Perceived doubling is 10dB... at least that is what the experts say... personally I think it is another of those subjective things about audio...
The idea is that 1 dB is the smallest increase in wide band noise that an average listener can perceive... and 10 of these units (10dB) is perceived as "twice as loud". 3dB however is clearly audible but not really twice as loud sounding but is twice the power and 10dB is ten times the power. Therefore to reach a level that is perceived as being twice as loud, whatever that means, does indeed require ten times the watts.
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