SOUND INTENSITY: TECHNICAL INFO

Sound Intensity Definition:
Sound Intensity is a time-averaged, directional quantity that measures the rate of energy flowing through a specified unit area.

Measurement Units:
Standard sound intensity measurements are in Watts per square metre, or similarily, decibels relative to one picoWatt per square metre.

How does Sound Intensity Measurement Work?
When acoustic energy flows through air, one result is a fluctuation of air pressure. A microphone measures sound in terms of these pressure fluctuations. However, sound pressure is only part of the whole picture. In some situations there is plenty of sound pressure, but little or no overall flow of acoustical energy. A sound field includes both acoustic pressure and particle velocity. Sound intensity is the product of sound pressure and acoustic particle velocity. It is measured in units of Watts per square metre or decibels relative to one picoWatt per square metre.

Acoustical particle velocity is difficult to measure. The most common method of measuring the particle velocity is a pair of phase-matched microphones locked at a fixed distance apart. By comparing the differences between the sound signal at the two microphones, the sound intensity analyzer can compute both the sound pressure and particle velocity at the mid-point between the two microphones, and can multiply the two together to compute the sound intensity. Because sound intensity is based on both acoustic pressure and velocity it has a direction. A two-microphone sound intensity probe measures only the component of the total sound intensity travelling parallel to the axis of the probe. As shown in the above illustrations, a sound originating in front of the probe will be measured as a positive intensity, whereas a sound from behind the probe will be negative.