Audio Engineering

Measurement 49

being measured). For 1 W using 10 ^12 W at 0.283 m, LW  LP  120 dB. For 1 W using
10 ^13 W at 0.282 ft, LW  LP  130 dB as found with the equation:

LLPW 10 log ( 4 πr^2 ) (2.27)

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 fi eld 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.

Table 2.4 : Common Decibel Notations and References

Quantity Standard reference Symbol Log multiplier

Sound pressure Water: 1 dyn/cm^2 SPL o r 20

Air: 0.0002 dyn/cm^2 LP

or 0.00002 N/m^2

Sound intensity 10 ^16 W/cm^2 10

10 ^12 W/m^2

Sound power 10 ^12 W (new) PWL 10

10 ^13 W (old) or Lw

Audio power 10 ^3 W dBm 10

EMF 1 V dBV 20

Amperes 1 mA 20

Acceleration 1 gRMS 20

Acceleration 1 g^2 /Hz 10

Spectral density

Volume units 10 ^3 W VU 10

Distance 1 ft or 1 m ΔDx 20

Noise-ref  90 dBm at 1 kHz dBm 10

dB Logarithm Multiplier Quantity

Standard Reference

   log