Amplifier power must double to increase a speaker's SPL by 3dB. 
 A speaker rated at 85 db for 1 watt needs 1 watt to produce 85 dB of sound, 
 2 watts to produce 88 dB, 4 watts to produce 91 dB, 
 8 watts to produce 94 dB, 16 watts to produce 97 dB, 32 watts to produce 100 db, 
 64 watts to produce 103 dB, 128 watts 106db, 256 watts 109 db, 512 watts 112 db, 
 1024 watts 115 db. Always look at the speaker's sensitivity ratings.
 Most PA speakers are 97-100 db for 1 watt, Guitar speakers can be 107-110 db.
 
 A speaker rated at 100db at 1 watt, 2 watts 103db, 4 watts 106db, 8 watts, 109db,
 16 watts 112db, 32 watts 115db, 64 watts 118db, 128 watts 121db, 256 watts 124db
 512 watts 127db, 1024 watts 130db 2048 watts 133db  
 Just math and science.
 
 Conversely, sound falls off fast. 
 Sound pressure levels (SPL) will attenuate by 6 dB with the doubling of the distance. 
 If a speaker were capable of 135 dB at 3 feet, then 6 feet would have an SPL of 129 dB. 
 By doubling the distance to 12 feet, the speaker would output 123 dB and so on. 
 Another consideration is that doubling up on the speakers will result in a +3 dB increase. 
 If a speaker has a peak output of 135, by adding another speaker the output would increase to 138 dB.

 Microphones work the same way but start on the screen, -18db so 1" away -21db or half the volume, 
 2" away -24db where the stage noise is usually on all mics when using stage monitors.