Average vs RMS

[Nigel Goodwin]

In your first post you quote that "RMS Power" is "wrong", but it's being used by the audio industry and has been used for a number of years now.

Hardly a 'number of years'

As far as I'm aware it's been used all the time the electronics audio industry has existed, and certainly well before WW2.

Also, as far as I'm aware, NOTHING else was ever used, until the American advertisers got involved, and started claiming amplifiers were 150W etc. - when they were only 5W or so - that was probably late 60's?, early 70's?.

 

[MrAl]

Hardly a 'number of years'

As far as I'm aware it's been used all the time the electronics audio industry has existed, and certainly well before WW2.

Also, as far as I'm aware, NOTHING else was ever used, until the American advertisers got involved, and started claiming amplifiers were 150W etc. - when they were only 5W or so - that was probably late 60's?, early 70's?.

Hello again,

I am not sure what you mean here. "A number of years" is a number of years, without specifying how many exactly. That could be 1 year to 200 years, but in this case it is the number of years since the Federal Trade Commission forced the audio industry to adopt the standard. But it's definitely less than the time the audio industry has been around. The reason for the ruling was because before that the various manufacturers would make up their own ratings and thus make their own equipment look better than some other makers equipment. It's also not one set number of years either because they sometimes update the ruling to help make it more beneficial to consumers given any changes in technology or usage over time. The last time i know of was in 2001 which was only about 15 years ago, but there could have been more recent changes that i dont know about. Also as far as i know the rule started in 1974.

 

[audioguru]

My little Name Brand 2.1 powered speaker system is advertised as, "150 Watts Total Output and Output RMS 75W". But inside it has a few small power amplifier ICs with little bits of metal glued on top as heatsinks and the transformer is marked "9VAC/1.1A" which is 9.9W . But the ICs are class AB that have about 60% efficiency so the actual output power is only 6W.

The difference between average power and RMS power is so small that it cannot be heard because our hearing's sensitivity to loudness is logarithmic so we can hear things that are very low level and hear very loud things. Double or half the power is just a small loudness difference. 10 times the power sounds twice as loud and 1/10th the power sounds half as loud.

My 52 years old Scott stereo receiver suddenly produced lots of hum. The 5000uF/50V main filter capacitor has no microFarads anymore. I tried to remove big capacitors from amplifiers I do not use anymore but they are glued down so I cannot remove them.

 

[MrAl]

My little Name Brand 2.1 powered speaker system is advertised as, "150 Watts Total Output and Output RMS 75W". But inside it has a few small power amplifier ICs with little bits of metal glued on top as heatsinks and the transformer is marked "9VAC/1.1A" which is 9.9W . But the ICs are class AB that have about 60% efficiency so the actual output power is only 6W.

The difference between average power and RMS power is so small that it cannot be heard because our hearing's sensitivity to loudness is logarithmic so we can hear things that are very low level and hear very loud things. Double or half the power is just a small loudness difference. 10 times the power sounds twice as loud and 1/10th the power sounds half as loud.

My 52 years old Scott stereo receiver suddenly produced lots of hum. The 5000uF/50V main filter capacitor has no microFarads anymore. I tried to remove big capacitors from amplifiers I do not use anymore but they are glued down so I cannot remove them.

Hi,

That's funny, but have you been reading the other posts here? RMS power is a misnomer taken literally but was still being used by the industry due to rules made by the FTC. RMS power in the context of what it really means is exactly the same as average power.

Let me guess, the back label reads, "Input: 120vac 60Hz, 10 watts rms", ha ha.

 

[The Electrician]

There's some discussion of all this elsewhere on the web:

https://www.n4lcd.com/RMS.pdf
https://hifi-writer.com/he/misc/rmspower.htm
https://www.eznec.com/Amateur/RMS_Power.pdf

Audio power amplifiers currently offered for sale in the U.S. are required to describe their power rating (if any rating at all is given) as "minimum sine wave continuous average power output, in watts," See: **broken link removed**

 

[Nigel Goodwin]

That could be 1 year to 200 years, but in this case it is the number of years since the Federal Trade Commission forced the audio industry to adopt the standard.

What has the Federal Trade Commission got to do with it? - it's not an 'American' Standard, and LONG predates the dates you suggested.

