stick the "t" from "willtravel" into "equasion" and call it even
what i think you might be asking, willravel, simply involves using logarithmic scales. speakers' efficiencies are measured at X dB/Watt @ 1 meter distance. SPL is a unit of pressure, hence it decays by 6 dB/decade (SPL = 10 log(P/ref)) versus power which decays 3 dB/decade (P = 20 log(W/ref)). If you've got a 90 dB efficiency speaker driven at 1 watt, then 1 meter from it you'll have an SPL of 90 dB. 2 meters away will be 84 dB, 4 meters away will be 78 dB, 8 meters away will be 72 dB, etc. If you want 90 dB at 2 meters, you'll need to increase your amplifier's power 4 times - rather than twice - because of the difference in calculating power and SPL. as you can see, it takes alot of power to bump up SPL just a little. If you take that logic one step further, you can see that LOTS of power is a good thing.