Acoustic Glossary


Sound Intensity : Definitions, Terms, Units, Measurements ...

Sound Intensity is the sound power per unit area, a sound energy quantity, the SI units are watts/m2

Sound Intensity is also the time averaged product of the sound pressure and particle velocity.

Sound intensity is a vector quantity with both magnitude and direction of sound energy flow.

Sound Intensity IEC Definition,

sound energy flux in a specified direction and sense through an area perpendicular to that direction, divided by the area


Sound Intensity = sound pressure x particle velocity = (Force/Area) x (Distance/Time) = Energy/(Area x Time) = Power/Area.

Sound Intensity Measurement Equipment.

Sound Intensity is also known as Sound Energy Flux Density and Sound Power Density.

Our ears can detect sound intensity levels as low as 0.000000000001 Watt/m2 and up to 20 Watts/m2 and more, a range which makes using absolute sound intensity values impractical for everyday use. A neat solution is the Sound Intensity Level descriptor, using the dB (decibel) scale.

Sound Intensity Level IEC Definition,

logarithm of the ratio of a given intensity of sound in a stated direction to the reference sound intensity. Sound intensity level in decibels is 10 times the logarithm to the base ten of the ratio.
Note : unless otherwise specified, the reference sound intensity is 1 pW/m2


Sound Intensity Level : LI is the logarithmic ratio of the Sound Intensity to the threshold of hearing and makes the values more manageable, i.e. 0 to 120+ dB, the table below demonstrates this point.

LI = 10 lg(I/Io) dB re 1 pW/m2

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*

The Reference Sound Intensity : Io = 1 pW/m2 = 1 x 10-12 W/m2 ≡ 0dB

See also other Standard Reference Levels

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 : SIL is measured with a sound intensity probe using two closely spaced precision, phase matched microphones. SIL describes as a function of frequency the direction and the amount of net flow of acoustic energy at a given position in a sound field.

Sound Intensity Level is also known as Sound Energy Flux Density Level.

See also the IEC Definition of Level

Active Intensity the propagating part of a sound field, producing a net flow of sound energy, predominates in the free-field.

Instantaneous Intensity IEC Definition,

acoustic energy flow rate in the direction of propagation per unit area normal to the direction of propagation Note – Instantaneous intensity is the product of instantaneous acoustic pressure and instantaneous particle velocity.

PI index see Pressure Intensity Index below.

Pressure Gradient the change in pressure with distance, from lower to higher pressure, or vice versa.

Used in the determination of Sound Intensity, the pressure gradient enables particle velocity to be measured.

Pressure Intensity Index : PI is the difference between the sound pressure and the sound intensity and is good guide to the quality of the measurement. Also known as Lk.

In a free-field environment, pressure = intensity so LPI = 0. In reactive 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.

Reactive Intensity the part of a sound field that does not contribute to the net flow of energy, however it influences the Pressure Intensity Index (PI) and therefore the 'quality' of the measurement.

Reactivity Index is the difference between the Sound Intensity Level and the sound pressure level.

Residual Intensity is the Sound Intensity level measured when the same signal is fed to both channels of a sound intensity measuring system. Ideally the Residual Intensity should be 'zero' but in practice the difference is due to phase mismatch between channels.

Because the microphones have to be included in the measurement of Residual Intensity, specialised calibrators are required - like the B&K 4297 Sound Intensity Calibrator.

Residual Pressure Intensity Index the residual pressure 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 an intensity analysing system.

The difference is also known as the Residual Intensity and some use the term Lkvo.

Sound Intensity Probe used to determine the sound power. Bruel & Kjaer's sound intensity probes consist of two closely spaced*, Phase matched microphones. This enables the pressure gradient to be measured and therefore the sound velocity to be calculated. It follows therefore that the intensity is the pressure multiplied by the calculated 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 EnergySound PowerSound Pressure


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