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RMS Power?

I often see power specifications like "100W RMS".  There is no such thing as RMS power.  Of course, you can calculate RMS value from any curve, including power curve, but it won't represent anything.  "Real" power representing heat dissipated in resistive load is "Average Power"   Pavg=Vrms*Irms.   In case of sinewaves Pavg=0.707Vpeak * 0.707Ipeak = 0.5Ppeak,  or Ppeak = 2Pavg. 

Term "RMS Power" or "watts RMS" is a mistake, very common in audio.
128x128kijanki
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kijanki OP
3,575 posts
 
Nobody interested?  Really?
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atmasphere
7,068 posts
 
The term refers to the fact that if you are to calculate the output power, then the AC voltage used in the calculation must be RMS volts.

To do this, first find the peak to peak AC voltage that is the output of the amplifier into a known load such as 8 ohms (usually non-inductive).

Then divide by 2 to get the peak voltage. Then multiply by 0.707 to obtain the RMS voltage. Then perform the power formula on the result.
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kijanki OP
3,575 posts
 
That’s true, but it is called average Power in every engineering textbook. Unit of watt already signifies power convertible to heat - a "real" power.

Pavg = 0.5Ppeak

Taking RMS of power curve would bring different, totally irrelevant result.

With sinewaves sin(x) for power would be squared sin^2(x)
|1. Square: (sin^2 (x))^2 = sin^4(x)
2. Mean: Integral of sin^4(x) over 0-pi is 3/8
3. Root: SQRT(3/8) = 0.61

Prms = 0.61Ppeak

This number does not represent anything useful.

Faulty term watts rms or rms power came most likely from around 1975 FTC standard for power rating. FTC realized, that they goofed up and 25 years later in 2000 they corrected it .
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