Sound Intensity Terms, Definitions, Units, Measurements ...
Sound Intensity (I)
is the sound power
per unit area
in W/m² and the area is measured perpendicular to the direction of the sound energy
Sound intensity (I) is a vector quantity, having both magnitude and direction of the sound energy flow.
Sound Intensity Definition IEC 801-21-38, sound energy flux
in a specified direction and sense through an area perpendicular to that direction, divided by the area.
Sound Intensity or acoustic intensity is also known as the sound energy flux density and the sound power density.
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)
is the logarithmic
ratio of the sound intensity
to the reference sound intensity
, making the absolute W/m² (watts per metre-squared) levels 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
130 dB = 10 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)
Sound Intensity Reference Level (Io) = 1 pW/m² = 1 x 10-12 W/m² ≡ 0dB
See 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 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
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 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 Measurements, sound intensity is the time averaged product of sound pressure and particle velocity
. Both quantities can be directly measured by using a sound intensity p-u probe comprising a microphone and a particle velocity sensor, or more commonly, estimated indirectly by using a p-p probe that approximates the particle velocity by integrating the pressure gradient between two closely spaced microphones.
See also sound intensity pressure index • sound intensity probes
Sound Intensity Pressure Index (LPI)
the difference between the
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 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.
Sound Intensity Probes
with two closely spaced** phase
matched microphones, are widely used, which enables the pressure gradient
to be measured and therefore the particle velocity
to be calculated as 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 •