Acoustic Glossary

Sound Intensity

Sound Intensity (I) is the sound energy flowing through a specified area, the product of the sound pressure and the sound particle velocity.

Sound Intensity is a vector quantity, having both magnitude and direction of the sound energy flow.

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

Sound Intensity Relationships;
Sound Intensity = sound pressure × particle velocity.
Sound Intensity = (sound pressure)² ÷ acoustic impedance
Sound Intensity = (particle velocity)² × acoustic impedance

Sound Intensity Level

Sound Intensity (SIL) is the logarithmic ratio of the measured sound intensity to the reference sound intensity in decibels.

See also our sound level calculations page and the IEC Definition of Level

Sound Intensity Levels are measured with a sound intensity probe to establish both the sound pressure and the particle velocity.

Active Sound Intensity

Active Sound Intensity is the propagating part of a sound field, producing a net flow of sound energy, predominant in the free sound field.

Effective Sound Intensity

Effective Sound Intensity is the root-mean-square of the instantaneous sound intensity, over a given period of time.

Instantaneous Sound Intensity

Instantaneous Sound Intensity is the intensity at a specific instant in time and point in a sound field.

Pressure Intensity Index

Pressure Intensity Index is the difference between the sound pressure and the sound intensity, a good guide to the quality of the measurement - also known as Lk and PI Index.

In a free-field environment the sound pressure = sound intensity so the pressure-intensity index = 0.

In reactive sound fields, the sound pressure may be 20 dB higher than the sound intensity. A high pressure-intensity index indicates that accurate sound intensity measurements will be difficult.

Previously known as the phase index or reactivity index - cite Bruel and Kjaer.

Pressure-residual Intensity 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 Sound Intensity

Reactive Sound Intensity the part of a sound field that does not contribute to the net flow of sound energy, however it influences the intensity pressure index and therefore the 'quality' of the measurement.

Reference Sound Intensity

Reference Sound Intensity (Io) = 10^-12 W/m² the threshold of hearing at 1Khz and 0 dB when calculating sound intensity levels in decibels, resulting in the more practical range of 0 to 120 dB.

See other • acoustic reference quantities.

Sound Intensity Measurement

Sound Intensity is the product of the sound pressure and particle velocity, so beyond the scope of normal sound level meters. However both quantities can be measured using a sound intensity probe and the associated instrumentation.

Sound Intensity Pressure Gradient

Sound Intensity Pressure Gradient is 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 Probes

Sound Intensity p-p probes with two closely spaced* phase matched microphones are widely used. Enabling 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 the probes are delivered with interchangeable spacers to enable measurements from 50 Hz to 10 kHz.

Sound Intensity p-u probes measure the sound pressure using a pressure microphone and the particle velocity directly using a particle velocity sensor