Sound Intensity

Sound Intensity
The sound intensity level is the acoustic power of a sound per unit of area in relation to a fixed reference.
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Sound Intensity
Sound Intensity is the Acoustic or Sound Power (W) per unit area. The SI-units for Sound Intensity are W/m2.
Sound Intensity Level
The dynamic range of human hearing and sound intensity spans from 10-12 W/m2 to 10 - 100 W/m2. The highest sound intensity possible to hear is 10,000,000,000,000 times as loud as the quietest!

This span makes the absolute value of the sound intensity impractical for normal use. A more convenient way to express the sound intensity is the relative logarithmic scale with reference to the lowest human hearable sound - 10-12 W/m2, 0 dB.
The reference is the threshold of hearing - 10-12 W/m2 - and is given the value of 0 dB.
Note! In US a reference of 10-13 watts/m2 may be used.
The Sound Intensity Level can be expressed as:
LI = 10 log(I/Iref) (1)
where
LI = sound intensity level (dB)
I = sound intensity (W/m2)
Iref = 10-12 - reference sound intensity (W/m2)
The logarithmic sound intensity level scale match the human sense of hearing. Doubling the intensity increases the sound level with 3 dB ( 10 log (2) ).
Example - Sound Intensity
The difference in intensity of 10-8 watts/m2 and 10-4 watts/m2 (10,000 units) can be calculated in decibels as
ΔLI = 10 log( (10-4 watts/m2) / (10-12 watts/m2) )
- 10 log( ( 10-8 watts/m2) / ( 10-12 watts/m2) )
= 40 dB
Increasing the sound intensity by a factor of
• 10 raises its level by 10 dB
• 100 raises its level by 20 dB
• 1,000 raises its level by 30 dB
• 10,000 raises its level by 40 dB and so on
Note! Since the sound intensity level may be difficult to measure, it is common to use sound pressure level measured in decibels instead. Doubling the Sound Pressure raises the Sound Pressure Level with 6 dB.
Sound Power, Intensity and Distance to Source
The sound intensity decreases with distance to source. Intensity and distance can be expressed as:
I = Lw / 4 π r2
where
Lw = sound power (W)
π = 3.14
r = radius or distance from source (m)
Sound Intensity and Sound Pressure
The connection between Sound Intensity and Sound Pressure can be expressed as:
I = p2 / ρ c (2)
where
p = sound pressure (Pa)
ρ = 1.2 = density of air (kg/m3) at 20oC
c = 340 - speed of sound (m/s)