RMS Power?

imhififan, Thank you for posting.  Example of the wrong average power calculation in the second article is very interesting.  Using averaged values of voltage and current produced error of 50%.  It is very common error.  People who design test and measurement equipment (often big companies) make this mistake all the time.  For instance, when calculating mechanical power, they take average (filtered) values of speed and torque and multiply them, instead of taking instantaneous values of both, multiply and average (filter) the product - mechanical power.  For constant torque and speed it does not make a difference, but as soon as oscillatory component appears error becomes significant.  Even single percent error is very important for efficiency calculation.  Of course it has nothing to do with audio, but shows that "Power" is a muddy subject.

Thanks for the nice words, Kijanki. But you’ve explained it well, IMO, as did the summary quoted above by Imhififan. And I of course agree with your point.

As you indicated, the references to RMS power or watts RMS that are often seen are actually references to average power, which are calculated as the product (multiplication) of RMS voltage and RMS current (assuming voltage and current are in phase with each other). And as you and the reference provided by Imhififan both said, RMS power is certainly something that can be calculated for a given power waveform, but it would be a different number than average power, and it would be a quantity that has no relevance or usefulness. While average power, being proportional to heat generation, is relevant and useful.

Widespread usage of incorrect terminology doesn’t make it correct. Just as widespread misuse of the English language doesn’t make it correct. For example, many people would say that they could care less about this topic, even though that is the opposite of what they mean :-)

Best regards,
-- Al

@almarg 

Widespread usage of incorrect terminology doesn’t make it correct. Just as widespread misuse of the English language doesn’t make it correct. For example, many people would say that they could care less about this topic, even though that is the opposite of what they mean :-)

IMO you have to be pragmatic about these things. The industry isn't going to shy away from 'RMS power' anytime soon. Its a 'charged term' that has a life of its own.

Other examples of charged terms:

'output impedance' is not the measure of the actual impedance of the output circuit, its the measure of the servo (feedback) gain of that circuit. Otherwise we would have a problem with Kirchoff's Law. Put another way, if adding loop negative feedback really lowered the output impedance of an amplifier, the result would be that the amp could make more power into lower impedances. But the fact is that the only way to do that is to add more output devices, more heatsinks (or bigger output transformers) and so on. But adding loop feedback definitely lowers output impedance! That is because 'output impedance' is a charged term.

'Conservative' (political term) refers to a person that will drive a Prius rather than a Ford Excursion, will work to protect the forest rather than log it, will work to reduce global warming etc, but most people think it means the opposite. 

(the latter is an example of how messed up our politics are that liberals are conservative and conservatives are liberal...)

We deal with charged terms that mean other than what we are saying all the time. Its part of what makes the human experience so fun :)

Output impedance term can be complex. Perhaps we should rate it in ohms RMS for clarity? :)