Sound Intensity Relationships;
Sound Intensity = sound pressure × particle velocity.
Sound Intensity = (sound pressure)² ÷ acoustic impedance
Sound Intensity = (particle velocity)² × acoustic impedance
The Sound Intensity Unit is the watt per square metre (W/m²), however the immense range of human hearing, 0.000000000001 W/m² up to 20 W/m² or more, means the W/m² is not practical for everyday use. The sound intensity level in decibels neatly solves this problem.
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 (LI) 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
Sound Intensity Level Definition
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 Related Terms - listed alphabetically
Absolute Sound Intensity under effective sound intensity
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.
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.
Used in the calculation of sound intensity levels in decibels for the more practical range of 0 to 120 dB, instead of 0.000000000001 to 1 W/m² - see our dB and W/m² comparison table.
Sound Intensity and DistanceSound Intensity Levels decrease by a factor of 4 each time the distance from the source is doubled. In decibels this is 10·log (4) = 6 dB, see the inverse square law.
Sound Intensity MeasurementsSound Intensity is the product of the sound pressure and particle velocity, beyond the scope of sound level meters. However both quantities can be measured using sound intensity probes and the associated instrumentation.
See also, sound intensity pressure index
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 ProbesSound Intensity p-p probes with two closely spaced* phase matched microphones are widely used, enabling the pressure gradient to be measured and 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.