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RMS wats to watts

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John Sorensen

New Member
Watts is watts is watts.

RMS (root-mean-square) is just a way of calculating power (or voltage or current) especially when a waveform is non-linear.

j.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
RMS watts are what you need (no pun intended).

I'm presuming you are refering to audio amplifier output powers?. The only power spec you should look at to compare them is RMS watts - anything else is meaningless, and can't be compared.

It seems mainly an American advertising idea to make amplifiers appear much larger than they are - they make up all sorts of idiotic specifications just to make the figure look bigger.

In the past I've seen 10W per channel amplifiers advertised as 100W in the USA - and I'm sure we've all seen computer speaker systems advertised as 400W! - funny when the drivers have 5W stamped on them.
 

Johnson777717

New Member
How are they getting away with these kinds of advertising specifications?

I'm looking at car stereo amp, stated for 750 watts, yet the specifications state 220 watts one channel bridged for 4ohm impedence. Where are they comming up with 750 watts? :shock:

How can they advertise 400 watt stereo speakers when the chip is 5W??? What the heck is going on!!! :evil: :evil: :evil: :evil: :evil:
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ron H said:
Just to nitpick - RMS power is technically a meaningless term. See Why there is no such thing as 'RMS watts' or 'watts RMS' and never has been.

I think he's talking a load of rubbish!. As HiFi magazines usually do.

RMS power is the only decent way to report amplifier specification, mainly because it's very easily done and can easily be checked.

To qualify it a bit more, it shouldn't just say '60 Watts RMS', although even like that it's more comparable than any other specification. What it should say is '60 Watts RMS, continuous power into 8 ohms, both (or all) channels driven at less than 0.006% distortion' - this is actually the spec from my old Kenwood amplifier.
 

Dean Huster

Well-Known Member
RMS Watts

First of all, J.C. Watts is an Oklahoma politician. I don't know WHY they named that unit of measure after him. Oh, wait. Some guy looking over my shoulder just corrected me.

Isaac Watts (1687-1748) was a great church hymn writer(e.g., O God Our Help In Ages Past, Joy To the World, et. al.), one of the great musicians of the world, and hence the term for watts as Peak Music Power and ..... oh, wait .... some other clown behind me is saying that's a bunch of bunk. He says ...

Oh, James Watt, the steam engine developer was the one for whom the watt was named and ..... wait a minute ..... what? Another opinion? What do you mean, it was named after the Watts area of racial riot fame? Power to the people? That's where it came from? Wait a minute ....

Now my daughter is telling me that she thinks the term is named after the guy who invented light bulbs .... you know .... his name is stamped on every one of them .... 40, 60, 75, 100 .... and I thought that had something do to with the price.

Wats dat? Oh. RMS Watts isn't the real term that should be used anyway. Neither is Peak Watts, or watts music power. It's supposed to be "Watts DARIOT" .... that's, "watts, Dean's and Ron's Idea Of Truth". Ron, I forget that between the two of us, we have all the world's knowledge. Yeah, folks, we're THAT old. Come to think of it, I just had my birthday yesterday, January 12th.

"Gimme dat ol' time Social Security, gimme dat ol' time Social Security, ..... it's [gotta] be good enough for me."

Dean
 

Roff

Well-Known Member
Nigel Goodwin said:
Ron H said:
Just to nitpick - RMS power is technically a meaningless term. See Why there is no such thing as 'RMS watts' or 'watts RMS' and never has been.

I think he's talking a load of rubbish!. As HiFi magazines usually do.

RMS power is the only decent way to report amplifier specification, mainly because it's very easily done and can easily be checked.

To qualify it a bit more, it shouldn't just say '60 Watts RMS', although even like that it's more comparable than any other specification. What it should say is '60 Watts RMS, continuous power into 8 ohms, both (or all) channels driven at less than 0.006% distortion' - this is actually the spec from my old Kenwood amplifier.
He's not the only guy to say this. I had never even thought about it until one of my engineer colleagues at work made the same statement. He had the fact hammered into his head many years ago by a college professor. If you want more evidence, do a Google search for "rms power meaningless" (without the quotes).
The point is, RMS power really refers to the average power dissipated by a resistive load which is driven by a given RMS voltage (or current). If you use the term literally, as in "the square root of the mean of the square of the instantaneous power", then you get a meaningless answer. For example, I ran a little sim on SwitcherCAD III (Linear Technology's free Spice-based simulator) in which a 1 volt peak sine wave drove a 1 ohm resistor. The average power is 0.5 watts, which SWCAD reports correctly. SWCAD will also dutifully calculate the RMS value of the instantaneous power (which is a double-freq sine wave with the +peak at 1w and the -peak at 0w) as being ~611mw. What does this mean? Nothing. As I said, RMS power, taken literally, is meaningless. However, most of us understand that "RMS power" really refers to the power dissipated by a resistor when an RMS voltage (or current) is applied to it. As I said, I was nitpicking. The term "RMS power" doesn't bother me, but it apparently annoys the hell out of some people (present company excluded).

There. Have I sufficiently obfuscated the issue?

BTW - Dean, thanks for supporting me (I think). And Russ - my avatar is an actual monument (made many years ago, of course) on the face of Mars, honoring me.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ron H said:
He's not the only guy to say this. I had never even thought about it until one of my engineer colleagues at work made the same statement. He had the fact hammered into his head many years ago by a college professor. If you want more evidence, do a Google search for "rms power meaningless" (without the quotes).

