acoustic resistance of the rhedium. 

 The specific acoustic resistance of various mediums is shown in Table 1.1. 

 1.9. Decibels (Bels). — In acoustics the ranges of intensities, pressures, 

 etc., are so large that it is convenient to use a scale of smaller numbers 

 termed decibels. The abbreviation db is used for the term decibel. The 

 bel is the fundamental division of a logarithmic scale for expressing the 

 ratio of two amounts of power, the number of bels denoting such a ratio 

 being the logarithm to the base ten of this ratio. The decibel is one tenth 

 of a bel. For example, with Pi and P^ designating two amounts of power 

 and n the number of decibels denoting their ratio: 



p 



n = 10 logio —, decibels. 1.55 



P% 



When the conditions are such that ratios of currents or ratios of voltages 

 (or the analogous quantities such as pressures, volume currents, forces and 

 particle velocities) are the square roots of the corresponding power ratios, 

 the number of decibels by which the corresponding powers differ is ex- 

 pressed by the following formulas: 



i\ 

 n = 20 logio — ' decibels 1.56 



n = 20 logio — > decibels 1.57 



ez 



where /1//2 and £■1/^2 are the given current and voltage ratios respectively. 

 For relation between decibels and power and current or voltage ratios 

 see Table 1.2. 



