How much Power do speakers really use ???????

Weiserb:
This is antectdotal;guys with physics degrees,please cut me some slack.
The softest sound,at the softest pitch,(a flute)is about 50 decibels. On a music score,that is notated pp. Each time you double the loudness,add 10 decibels. That is p is 60db,mp is 70db,mf is 80db,f is 90db,and ff is 100 db. A modern orchestra with everybody playing as loudly as they can at the same time is 100 decibels. Some rock bands play at 110db.
If your speaker is rated at 8 ohms resitance, count 1,2,4,8,etc. If your speakers is rated at 4 ohms,count 2,4,8,16,etc. If it's rated at 6 ohms,count 1 1/2,3,6,12,etc. Everytime you move up a "Counting step" add three decibel watts.
Say your speaker is rated 90db at 8 ohms. At one meter,the speaker will have a loundness of 90 decibels being fed one watt. At 2 watts,93 db;at 4 watts,96 db;and so forth. At 90 db into 4 ohms,it would be 90db at 2 watts;93db at 4,and so forth.
It's true,as Sean says, that different speakers have different rates of change at different distances,but anecdotaly,I'd allow 6 db'ds of headroom.

Ok, I've got a question. I've got a pair of B&W 803's in a 12x20ft room. Moderately furnished. I play classical, jazz and hard rock almost about evenly. I like my music at concert levels some of the time and others loud enough so I have to say "what" when someone interrupts me. I rarely listen to music below that, but on occasion.

I am interested in buy a CJ MW60 or Premier 11a. Enough power? How about a Rogue 120 Magnum?

Busaganashi:
To double check my facts,I looked up the B&W Nautalis 803s on their web site;they are rated 90db into 8 ohms.
So 90db would require 1 watt,93db-2 watts,96db-4 watts,99db-8 watts,102db-16 watts,105db-32 watts,108db-64 watts,and 111 db-128 watts.
Sounds to me as if 110-120 Clean watts would fill the bill-with headroom to spare.

So it sounds like the Rogue 120's or perhaps a CJ Premier 12 or 8 is what I need then. Thanks.

I would beg to differ with the conclusion that Pragmatist came up with regarding the B&W's. The figures that he quotes are for a listening distance of one meter from the speakers AND do not take into account speaker compression. I don't know of a speaker made that is linear in terms of input vs output, especially when one starts hitting them with some juice. This has to do with thermal losses in the drivers and crossover components and is pretty much unavoidable to one extent or another.

As such, one should not expect to hear anywhere near the spl's quoted at their seated listening position unless one sits very near their speakers and has a very small room. To achieve the figures quoted at a typical listening distance, i would count on needing at least twice the power ( and more realistically four times that amount ) in order to keep the amp from getting nasty due to being pushed too hard. This is especially true if running a typical SS design.

My experience with speakers rated at 96 dB's and an amplifier rated at 120 wpc in a similarly sized room supports the above statements that i made. I kept popping tweeter diaphragms from driving the amp into hard clipping. Given that these speakers are four times as sensitive as the B&W's in question, i would not count on having enough muscle and doing it cleanly unless you had quite a bit of power. Sean
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