Brüel & Kjær 2240 Guide De L'utilisateur page 43

Our hearing covers a surprisingly wide range of sound pressures – a ratio of over a million to one. The
'F' time weighting:
frequency:
dB scale makes the numbers manageable
A time weighting (sometimes called a 'time constant')
defines how the exponential averaging in root-mean-square
(RMS) measurement is done. It defines how the heavily
fluctuating sound pressure variations are smoothed or
averaged to allow useful readings. The standards define
three time weightings: F (Fast), S (Slow) and I (Impulse).
Most measurements are carried out using the 'F' time
weighting, which uses a 125 ms time constant.
The number of pressure variations per second. Frequency
is measured in hertz (Hz). The normal hearing for a healthy
young person ranges from approximately 20 Hz to
20000 Hz (20 kHz).
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