Sound intensity = sound pressure × particle velocity, see also sound intensity measurements
Sound intensity = (sound pressure)² ÷ acoustic impedance
Sound intensity = (particle velocity)² × acoustic impedance
Sound Intensity SI units are W/m² (watts per metre-squared), however our ears can detect sound intensities as low as 0.000000000001 W/m² and up to 20 W/m² or more, a range which makes using W/m² values impractical for everyday use. The neat solution is sound intensity levels, which use the dB (decibel) scale, to compress the immense range to more manageable numbers and detailed below.
Sound Intensity Level (LI) = 10 log (I/Io) dB re 1 pW/m²
123 dB = 2 W/m²
120 dB = 1 W/m²
100 dB = 0.10 W/m²
80 dB = 0.0001 W/m²
60 dB = 0.000001 W/m²
40 dB = 0.00000001 W/m²
20 dB = 0.0000000001 W/m²
0 dB = 0.000000000001 W/m² = 10-12 W/m² = reference level (Io)
Sound Intensity Reference Level (Io) = 1 pW/m² = 1 x 10-12 W/m² ≡ 0dB
See other standard reference levelsSound Intensity Level (LI), uses the 10 lg equation so, as a rule of thumb:
3 dB = a factor of 2 in sound intensity
10 dB = a factor of 10 in sound intensity
20 dB = a factor of 100 in sound intensity
30 dB is a factor of 1000
40 dB is a factor of 10000
Sound Intensity decreases by a factor of 4 each time the distance from the source is doubled, see the inverse square law. In decibels this is 10·Log (4) = 6 dB.See also our sound level calculations page
Sound Intensity Level Definition IEC 801-22-06, logarithm of the ratio of a given intensity of sound in a stated direction to the reference sound intensity. Such intensity level in decibels is ten times the logarithm to the base ten of the ratio, and is also known as the sound energy flux density level.● Note: unless otherwise specified, the reference sound intensity is 1 pW/m²
See also the IEC Definition of Level
Sound Intensity Levels are measured with a sound intensity probe
Related Terms - listed alphabetically
Absolute Sound Intensity is measured in watts/m²
● Note : instantaneous intensity is the product of instantaneous acoustic pressure and instantaneous particle velocity.
The difference is also known as the residual intensity and some use the term Lkvo
Because the microphones have to be included in the measurement of residual intensity, specialised calibrators are required - like the Bruel & Kjaer sound intensity calibrator.
Residual Intensity under pressure-residual sound intensity index
In a free-field environment sound pressure = sound intensity so the sound intensity pressure index = 0.
In reactive sound fields the sound pressure may be 20 dB higher than the sound intensity, so a high LPI means that accurate sound intensity measurements will be difficult.
** The frequency range is dependent on the distance between the microphones, so probes are delivered with interchangeable spacers to enable measurements from 50 Hz to 10 kHz.
Other Related Terms, sound energy • sound power • sound pressure