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9. Measurement Engineering
Fundamentals
Pressure and Sound
Noises are changes in the sound pressure in the air. When
conditions are normal, the air pressure is 1013 mbar about which
the sound pressure of the noise source fluctuates. The human ear
senses these pressure fluctuations and converts them to nerve
pulses. The ear is like a pressure sensor with an enormous
dynamic range. The quietest noise which can be heard by
humans causes pressure fluctuations of 0.0002 µbar
(corresponding to 0 dB), the loudest noise (which can be heard
without pain) has a sound pressure level of 635 µbar
(corresponding to 130 dB).
This corresponds to a pressure difference of around 3,000,000-
fold. Since stating the pressure in mbar would result in very long
figures, logarithmic notation is used and calculation is with level
values. In this way, a level increase of 20 dB corresponds to a 10-
fold increase in pressure. A sound level meter to EN 60651
measures the frequency weighted actual value of the sound level,
it is a measure of the total sound energy converted during
measurement time.
9. Measurement Engineering Fundamentals
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