Understanding sound pressure, decibels, and scaling for accurate measurements
The A111 supports IEPE-based microphones. It provides a 4.5 mA constant current, suitable for approximately 99% of industrial microphones.
Sound Pressure
Sound pressure, also called acoustic pressure, is the local deviation from the ambient (average or equilibrium) atmospheric pressure caused by a sound wave. In air, sound pressure is measured using a microphone, and in water with a hydrophone. The SI unit of sound pressure is the pascal (Pa).
Decibels (dB)
Sound pressure is often expressed in decibels (dB) to represent the ratio of a measured sound pressure to a reference pressure on a logarithmic scale. The logarithmic scale is used because the human ear perceives sound intensity logarithmically, and because it allows very large ranges of pressure to be represented compactly.
Microphone Sensitivity
Microphones typically output in Pa/mV and must be scaled correctly. The formula to calculate sound pressure level (Lp) in decibels is:
Lp = 20 × log10 (p/pref)
Where:
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Lp = sound pressure level in dB
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p = measured sound pressure in Pa
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pref = reference sound pressure, 20 μPa in air (the threshold of human hearing at 1 kHz)
Explanation of the formula:
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The ratio p/pref compares the measured pressure to the reference pressure.
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The logarithm (base 10) compresses large ranges of pressure into a manageable scale.
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The factor 20 (not 10) is used because sound pressure is proportional to the square root of sound intensity, and in logarithms, this introduces the factor 20 instead of 10.
Example Calculation
If a microphone measures a sound pressure of 2 Pa:
Lp = 20 × log10 (2 / 0.00002) = 20 × log10 (100,000) ≈ 100 dB