The human ear can detect sound intensity levels as low as 0.000000000001 Watt and up to 20
Watts 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 threshold of hearing, reference level Io. The logarithmic scaling makes the values more manageable, 0 to 120+ dB, the table below demonstrates this point.
LI = 10 lg(I/Io) dB re 1 pW/m2
140 dB = 100 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*
The Standard Reference Sound Intensity : I0 = 1 pW/m2 = 1 x 10-12 W/m2 = 0dB
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 Level (SIL) is measured with a Sound Intensity Probe, using two closely spaced precision, phase matched microphones. SIL describes as a function of frequency the direction and the amount of net flow of
Acoustic Energy at a given position in a Sound Field.
Active Intensity : the propagating part of a sound field, producing a net flow of
Sound Energy, predominates in the
PI index : see Pressure Intensity Index below.
Pressure Gradient : the change in pressure with distance, from lower to higher pressure, or vice versa.
Used in the determination of Sound Intensity, the pressure gradient enables
Particle Velocity to be measured.
Pressure Intensity Index : PI : is the difference between the
Sound Pressure and the sound intensity and is good guide to the quality of the measurement. 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.
Reactive Intensity : the part of a sound field that does not contribute to the net flow of energy, however it influences the Pressure Intensity Index (PI) and therefore the 'quality' of the measurement.
Reactivity Index : is the difference between the Sound Intensity Level and the Sound Pressure Level.
Residual Intensity : is the Sound Intensity level measured when the same signal is fed to both channels of a sound intensity measuring system. Ideally the Residual Intensity should be 'zero' but in practice the difference is due to phase mismatch between channels.
Residual Pressure Intensity Index : the residual pressure 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 an intensity analysing system.
The difference is also known as the Residual Intensity and some use the term Lkvo.
Sound Intensity Probe : used to determine the
Sound Power. Bruel & Kjaer's sound intensity probes consist of two closely spaced*, Phase matched microphones. This enables the Pressure Gradient to be measured and therefore the Sound Velocity to be calculated. It follows therefore that the intensity is the pressure multiplied by the calculated 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.