However, if American manufacturers used to make up their own standards it perhaps explains the laughable specifications on American equipment in the 70's

 

[audioguru]

It looks like the Sylvania 2.1 amplified speaker system I showed did not violate the American rules by lying about 150 Watts when the system produces only 6W because it was ordered from China and I am sure they have an e-mail ordering a 150 Watts system.

 

[MrAl]

What has the Federal Trade Commission got to do with it? - it's not an 'American' Standard, and LONG predates the dates you suggested.

However, if American manufacturers used to make up their own standards it perhaps explains the laughable specifications on American equipment in the 70's

Hi,

Not sure what you are suggesting here, like the standard was around since the cave men? (he he)
When they banged rocks together to call each other they could not be sure which rocks made the loudest sound, so they came up with "RMS Power", which to them meant, "Rock-Man-Sounds Power"

I am naturally mostly concerned with what happens here in the USA, but yes if you live in another country then you'd have to check that out to see what they call it there and when it started. Also, check around Stonehenge for clues from the past

From the Electrician's post it appears that the FTC has made an amendment to call it "average power" now, which is more technically non confusing.

 

[MrAl]

It looks like the Sylvania 2.1 amplified speaker system I showed did not violate the American rules by lying about 150 Watts when the system produces only 6W because it was ordered from China and I am sure they have an e-mail ordering a 150 Watts system.

Hi again,

I would not be surprised if they lied about it and knew it was a lie.

Their excuse would be one of:

"Well if anyone actually believes that rating then they dont know much about audio power."

Or:
"Sorry, the type setter forgot to put in the decimal after the 1."

Or:
"We use the metric system here, and always have."

Or even:
"It rained yesterday."

 

[tomizett]

Interesting topic here guys - I've been saying "RMS power" since forever without giving it a second thought; it seems I may have been wrong!

To try and get a clarification on the original question (as I understand it, at least) am I correct in saying we are all agreed that "Average Power" and "RMS Power" are the same thing. Even if the nomenclature is not correct, these two terms *don't* represent different measurements that might produce different results when carried out on the same piece of equipment?

 

[MrAl]

Interesting topic here guys - I've been saying "RMS power" since forever without giving it a second thought; it seems I may have been wrong!

To try and get a clarification on the original question (as I understand it, at least) am I correct in saying we are all agreed that "Average Power" and "RMS Power" are the same thing. Even if the nomenclature is not correct, these two terms *don't* represent different measurements that might produce different results when carried out on the same piece of equipment?

Hi,

Yes. RMS Power refers to the power when using RMS values to calculate it. This would differ from using peak values to calculate the power.

For example, if the RMS voltage is 10 volts and the RMS current is 1 amp, then the RMS power is 10*1=10 watts.
If we used peak values, then the peak voltage is 14.14v and the current is 1.414 amps which comes out to 20 peak watts, and if we used the RMS conversion factor of 0.7071 we would get 14.14 watts which would not be correct, but if we knew what we were doing we would realize that the two waveforms when multiplied together do NOT form a sine wave and so we can not use the conversion factor of 0.7071 (see below 'crest factor'), we must use the conversion factor of 0.5 instead. That would give us 10 watts too.

Another way of looking at it is that when we do a conversion using a simple constant factor like K=0.7071 we have to be aware of the wave shape, and that would be called the "Crest Factor". What we learned is that we can not blindly apply a crest factor to a problem WITHOUT knowing the wave shape. Obviously the crest factor for a squared sine is 0.5 and so we must use that factor instead to calculate the RMS power. Interestingly when we do take the time to observe the wave shape and correct crest factor, we do in fact get the right value which in the above example was 10 watts. So "RMS Watts" does in fact make sense, we just have to understand that when we use a short cut like the 0.7071 rule it is actually a crest factor and since that applies only to a sine wave we have to find the right factor for this new wave, which is a squared sine wave and therefore we must look up the correct crest factor for that wave instead and observe that we are calculating power not an actual RMS value.

So we have:
RMS Voltage=PeakVoltage*CrestFactor
RMS Current=PeakCurrent*CrestFactor
and lastly we have:
RMS Power=PeakPower*CrestFactor

so there is really no difference except we must have the right crest factor to fit the waveshape and application we are dealing with. The crest factor in this case may have to be called something else however such as the crest factor squared. Also, the actual crest factor may be 1/K as applied here.

And yes, RMS Power in this context means the same as Average Power and it looks like the standard at least in some places in the world was changed to read "Average Power" instead.

