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  #1  
Old 04-17-2009, 12:37 PM
mrmjp mrmjp is offline
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What does RMS mean in wattage terms?

So having an amp that is 20 watts RMS, means what really in wattage terms? I know what RMS(Root Means Square) but not what it translates to.
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  #2  
Old 04-17-2009, 12:48 PM
Frankee Frankee is offline
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An RMS comparison just equates what the AC value of the voltage would be if compared to an equal DC source. Peak AC x .707 = RMS equivalent.....or........RMS value x 1.414 = Peak AC.
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Old 04-17-2009, 02:06 PM
collinsamps collinsamps is offline
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RMS stands for Root Mean Square. It's another way to say "Average" amount just like RMS voltage is an average of voltage peak to peak for example.

Since so many companies like to fudge their own RMS ratings and say it's much more wattage than it actually is (bigger is better to people still trying to shove a 502 big block crate motor in a chevy chevette) the only Legal representation of RMS Wattage is one that has a UL seal of approval.

Looking at peak wattage lets you know more about what the amp is capable of and a better way to select speakers, which is confusing since guitar amps are usually rated in peak wattage and not RMS.

Most everyone in the business rounds wattage up to the next highest number ending in 0 or 5 and you seldom see an amp putting out what it's actually sold as. You also see "cool" numbers like 44 magnum, lucky 7, 38 special, blackjack 21, Atomic 16, etc etc etc..........which also may or may not be what the amp is actually putting out but it sure sounds cool and probably helps the sales.

Sometimes if you look at max tube dissipation and what they are advertising you can figure out that it's all about advertising and not reality.

The difference between 100 & 50 watts being 3db to the human ear it just doesn't matter in the end. You can't hear the difference in a few watts.

Last edited by collinsamps; 04-17-2009 at 02:33 PM.
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  #4  
Old 04-17-2009, 02:55 PM
Jay Mitchell Jay Mitchell is offline
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Quote:
Originally Posted by collinsamps View Post
RMS stands for Root Mean Square.
Correct. However, as used in audio, RMS only applies to voltage or current, not power. The term "RMS power," although widely used, is incorrect. The correct term is average power, which is calculated using RMS voltage or current and nominal impedance:

Pav = VRMS^2/R = IRMS^2*R.

Example: 28.28 volts RMS applied to a nominal 8 ohm load produces 100 watts average - not RMS - power. There is an RMS power figure that would result from that scenario, but it is a different number than 100 watts, and furthermore is not physically meaningful. Therefore, RMS power is never calculated.

Quote:
It's another way to say "Average" amount
Nope. "Average" in signal processing terms is the mean value, whereas RMS is the Root Mean Square value. The two are different.

Quote:
the only Legal representation of RMS Wattage is one that has a UL seal of approval.
Wrong. Completely wrong. A UL listing (not "seal of approval") is about product safety only. It says nothing about the power an amp will deliver to a speaker.

Quote:
The difference between 100 & 50 watts being 3db to the human ear it just doesn't matter in the end. You can't hear the difference in a few watts.
Correct.
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Last edited by Jay Mitchell; 04-17-2009 at 05:43 PM.
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  #5  
Old 04-17-2009, 03:32 PM
Tone_Terrific Tone_Terrific is offline
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Quote:
Originally Posted by Jay Mitchell View Post

Pav = VRMS^2/R = IRMS^2*R.

Example: 2.828 volts RMS applied to a nominal 8 ohm load produces 100 watts average - not RMS - power. There is an RMS power figure that would result from that scenario, but it is a different number than 100 watts, and furthermore is not physically meaningful. Therefore, RMS power is never calculated.
.
Thanks for the definition. Just bounce that decimal point over to correct the typo. 28.28V
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Old 04-17-2009, 03:37 PM
gixxerrock gixxerrock is offline
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The 0.707 number only technically applies to a sin wave. Adequate for specifications. For any real world signal, the definition is more complicated and involves calculus.
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Old 04-17-2009, 05:44 PM
Jay Mitchell Jay Mitchell is offline
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Quote:
Originally Posted by Tone_Terrific View Post
Thanks for the definition. Just bounce that decimal point over to correct the typo. 28.28V
Done. Thanks for catching that.
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  #8  
Old 04-17-2009, 05:48 PM
Jay Mitchell Jay Mitchell is offline
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Quote:
Originally Posted by gixxerrock View Post
The 0.707 number only technically applies to a sin wave. Adequate for specifications. For any real world signal, the definition is more complicated and involves calculus.
Yep. The mathematical functions for which the acronym stands (root mean square) are performed in reverse order. First square the function, then take its mean value over the period of time of interest - this requires integration - then take the square root of the result.
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  #9  
Old 04-17-2009, 06:09 PM
Frankee Frankee is offline
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Quote:
Originally Posted by gixxerrock View Post
The 0.707 number only technically applies to a sin wave. Adequate for specifications. For any real world signal, the definition is more complicated and involves calculus.
Easier with trig/vector/power triangle...similar to power factor calculation between VA (apparent) and wattage (actual). The RMS value can be substituted.
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  #10  
Old 04-17-2009, 06:30 PM
hasserl hasserl is offline
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Quote:
Originally Posted by Jay Mitchell View Post
Correct. However, as used in audio, RMS only applies to voltage or current, not power. The term "RMS power," although widely used, is incorrect. The correct term is average power, which is calculated using RMS voltage or current and nominal impedance:

