Sound intensity is measured in db (Decibel). The decibel scale gives sound of all frequencies equal weight,
while human hearing does not. Our hearing sensitivity varies with frequency: It is most sensitive in the
middle range (between 400~4000 Hz), but much less sensitive in the low frequencies, and less sensitive
again in the high frequencies.
To compensate for the non-linear frequency response of human hearing, the "A" weighting scale was developed
for sound level measurements. Such measurements are expressed as dBA instead of dB and allow sounds of
different frequency balances to be fairly compared for relative loudness.
dB is a logarithmical measure unit; each doubling of identical noise sources results in a 3 dB
increase in noise. The human ear will not sense this as double as loud.
1 dB is generally the smallest difference that can be perceived by human beings
3 dB difference is clearly audible for just about anyone with normal hearing
10 dB difference is generally perceived as being twice or half as loud.
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