Acoustic Glossary


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

Sound Intensity is the sound energy flow in watts per square metre (W/m²), measured in the direction of the sound wave propagation.

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 Relationships:
Sound intensity = sound pressure × particle velocity, see also sound intensity measurements
Sound intensity = (sound pressure)² ÷ acoustic impedance
Sound intensity = (particle velocity)² × acoustic impedance

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 (watts per metre-squared)
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 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 Intensity Definition IEC 802-01-10, 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.

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 indexsound intensity probes

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, 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 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.

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

Other Related Terms, sound energysound powersound pressure

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