An introduction to decibel, sound power, sound intensity and sound pressure
Decibel is a logarithmic unit used to describe physical values like the ratio of the signal level - power, sound pressure, voltage or intensity.
The decibel can be expressed as:
decibel = 10 log(P / Pref) (1)
P = signal power (W)
Pref = reference power (W)
Sound Power Level
Sound power is the energy rate - the energy of sound per unit of time (J/s, W in SI-units) from a sound source.
Sound power can more practically be expressed as a relation to the threshold of hearing - 10-12 W - in a logarithmic scale named Sound Power Level - Lw:
Lw = 10 log (N / No) (2)
Lw = Sound Power Level in Decibel (dB)
N = sound power (W)
- The lowest sound level that people of excellent hearing can discern has an acoustic sound power about 10-12 W, 0 dB
- The loudest sound generally encountered is that of a jet aircraft with a sound power of 105 W, 170 dB.
The Sound Intensity Level can be expressed as:
LI = 10 log(I / Iref) (3)
LI = sound intensity level (dB)
I = sound intensity (W/m2)
Iref = 10-12 - reference sound intensity (W/m2)
Sound Pressure Level
The Sound Pressure is the force (N) of sound on a surface area (m2) perpendicular to the direction of the sound. The SI-units for the Sound Pressure are N/m2 or Pa.
The Sound Pressure Level:
Lp = 10 log(p2 / pref2) = 10 log(p / pref)2 = 20 log (p / pref) (4)
Lp = sound pressure level (dB)
p = sound pressure (Pa)
pref = 2 10-5 - reference sound pressure (Pa)
- If the pressure is doubled, the sound pressure level is increased with 6 dB (20 log (2))