Sound Intensity and related terms, definitions, units etc.,
See also • sound intensity measurement equipment.
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.
Sound Intensity Level (LI) 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/m2
So 140 dB = 100 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
See also • other Standard Reference Levels
Sound 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 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
Pressure Gradient under Sound Intensity Pressure Gradient
Pressure Index under Sound Intensity Pressure Index
Pressure-residual 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.
under Pressure-residual Intensity Index
Sound Intensity Pressure Gradient the change in sound pressure with distance. So if the 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.
In reactive sound fields the sound pressure may be 20 dB higher than the intensity, so a high LPI means that accurate sound intensity measurements will be difficult.
Sound Intensity Probes 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.
Related Terms •
Sound Energy •
Sound Power •
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