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 descriptor, 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
See also other Standard Reference LevelsSound Intensity Level : LI uses the 10 lg equation so, as a rule of thumb:
Sound Intensity Level Definition : 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. (IEC 801-22-06)
● Note : unless otherwise specified, the reference sound intensity is 1 pW/m2
Sound Intensity Level is also known as the sound energy flux density level
Sound Intensity Levels are measured with a sound intensity probe
See also the IEC Definition of Level
Instantaneous Sound Intensity Definition : acoustic energy flow rate in the direction of propagation per unit area normal to the direction of propagation (IEC 802-01-10)
● 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 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.