Audio Engineering

42 Chapter 2

The power output of Boulder Dam is said to be approximately 3,160,000,000 W.
Expressed in dBm, this output would be

10

316 10

10

125

9

log 3

  dBm.

2.5 The Decibel in Acoustics—LP,LW, and LI ..............................................................

In acoustics, the ratios encountered most commonly are changes in pressure levels. First,
there must be a reference. The older level was 0.0002 dyn/cm^2 , but this has recently been
changed to 0.00002 N/m^2 (20 μ N/m^2 ). Note that 0.0002 dyn/cm^2 is exactly the same sound
pressure as 0.00002 N/m^2. Even more recently the standards group has named this same
pressure pascals (Pa) and arranged this new unit so that

20

0 0002

μPa 2

dyn

cm

.

. (2.17)

This means that if the pressure is measured in pascals,

L

xPa

P^20 log 20 μPa. (2.18)

If the pressure is measured in dynes per square centimeter (dyn/cm^2 ), then

L

x

P^20 0 0002

2

log. 2.

dyn/cm

dyn/cm

(2.19)

Table 2.2 : Root Mean Square Voltages Used as Nonstandard References

Voltage (V) Meter indication Apparent level (VU) User

1.950 (^0)  8 Broadcast
1.230 (^0)  4 Recording
0.245 (^0)  10 Home recording
0.138 (^0)  15 Musical instruments