All they appear to be doing is arguing about the words!. 'RMS Watts' is defined as "the value giving the same heating effect as the equivalent DC power". It's easily measurable, easily calculable, and easily used to compare powers between amplifiers.

The point is, RMS power really refers to the average power dissipated by a resistive load which is driven by a given RMS voltage (or current). If you use the term literally, as in "the square root of the mean of the square of the instantaneous power", then you get a meaningless answer. For example, I ran a little sim on SwitcherCAD III (Linear Technology's free Spice-based simulator) in which a 1 volt peak sine wave drove a 1 ohm resistor. The average power is 0.5 watts, which SWCAD reports correctly. SWCAD will also dutifully calculate the RMS value of the instantaneous power (which is a double-freq sine wave with the +peak at 1w and the -peak at 0w) as being ~611mw. What does this mean? Nothing.

I'm somewhat bemused by the result from SWCAD - but I never use such things.

It's very simple to work out (for sine waves), first convert the peak voltage to RMS by dividing it by 1.414 - 1/1.414=0.707.

Then square it and divide by the load (0.707*0.707)/1 = 0.5W.
 

Roff

Well-Known Member
'RMS Watts' is defined as "the value giving the same heating effect as the equivalent DC power".
While I agree that this is basically a semantic argument, the definition above is actually the definition of average power. Why do we need two names for the same thing? Ok, I know, we have two (or more) names for lots of things.

I didn't mean to leave the impression that I didn't know how to calculate sinewave average power. I was simply illustrating that, if you apply the RMS operation to instantaneous power, you get the wrong answer.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ron H said:
While I agree that this is basically a semantic argument, the definition above is actually the definition of average power. Why do we need two names for the same thing? Ok, I know, we have two (or more) names for lots of things.

I didn't mean to leave the impression that I didn't know how to calculate sinewave average power. I was simply illustrating that, if you apply the RMS operation to instantaneous power, you get the wrong answer.

I don't see the reasoning behind 'average power' at all, it doesn't make any sense! - RMS voltage is a mathmatical expression, for a sinewave it's 0.707 of the peak voltage - Watts RMS is calculated from that voltage.

As I see it 'average power' means exactly what it says - say 100W RMS for 10 minutes, then turned off for 10 minutes = 50W average power.

I don't see how you can apply the RMS power operation to 'instantaneous power', the whole point of Watts RMS is that it's continuous sinewave output.

Actually, I've never even seen a specification 'average power', is it another purely American term?.
 

ALARMED

New Member
Hello! -My first post- Would anyone have a good clarification between volt-amps (VA) and watts? Seems to me they should be the same, but I've heard differently.
 

Styx

Active Member
Err true VA (or Volt-Amps) and Watts are the same units.
However this is more to do with Industry. In home we are charged by the kWh regardless of what we draw - basically the arguement is that a house does not draw that much power and thus cannot greatly effect teh THD of the main utility. This has recently changed due to all the diode circuits present in a house and appliance builders now have maximum distorted current waveform they can draw (hence the ferrite chokes around the PSU of new appliances)

In industry they are charged by the Volt-Amps. This is due to the fact that industry are more likely to have large capacitive loads or inductive loads (as seen by the utility supply) and also use alot of power. This will cause leading and lagging power to be drawn. By charging by Volt-Amps and not Watts it punishes people who draw crap power from the supply and thus messing everyone else up at the point of common coupling - Again to do with THD (Total Harmonic Distortion) - how much like a sine wave the supply's voltage and current waveform is.
 
Watts RMS

Really quick question... Why can't he just convert watts rms to watts peak which is as simple as multiplying his watts rms by 1.414 or dividing his watts rms by .707
 

ljcox

Well-Known Member
Styx said:
Err true VA (or Volt-Amps) and Watts are the same units.

This statement is only true if the phase angle is zero!

Volt Amp is called "Apparent Power" since it is simply the current multiplied by the voltage (assuming an AC sinewave source).
The actual power is the energy consumed by the load per second.
Actual Power = Apparent Power * cos(theta) where theta is the phase angle.

Len
 

spuffock

Member
The calculation for american peak watts is as follows. First find by experiment the highest impedance load that will make a noticeable sound(Rnn), then using a storage scope determine the best microsecond short circuit current (IpkDESTRUCT) , then : WATTS(PIMP)=(IpkDESTRUCT)^2 * (Rnn) ohms. You can only measure this once.
 

pike

Member
Re: Watts RMS

Painandsuffering said:
Really quick question... Why can't he just convert watts rms to watts peak which is as simple as multiplying his watts rms by 1.414 or dividing his watts rms by .707

i believe you divide by 1.414 not .707

As for the RMS ratings and stuff, dont let the watts suck you into buying an expensive rip off. I bought a $300 US amplifier, rated at 600 watts, when i looked around for the RMS rating i found out it was about 300 watts.

When i took it home to test it out, i found out it was 300 watts RMS at 10% THD. you heard right 10% THD !!!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: Watts RMS

pike said:
When i took it home to test it out, i found out it was 300 watts RMS at 10% THD. you heard right 10% THD !!!

Always look for 'proper' specs (particularly with equipment of American origin!). The only figures worth looking at are:

xxx watts RMS continuous, both channels driven, from xxHz to xxKHz, at less than 0.xxx% distortion.

If any of these figures are missing, it's probably because it's a poor result 8)
 
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