Audio Engineering

48 Chapter 2

2.10 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.

For LP  20 log ( x Pa/0.00002 Pa), use Eq. (2-29).

LxP(Pa)dB20log (^94) (2.26)

2.11 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 fi eld (i.e., the number values are the same but, of course, different quantities are

WRor (PF)WZ

EI or EI(PF) W W

WW

W W

W W

WW

I orI(PF)

IR or IZ E

(^2) E 2
or (PF)
I^2 orI^2 (PF)
E
I
R or Z
R or
E or E(PF)
W I
E
E^2 E^2
RorZ(PF)
E
R
E
orZ
I^2 R or I^2 Z(PF)
Z(PF)
Figure 2.5 : Ohm’s law nomograph for AC or DC.