Acoustic Glossary


Sound Intensity and related terms, definitions, units .... including

absolute sound intensityactive sound intensityeffective sound intensityinstantaneous intensity definitionpressure residual intensity indexreactive intensityreactivity indexsound intensitysound intensity definitionsound intensity levelsound intensity level definitionsound intensity pressure gradientsound intensity pressure indexsound intensity probes

Sound Intensity

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

Sound Intensity is 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 or Acoustic Intensity is also known as sound energy flux density and sound power density.

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 Measurement Equipment.

Our ears can detect sound intensities 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 parameter, detailed below, using the dB (decibel) scale.

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 120+ dB, as the list below demonstrates.
Sound Intensity Level : 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

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 is also known as the sound energy flux density level

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.

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

Absolute Sound Intensity

is measured in watts/m2

Active Sound Intensity

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

Effective Sound Intensity

is the RMS (root-mean-square) of the instantaneous sound intensity, over a given period of time

Instantaneous Intensity Definition

IEC 802-01, acoustic energy flow rate in the direction of propagation per unit area normal to the direction of propagation.

Note 1 instantaneous intensity is the product of instantaneous acoustic pressure and instantaneous particle velocity.

Pressure Gradient is under Sound Intensity Pressure Gradient

Pressure Index is 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.

Reactive 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.

Reactivity Index

is the difference between the sound intensity level and the sound pressure level.

Residual Intensity Index is under pressure residual intensity index above

Sound Intensity Pressure Gradient

is 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

is 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.

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.

Sound Intensity Probes

comprise 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 EnergySound PowerSound Pressure

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