# Acoustic Glossary

## Sound Intensity Terms and Definitions ...

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 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 Units are watts per square metre (W/m²). However the range we hear 0.000000000001 W/m² up to 20 W/m² or more, means W/m² are not practical for everyday use - the sound intensity level in decibels solves this problem.

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

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

Sound Intensity Level Formula, Li = 10 log (I/Io) dB, where I is the sound intensity in W/m² and Io is the reference sound intensity of 10-12 W/m², converting the watts per square metre levels into the more manageable range of 0 to 140 dB as the following list demonstrates.

140 dB = 100 W/m² (watts per metre squared)
130 dB = 10 W/m²
123 dB = 2 W/m²
120 dB = 1 W/m² ≡ threshold of pain
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 the threshold of hearing

Sound Intensity Level uses the 10 log 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

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.

Sound Intensity Related Terms - listed alphabetically

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

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

Instantaneous Sound 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.

Lk : under pressure intensity index

Phase Index : under pressure intensity index
PI Index : under 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 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 sound energy, however it influences the intensity pressure index and therefore the 'quality' of the measurement.

Reactivity Index : under pressure intensity index

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.

Residual Sound Intensity : under pressure-residual intensity index.

Sound Intensity Inverse Square Law : under Inverse Square Law

### 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 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 Pressure Intensity Index : under pressure intensity index

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

Sound Intensity Reactivity Index : under pressure intensity index.

Other related termssound energysound powersound pressure 