 1decibel (dB) is roughly the smallest change we can perceive.
 Doubling the amplifier wattage results in ONLY a 3dB increase in
volume ("slightly louder"). Switching from a 50 watt amplifier
to a 100 watt amplifier or from a 100 watt amplifier to a 200 watt
will only increase the volume by 3 dB as long as you use the same
speakers.
 In order to double the current volume requires
roughly a 6 to 10 dB increase. A 6 dB increase
requires 4 times the power! and a 10 dB increase
requires a 10fold increase in amplifier power.
Perceptions of Increases in Decibel Level
Decibel: The decibel (abbreviated dB)
is the unit used to measure the intensity of a sound.
(Key points):
1) On the decibel scale, the smallest audible sound (near total silence)
is ~01 dB.
2) A sound 10 times more powerful is 10 dB.
3) A sound 100 times more powerful than near total silence is 20 dB.
4) A sound 1,000 times more powerful than near total silence is 30 dB.
* Near total silence  0 dB
* A whisper  15 dB
* Normal conversation  60 dB
* A lawnmower  90 dB
* A car horn  110 dB
* A rock concert or a jet engine  120 dB
* A gunshot or firecracker  140 dB
Imperceptible Change  1dB
Barely Perceptible Change  3dB
Clearly Noticeable Change  5dB
About Twice as Loud  10dB
About Four Times as Loud  20dB
[Any sound above 85 dB can cause hearing loss, and the loss is related
both to the power of the sound as well as the length of exposure.]
dB SPL laws
Impact of the various changes on sound levels
Double amplifier wattage  increase of
+3 dB
Double number of speakers  increase of
+3 dB
Double distance from sound source  Decrease
of  6 dB
Halve distance from sound source 
Increase of +6 dB
Double driver excursion  increase of
+6 dB (note: Equivalent to doubling
wattage and # of speakers)
Placement of Speakers
Place speaker on the floor  increase of
+3 dB  half loading
Place speaker on floor, and against a wall
 increase of +6 dB  quarter loading
Place speaker on floor, near a wall, and in the corner
 increase of +9 dB eighth loading
