| The dB or decibel is a logarithmic scaling of the sound pressures we hear.
The actual pressures we can hear is enormous 10 bars = 114dB rather loud right down to 20 microbar = 0dB, the threashold of hearing. Barometer also measures pressure in bars or millibars
To discuss values from a few millionths of a bar up to 10 bar was difficult so they decided to convert the measured pressures into logarithmic values and so finished up with values of 0 - 12 bels
This was reduction down to 12 steps was a bit too crude, so they simply multiplied the values by 10 and got the usual sound range 0 - 120 dB [threshold to pain]
An increase of 3dB = twice as much acoustic power or watts. If you have a klaxon of 80dB and you put another one next to it a sound level meter would read 83dB because it's a logarithmic scale not 160.
But you ears would not perceive the 2 klaxons together as being twice as loud as ears are non linear. In practice a 10dB increase 'sounds' twice as loud although the acoustic power or energy is many times that.
For the same reason, an increase of 1dB is difficult to detect by ear
Hopefully you are not totally confused.
A simple and effective meter to use for m'cycle exhausts is something like the Nor 102. Meters like this cost £1000 or more. You can buy meters from Maplins etc for as little as £60 but they are indicators not meters  | 
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