It seems that you are asking a question about loudspeakers more than the amp... here is a good web page to calculate these things:
Efficiency and sensitivity conversion - loudspeaker percent and dB per watt and meter loudspeaker efficiency versus sensitivity vs - sengpielaudio Sengpiel Berlin
For typical home speakers, you are looking at significantly less than 1% efficiency, meaning that more than 99% of the power delivered by the amplifier is dissipated as heat.
-Charlie
What I find interesting, and related to this topic, is the power ratings found on some tweeters. For instance, a rating of 100W or more on a tweeter is misleading. If you delivered 100W of power to a tweeter it would be toast in a few minutes. Think of how hot a 100W incandescent light bulb gets! Really they mean that the system power can be 100W when the tweeter is used with a conservative crossover (but what that is really is almost never described). Because of the small size of the voice coil and former, the "real" power handing of the tweeter is more like 10W max. A crossover reduces the power to the tweeter by restricting the signal bandwidth to it, and this lowers the 100W total input to something on the order of 10W to the tweeter, less if "crossed high".
-Charlie
A very common mistake to think that. And if the signal was direct current and the speaker a pure resistance. That would indeed be the case.
However, an audio signal is by nature alternating current and a dynamic loudspeaker is a reactive load which if it wasn't for parasitic effects would show a perfect 90 degree phase shift as any other inductor will.
In effect a loudspeaker is a resistive reactive load.
Please read on this basic principle of alternating current electronics. For example here gives a good brief overview: Power in resistive and reactive AC circuits : POWER FACTOR
Huh? Nothing CharlieLaub said indicates he doesn't understand impedance, (perhaps better than most). What are you trying to say; what's your point? That some of the (in)efficiency is not only due to just power lost as heat in the voicecoil? Many AC heaters are somewhat inductive...some make beautiful music...
Maybe, but that doesn't change that fact that the spec sheet for a driver shows an efficiency figure right on it, mechanism be dammned!
The o/p did not ask for a dissertation on reactive loads or power factor. I assume that the answer expected (via our replies) should not be on that level.
-Charlie
Ah. But you see that is integral part of where the power goes in the system.
The efficiency in a speaker specification only shows the amount of power that is converted to sound. To assume that the remaining power is lost as heat is false as a speaker is a resistive reactive load, and not a resistive load.
The o/p actually asked what percentage of the power sent to the speaker is dissipated as heat. And the answer is that depends on the speaker and the relationship between it's reactive and resistive load elemtents. The efficiency of the speaker can in fact be ignored as the power converted to sound is a subset of the resistive load, remembering that the reactive load element exhibits zero loss.
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