Overpowering Speakers

Sebrof is also correct that you're very rarely sending the full rated power of your amplifier to the speakers. An EE buddy of mine told me that a rule of thumb on a reciever that shows volume in -dB is that full power is at 0dB and, I believe, half power was something like -10dB. Someone can confirm my memory, but I believe it takes twice the power to get twice the volume and double the volume is 10dB. I have CDs that I can listen to at 0dB, but very few. The majority of them I listed to below -10dB.

You have to double your power to get a 3db gain. It takes 3db to get a noticable difference in volume. If you are listening to a speaker of 90db @ 1 watt, 2 watts give you 93db, 4 watts give you 96db, 8 watts give you 99db, 16 watts give you 102db, 32 watts give you 105db, 64 watts give you 108db, 128 watts give you 111db, 256 watts give you 114db.... 121db is our threshold of pain. This tells you how speaker sensitivity dramitically effects how much power is needed.

Just a quick add on to make the point of sensitivity and power. All other aspects being equal, a 93db sensitivity speaker will play a volume of 111db using 64 watts. A speaker with 84 db sensitivity will play a volume of 111db on 256 watts.
Your speaker choice makes all the difference in the world on how much power is required to hit the volume levels that you seek.

Timlub, I'm pretty sure you will need over 400 watts to drive an 84 db speaker to 111 db.

Here's a useful web page for things like this:

http://www.sengpielaudio.com/calculator-gainloss.htm

take a look at the chart at the bottom of the page.

I appologize, I had just run the numbers off the top of my head, but the formula is fact, not opinion, so
84 @1 watt 105 @256
87 @2 watts 111 @512 watts @ 84db sensitivity
90 @4 111 @ 64 watts @ 93db sensitivity
93 @8 I missed one line in figuring.
96 @16 110 @ 16 watts @ 98db sensitivity speakers
99 @32
102@64
105@128
Thanks Atmasphere, your chart is correct, but I was quoting how much power it took to achieve a 3db gain, not how much voltage it takes to double your volume. My numbers are correct with all things being equal.