What does RMS mean in wattage terms?

Discussion in 'Amps and Cabs' started by mrmjp, Apr 17, 2009.

  1. mrmjp

    mrmjp Member

    Joined:
    May 16, 2008
    Messages:
    834
    Location:
    Seattle, Wa
    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.
     
  2. Frankee

    Frankee Low-rent hobbyist Gold Supporting Member

    Joined:
    Jan 31, 2008
    Messages:
    15,495
    Location:
    92118
    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.
     
  3. collinsamps

    collinsamps Member

    Joined:
    Apr 14, 2009
    Messages:
    41
    Location:
    Oklahoma
    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.[FONT=arial, helvetica]

    [/FONT]The difference between 100 & 50 watts being 3db to the human ear it just doesn't matter in the end.[FONT=arial, helvetica] [/FONT] You can't hear the difference in a few watts.[FONT=arial, helvetica]
    [/FONT]
     
    Last edited: Apr 17, 2009
  4. Jay Mitchell

    Jay Mitchell Member

    Joined:
    Oct 13, 2007
    Messages:
    3,242
    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.
     
    Last edited: Apr 17, 2009
  5. Tone_Terrific

    Tone_Terrific Member

    Joined:
    Dec 26, 2004
    Messages:
    15,621
    Location:
    Canada-GTA
    Thanks for the definition. Just bounce that decimal point over to correct the typo. 28.28V
     
  6. gixxerrock

    gixxerrock Member

    Joined:
    Apr 25, 2005
    Messages:
    3,007
    Location:
    Parksville, B.C.
    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.
     
  7. Jay Mitchell

    Jay Mitchell Member

    Joined:
    Oct 13, 2007
    Messages:
    3,242
    Done. Thanks for catching that.
     
  8. Jay Mitchell

    Jay Mitchell Member

    Joined:
    Oct 13, 2007
    Messages:
    3,242
    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.
     
  9. Frankee

    Frankee Low-rent hobbyist Gold Supporting Member

    Joined:
    Jan 31, 2008
    Messages:
    15,495
    Location:
    92118
    Easier with trig/vector/power triangle...similar to power factor calculation between VA (apparent) and wattage (actual). The RMS value can be substituted.
     
  10. hasserl

    hasserl Member

    Joined:
    Oct 24, 2004
    Messages:
    4,695
    Location:
    So Cal
    :D Correct, there is no such thing as RMS Watts.
     
  11. VaughnC

    VaughnC Supporting Member

    Joined:
    Jan 4, 2002
    Messages:
    10,261
    Location:
    Pennsylvania
    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?
     
  12. jumpbluesdude

    jumpbluesdude Member

    Joined:
    Mar 5, 2009
    Messages:
    2
    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.
     
  13. Jay Mitchell

    Jay Mitchell Member

    Joined:
    Oct 13, 2007
    Messages:
    3,242
    That is correct. It is the physical, as opposed to mathematical, definition of the meaning of RMS voltage.

    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.
     
  14. 6789

    6789 Member

    Joined:
    Mar 6, 2005
    Messages:
    2,715
    www.resonantelectronic.com
    The Barely Legal 18

    Her mom, the Cougar 36 or maybe it's the Cougar 46 is in the works.
     
  15. Franktone

    Franktone Member

    Joined:
    Feb 28, 2008
    Messages:
    2,018
    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.
     
  16. gixxerrock

    gixxerrock Member

    Joined:
    Apr 25, 2005
    Messages:
    3,007
    Location:
    Parksville, B.C.
  17. LSchefman

    LSchefman Supporting Member

    Joined:
    Jan 4, 2002
    Messages:
    13,457
    Yeah, well. I haven't had a chance to spout off, so screw you. ;)

    In the old days of hi-fi, so many amp manufacturers fudged their "peak" power numbers (numbers that are relatively meaningless) that the better manufacturers started using RMS because it really told the consumer more about the power output they could expect. This move was strongly supported by the old audio mags back in the day, and it became a hi fi standard.

    For guitar amps, peak power probably DOES mean something, if it's honestly measured, since you're not talking about music being played at a relatively constant volume, you actually WANT there to be large peaks and valleys in output.

    And none of this really matters.
     

Share This Page