A Decibel is a Decibel is a Decibel
The decibel is always a power ratio; therefore, when dealing with quantities that are not power ratios, that is, voltage, use the multiplier 20 in place of 10. As we encounter each reference for the dB, we will indicate the correct multiplier. Table 2.4 lists all the standard references, and Tables 2.5 through 2.8 contain additional information regarding reference labels and quantities. The decibel is not a unit of measurement like an inch, a watt, a liter, or a gram. It is the logarithm of a nondimensional ratio of two power-like quantities.
Older References
Much earlier, but valuable, literature used 10-13 W as a reference. In that case, the LP value approximately equals the LW value at 0.282 ft from an omnidirectional radiator in a free field (i.e., the number values are the same but, of course, different quantities are
where LW is 10 log the wattage divided by the reference power 10-13 and r is the distance in meters from the center of the sound source.
Figure 2.6 requires that you either know the distance from the source or assumes you are in the steady reverberant sound field of an enclosed space. LP readings without one of these is meaningless.
Figure 2.7 shows typical power and LW values for various acoustic sources.
