Λοιπον βρηκα κ τ αθρο που ελεγε "Your speaker system needs to cover more range than a Guitar or Bass Guitar, so it needs to be at least a 2 way system (A Woofer and a Horn, or a Woofer and a Tweeter). A good rule of thumb is to consider is that (on average) signals below 500 Hz will require 70% of the power and all signals above 500 Hz will require 30% of the power. Sub-Woofers are stand alone and not considered in the 70/30 rule. If your PA system power amp is 100 watts RMS, then its peak output is actually 200 watts. While the 70/30 rule implies that you could get away with 70 watt woofers, you should match the Woofer to the PAs output if there is a potential for a lot of Bass in your signal (if you have a Keyboard player plugged into the PA system, for example), to be on the safe side. If your PA is for Vocals only (such as a lecture hall, auction house or outdoor location), then your signal does not fit the 70/30 rule; in these cases the ratio is closer to 50/50.
Why do your need these ratios? Because when you buy the horn or tweeter you will find that they cannot handle as much power as the Woofer can - while it may seem that you should try to match these at the same power level, the reality is that you won't. Following the 70/30 rule, in the case of a 100 watt power amp, the signals above 500 Hz will only see 30% of the power - or 30 watts. The higher that you go on the frequency scale, the less power the speakers will see. At 5000 Hz, the tweeters will only see approximately 10% of the total power. This is why you see Midrange horns and tweeters at lower power ratings than Woofers."
http://www.colomar.com/Shavano/power_rating_speakers.html