Pav = VRMS^2/R = IRMS^2*R.

Example: 28.28 volts RMS applied to a nominal 8 ohm load produces 100 watts average - not RMS - power. There is an RMS power figure that would result from that scenario, but it is a different number than 100 watts, and furthermore is not physically meaningful. Therefore, RMS power is never calculated.

Nope. "Average" in signal processing terms is the mean value, whereas RMS is the Root Mean Square value. The two are different.

Wrong. Completely wrong. A UL listing (not "seal of approval") is about product safety only. It says nothing about the power an amp will deliver to a speaker.

Correct.
Correct, there is no such thing as RMS Watts.
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  #11  
Old 04-17-2009, 09:37 PM
VaughnC VaughnC is offline
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Back in tech school we were taught that RMS volts came about from how much AC voltage is required to heat up a resistance to the same temperature as a DC voltage. So, for instance, a 10V RMS AC sine signal will heat up the same value of resistance to the same temperature as 10V DC....which turns out to be .707Xpeak AC value.

So, RMS power doesn't make sense other than to say it's .707Xpeak power....so why don't amp manufacturers just say peak power and be done with it?
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  #12  
Old 04-17-2009, 09:52 PM
jumpbluesdude jumpbluesdude is offline
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Talking

As I remember it, when it comes to amps, it is the amount of power before distortion. So a 100 watt amp(peak) will give you 70.7 watts of clean power(RMS). What others have said is correct, but this is kinda the layman way to put it. Remember when a good amp design wasnt supposed to have distortion? Then your older than me!!!! HAHAHAHA.
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  #13  
Old 04-18-2009, 06:43 AM
Jay Mitchell Jay Mitchell is offline
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Quote:
Originally Posted by VaughnC View Post
Back in tech school we were taught that RMS volts came about from how much AC voltage is required to heat up a resistance to the same temperature as a DC voltage.
That is correct. It is the physical, as opposed to mathematical, definition of the meaning of RMS voltage.

Quote:
So, RMS power doesn't make sense other than to say it's .707Xpeak power....
That is incorrect. RMS voltage = .707 * peak voltage, but only in the special case of a sine wave (i.e., a single frequency). Average (not RMS) power = .5 * peak power, but only in the above special case. "RMS" power, were you to bother to calculate this physically meaningless value, would be neither of the above.

In general, the relationship between peak and average voltage (and therefore power) is completely dependent on the dynamics and frequency content of the program material. This very important ratio has a name: Crest Factor, and it is the reason that a single number for power never tells you everything you need to know about any piece of equipment, including speakers as well as amplifiers.
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  #14  
Old 04-18-2009, 07:06 AM
6789 6789 is offline
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Quote:
Originally Posted by collinsamps View Post
Most everyone in the business rounds wattage up to the next highest number ending in 0 or 5 and you seldom see an amp putting out what it's actually sold as. You also see "cool" numbers like 44 magnum, lucky 7, 38 special, blackjack 21, Atomic 16, etc etc etc..........which also may or may not be what the amp is actually putting out but it sure sounds cool and probably helps the sales.
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  #15  
Old 04-18-2009, 07:18 AM
Franktone Franktone is offline
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Some audiofile companys test their amplifiers using square waves and release the distortion characteristics according the square wave specification. Perhaps this may be a better way of rating guitar amplifiers. Or perhaps our amplifiers should be tested and analysed using a Fourier series of frequencies (probably somewhat similar to the square wave pattern) since a Fourier series of waves could be an infinite combination of possible frequencies.
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