Sound Intensity Relationships :
Sound intensity = sound pressure × particle velocity
Sound intensity = (sound pressure)2 ÷ acoustic impedance
Sound intensity = (particle velocity)2 × acoustic impedance
Our ears can detect sound intensities as low as 0.000000000001 Watt/m2 and up to 20 Watts/m2 or more, a range which makes using Watts/m2 values impractical for everyday use. The neat solution is the Sound Intensity Level, which uses the dB (decibel) scale, to compress the range to more manageable numbers and discussed below.
Sound Intensity Level (SIL) is the logarithmic ratio of the sound intensity to the reference sound intensity and makes the values more manageable, i.e. 0 to 140 dB, as the list below demonstrates.
Sound Intensity Level (LI) = 10 log(I/Io) dB re 1 pW/m2So 140 dB = 100 W/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*
Reference Sound Intensity (Io) = 1 pW/m2 = 1 x 10-12 W/m2 ≡ 0dB
See also • 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 (Watts/m2) 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
Active Sound Intensity is the propagating part of a sound field, producing a net flow of sound energy, predominates in the free sound field.
Effective Sound Intensity is the root-mean-square of the instantaneous sound intensity, over a given period of time
instantaneous acoustic pressure and instantaneous particle velocity.
Pressure Residual Sound Intensity Index for a given measurement system, is the difference between the indicated sound intensity level and the measured sound pressure level, when exactly the same signal is fed into the two channels of the sound intensity analysing system. Ideally the pressure residual intensity should be 'zero' but in practice the difference is due to any phase mismatch between channels.
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.
Reactive Sound Intensity the part of a sound field that does not contribute to the net flow of energy, however it influences the sound intensity pressure index and therefore the 'quality' of the measurement.
Residual Intensity under pressure-residual sound intensity index
Sound Intensity Pressure Gradient the change in sound pressure with distance. So if the sound intensity pressure gradient is determined during sound intensity measurements, the particle velocity is also known.
Sound Intensity Pressure Index (LPI) the difference between the sound pressure and the sound intensity and is good guide to the quality of the measurement and is also known as Lk and PI Index.
In a free-field environment the sound pressure = sound intensity so the sound intensity pressure index = 0.
Sound Intensity Probe, include two closely spaced** phase matched microphones, which enables the pressure gradient to be measured and therefore the particle velocity to be calculated. The sound intensity is the sound pressure multiplied by the particle velocity at any given position.
** 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.
Sound Intensity Reactivity Index is the difference between the sound intensity level and the sound pressure level.
Related Terms • Sound Energy • Sound Power • Sound Pressure