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Hadn't thought about that actually... I still highly doubt you would use more than 1 watt.
http://myhometheater.homestead.com/splcalculator.html is giving me 85 db at 15 ft not considering positioning. (using 96 db efficiency)
85 db efficiency gets about 75 db at 15 ft which is quite loud but not unfeasible for some people here to listen to.
Transients are 1 big reason heaps of power might be required. Not nearly so much with 96+ dB/Watt speakers, but speakers which hit that number
honestly (e.g. not taking room gain into account, which would be doubled by that online calculator) are rare, and you'd still need well above 1 watt to make "lifelike" levels for music with a healthy crest factor (overly compressed/brickwall'd recordings, e.g. RHCP, need not apply). A continuous/sustained 106dB level is going to be too damn loud for most folks. However, 106dB peaks on music with a 20-30dB crest factor (i.e. 75dB-85dB average levels) is quite enjoyable, and some would argue it's the proper way to listen to many fine classical recordings (or even higher). Many amps only supply an extra 3dB or so of power for peaks (again, if they're
honest) vs. their continuous power rating. So the crest level -- not the continuous level -- is almost always going to be the limiting factor, and thus what we should use in these calculations (which btw are just "good" estimates). So let's do those calculations again with a target of 106dB peaks:
1 Watt gives us 96dB @ 1m. Let's assume only 3 meters distance:
=> 6 * log2(1m/3m) = -9.5 dB (loss of almost 10 dB)
=> +6dB for stereo speakers
=> +3dB for estimated room gain
Add them up: now we've about broken even, at 95.5dB for our 1 Watt. We'll need to add power (over 1 Watt) to hit 106dB peaks:
106dB - 95.5dB = 10.5dB
10.5dB = 10 * log10(P/1Watt), where P is the power (in Watts) we're solving for:
P = 11.2 Watts (peak)
So we'd need an amp that can provide 11 Watts peak in this situation (btw the requirement would be a stout 87 Watts if our speakers were only an ordinary 87dB/Watt!). That's a lot more than 1 Watt, but still not much. What about 112dB peaks (still well below the levels of some venues)? That would require 44 Watts (96dB speakers) or 348 Watts (87dB speakers).
Now here's where audiophile madness comes into play: many stereo amps -- especially tube amps or single-ended amps -- rise in distortion up to their rated power. Also, speakers are not a purely resistive load, and can be much more demanding on an amp than their nominal impedance would indicate. The more headroom you have, the lower your distortion is likely to be -- though obviously this yields reduced gains as your power requirements become so minimal that the noise floor begins to dominate. How much headroom you decide you'll need is a very subjective call -- but this being the summit-fi forum, I assume we'll have plenty of "enough is never enough" kind of guys/gals around here. A modest 10dB of headroom would require 110 Watts of peak power capability in our example (440 Watts for 112dB peaks), or 870 Watts (3,480 Watts for 112dB peaks) with an 87dB/Watt speaker! These multiply out very quickly...
Either way: yes, 250 Watts is a LOT for a 96dB/Watt speaker. A stout 60 Watt/ch amp would be unlikely to ever clip audibly in my situation (and you DO want to avoid clipping, even by accident, and even with tube amps), even with my "let's push the volume" tendencies. I'm a big fan of efficient speakers -- I think it's impossible to get back everything by just bludgeoning the speakers with brute power, once you give up efficiency (e.g. heat dissipation issues) -- and just happened to have really powerful tube amps at the time I upgraded speakers. I'd consider less powerful tube amps in the future (the Atmasphere 60 Watt OTL monos look niiice), but for now it sounds superb so I'll ride out this rare streak of contentedness.
