Decibels, dB, are a measure of loudness. Unlike other measurements, decibels are logarithmic in scale not linear.
Speed is linear, 40km/h is twice as fast 20km/h. Time is linear, 60 seconds is twice as long as 30 seconds.
Decibels arent, 80dB isnt twice as loud as 40dB. 80dB is twice as loud as 77dB !
This is how "decibels" interprets into "loudness":
+1db is 1.25x louder
+3db is 2x louder
+ 20db is 100x louder !
So to get twice as loud as your buddy you only need to gain 3dB's, but how ? Easy, you have a few options..
1. Double the power into your existing subs. Only have 500WRMS ? Get another 500WRMS !
2. Double the cone area and power. Just adding a second sub doesnt work. Why ? Because you half the power going into the first one !
3. Tune the enclosure using our nifty "SPL tuning guide"
If you have a quality amp that can handle low impedences, you can cheat a bit. Have a duel 2ohm woofer wired in series for 4ohm and 1ohm stable amp ? Parallel the coils for a 1ohm load !
Same deal with adding an extra sub, or more... load down the amp if possible and safe and voila ! More power and more cone area.
So what have we learnt ? dB's are all 2's and 3's !
2x Loud = +3dB.
2x Power = +3dB.
2x Cone = +3dB (with 2x power)
0.5x ohms = 2x Power (half the ohms load = 2x the power !)