Originally Posted by bbrio
I'm simply concerned that we rarely see any scientifically determined measurements of "distortion".
I recall I've come across some good stuff before about this, but I'll have to dig it up. I suspect some of the better educated forum members might have such resources more at hand.
Compression is a form of distortion, in that what goes in is not what comes out. The compression/distortion effects can be seen potentially from two sources: the amplifier and the speaker.
Your speaker is sensitive enough for the volume you're attempting as long as the power delivered to the speaker doesn't result in compression/distortion effects. At some percentage of its power handling these will appear. (Still waiting to hear others confirm a generalization of where this occurs). Your amp is powerful enough as long as the watts you're being asked to output doesn't send it into clipping.
The problem is, we over-drive speakers without realizing it, and we clip amplifiers without realizing it!Barely Good Enough
With 86db sensitivity speakers, you can listen as -10dBfs, where your amp will be at 1 watt for 75db average level. To get the 20db dynamic peaks (95db) at that volume level your amp will be tasked with 100x the watts, or 100 watts. Assuming your amp really can output 100 watts with the necessary channels driven for that program material, and the speakers' power handling are several times higher than 100 watts, then I think there won't be compression/distortion.Low Power-Handling Speakers
With speakers whose power handling is only 150 watts, I think you'll have distortion. (but I'd like to hear from others what they think the extent would be. Is the speaker-loafing distortion of 5% going to rise to 10%? 20? Will you still get the 95db peaks?)Too Weak an Amp
If we instead want to listen to -5dBfs, that's only 5dB higher. But this would require 340 watts! While I've heard of amps that can provide 1dB to 3dB of peak power (dynamic headroom), the length of time may well be small for repeated or sustained peaks of a few seconds. The capacitors are quickly drained. 3dB would only get you to 200 watts, anyway. You're going to be clipping, where you're producing square waves that are twice as hard on the speakers. The sound is strained, harsh, grainy, and it makes people say, "Wow, that's LOUD (uncomfortable)!" Instead of the ideal, "Wow, that's powerful, and incredible!"
I'm sure you can imagine an in-between case, where the amp was really 100 watts two channels driven, but 70 watts three channels driven. And it'll be even a little less to the front three, when the surrounds are also probably being somewhat powered.
Some great info here: http://www.rocketroberts.com/techart/powerart_a.htm"Too Loud", or Too Distorted?
It may sound crazy to want -5db, but in a large room, or an acoustically treated room, it's not as loud as you think, and if we've heard clean dynamics before, we'd want them in our own room too! The absorbtion sucks up more of the reflected sound, and that lack of reflections lowers the overall level versus an untreated room.
When it's not distorted, people are surprised at how effortless it sounds. If you take a great singer who only does well at low volumes, and ask her to belt out operatic levels, she'll sound terrible. "Too loud!" you tell her. But really, it's that you don't like how she
When grand pianos get up to 109dB (for brief peak periods), it's not unreasonable to ask for peaks approaching that in our own rooms.
Originally Posted by bbrio
So, if I really liked the sound of a certain speaker, but it was not efficient, I would simply plan for a higher rated amp. I would NOT base my speaker choice on what my amp is capable of.
But, depending on the speaker, room, and our occasional listening desires, that amp might have to be 5,000 or 4,000 watts!