 

[Nigel Goodwin]

Interesting topic here guys - I've been saying "RMS power" since forever without giving it a second thought; it seems I may have been wrong!

Not at all, that's how it's always been (and still is), it allows you to accurately compare different systems, and to easily measure that the system meets the claimed specification.

To try and get a clarification on the original question (as I understand it, at least) am I correct in saying we are all agreed that "Average Power" and "RMS Power" are the same thing. Even if the nomenclature is not correct, these two terms *don't* represent different measurements that might produce different results when carried out on the same piece of equipment?

I don't really see how 'average power' is at all useful?, unless of course it's simply a short term for something much longer that explains it? - like RMS power is.

 

[The Electrician]

Wikipedia has an article that discusses various kinds of "audio" power, e.g., peak power, music power, average power, etc.: https://en.wikipedia.org/wiki/Audio_power

This article mentions the 1974 FTC rule that required use of the term "RMS power" in published ratings, but it fails to note that the requirement has changed, and the current FTC rule now requires different phraseology, namely "sine wave continuous average power".

 

[nsaspook]

**broken link removed**

 

[MrAl]

Not at all, that's how it's always been (and still is), it allows you to accurately compare different systems, and to easily measure that the system meets the claimed specification.



I don't really see how 'average power' is at all useful?, unless of course it's simply a short term for something much longer that explains it? - like RMS power is.

Hi there,

"Average Power" is useful because that measurement/calculation is more technically correct in as much as the wording goes, or at least less confusing.

There is a mathematical definition of average power that can not be confused with anything else, and that is the power calculation we use to calculate the actual power in a device of any kind. The definition is:
Pavg=(1/T)*integral of p(t) with respect to t
where t is time and T is the total period, and p(t) is the instantaneous power which could be for example v(t)*i(t) which is the voltage times the current in time.

Also as Electrician noted, the FTC changed the definition for audio work to average power now maybe to be less confusing and thus remove any remaining wiggle room for the manufacturers.

For example if Pavg was 1000 watts and we ran that device for 20 hours and paid 10 cents per kilowatt hour to the electric company, we would be paying them 20*0.10=$2.00 for that 20 hours usage. If we later ran it for another 20 hours we'd be paying another 2 dollars.
So it's the actual power and so that means the actual heat generated by a load would be based on that measurement/calculation.

 

[Nigel Goodwin]

So it's the actual power and so that means the actual heat generated by a load would be based on that measurement/calculation.

Which is EXACTLY what RMS power is - without the confusing mention of 'average'.

 

[MrAl]

Which is EXACTLY what RMS power is - without the confusing mention of 'average'.

Hi there Nigel,

Sorry, i dont understand what could be confusing about "average power" when it is a well defined quantity in electrical engineering.
For example an EE handbook would contain the formula given previously, which is the mean of the instantaneous power taken over the total time period. So it's a very universal quantity rather than a quantity specific to one given application like audio. This means it could be used in say power line calculations as well as audio work. In each case we would get a result that would match any other person's result on earth, or really anywhere in space too

 

[MikeMl]

Which is EXACTLY what RMS power is - without the confusing mention of 'average'.

Ok Nigel,

How would you describe the power produced by the dotted box (or dissipated by the resistor) in this simple circuit?




Here is my (unambiquous) description of what I see:
With the 50% duty-cycle shown, the power dissipated by the resistor is 5W.
If the switch was closed 100% of the time, the power dissipated by the resistor is 10W.
The instantaneous peak power is 20W, as shown by the green trace.

Notice that I didn't mention RMS or Average....

 

[Nigel Goodwin]

Ok Nigel,

How would you describe the power produced by the dotted box (or dissipated by the resistor) in this simple circuit?

I wouldn't (in the context of this thread), as it has no bearing on it.

However, you 'could' use the 'average' from it (as you've done below), which is why 'average' is a confusing term, and has no bearing on the power output measurement of an amplifier.

Here is my (unambiquous) description of what I see:
With the 50% duty-cycle shown, the power dissipated by the resistor is 5W.
If the switch was closed 100% of the time, the power dissipated by the resistor is 10W.
The instantaneous peak power is 20W, as shown by the green trace.

Notice that I didn't mention RMS or Average....

 

[MikeMl]

I wouldn't (in the context of this thread), as it has no bearing on it....

Huh? It has everything to do with power measurement, unless, I guess, you are an audiophile...