Understanding Decibels (dBs)
When considering noise measurement, the range
of levels of interest is in the ratio of about
1,000,000,000,000,000,000 to 1. With numbers of this range and
magnitude it is much more convenient to use a logarithmic scale to
provide numbers which are far easier to comprehend and use.
The decibel scale is thus a logarithmic ratio
between any two sound levels. For example, the ratio given above
may be written as 180dB. The addition or subtraction of decibels is
different to normal linear calculations. Adding 3dB to an existing
noise level doubles it. So 93dB of noise is twice 90dB of noise.
Adding two noise levels of 80dB does not give a noise level of
160dB, but gives a level of 83dB; i.e. twice the 80dB noise
level.
The decibel measure is also applied to hearing
protected headsets to describe their 'average attenuation' or
'insertion loss' (noise reducing effect). For example, if an
individual is working in a noise level of 105dB at a frequency of
1kHz then wearing a protective headset with an average attenuation
of 30dB at 1kHz reduces the effective noise level to 75dB at that
frequency.
Further reading
Wikipedia Link http://en.wikipedia.org/wiki/Decibel
Salford University http://www.acoustics.salford.ac.uk/acoustics_info/decibels/