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Discussion in 'Amps and Cabs [BG]' started by Somebassguy, Jan 4, 2004.

  1. Somebassguy


    Nov 5, 2003
    Everett Wa
    Could someome please explain to me the difference between RMS wattage and peak (max)wattage?
  2. notanaggie

    notanaggie Guest

    Sep 30, 2003
    There ain't no "rms" wattage. There is "continuous average power", which is what most folks mean when they say "rms" watts.

    That is the continuous output, power that would light a light bulb and the light would be steady.

    Peak power is the power that is available only for a short time. Kind of as if the amp could only light the bulb dimly if asked to run it continuously, but if it were to not light it most of the time, then every so often it could flash the bulb brightly for an instant.

    The flashes would be peak power, power bigger than the continuous power, but which the amp cannot sustain for long.

    Peak power is lots cheaper than continuous power, btw....
  3. Well, that explanation was sorta correct. "RMS watts" is a misnomer, but it has a scientific background:

    An amplifier's power output is measured by putting an input signal (usually a 1000 hertz sine wave) into the amp's input, with tone controls flat. The amp's output (into a dead load) is observed on an oscilloscope and/or with a distortion meter. When the amp visibly "clips" it can be seen on the scope: the output signal goes from a perfect sine wave, into a sine wave with the tops and bottom of the signal flattened or "clipped" off. Now the amplitude of the sine wave is measured, on the scope--the distance from the top to the bottom. Divide this by two, then multiply that by half of the square root of two, or .707, this is the RMS (root-mean-squared)voltage of the output signal at clipping. Knowing the resistance of the dead load, the so-called RMS power at clipping is the RMS voltage squared, divided by the resistance. Although complicated at first glance, it's easy and very repeatable, and is a good indicator of an amp's performance.

    On the other hand, some companies want to claim more power, and they use a "peak power" rating. "Peak" power doesn't have a well defined way of measuring it, different companies can use weird ways of getting a measurement of a very very brief surge of power. In fact some companies will use any testing method that gives them a big number. Hence you see $20 car amplifiers that claim 1000 watts--sure, for a thousandth of a second it produced a surge that was mysteriously calculated to equal whatever number they wanted to advertise. For all intents and purposes, ignore peak power ratings.
  4. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    I beg to differ. RMS watts is not a misnomer. RMS watts has a precise technical meaning. One RMS watt produces the same amount of heat in a resistor as the equivalent DC power (1 watt DC, which can be calculated according to the formulas derived from Ohm's law, so P=IE, or P=E^2/R, that kind of thing).

    The key thing here to understand is the "time factor". In other words, I could heat a resistor for a microsecond at an incredibly high voltage, and if I looked at it 2 seconds later I might measure the same amount of heat produced, even though the signal has only been applied for a very short time.

    So that's where the "average power" concept comes in. All that means is that while you're playing in a "normal" way, all the instantaneous peaks and valleys of your signal average out to that power level.

    The RMS (or "average", in the sense just mentioned) power delivered to a speaker will determine how hot the voice coil gets after you've been playing for an hour.

    The "peak" (or "instantaneous") power will determine how far your speaker cone tries to jump out of its socket when you hit a loud note on your low B string.

    Both types (or concepts) of power are important.
  5. Hehe, do love those discussions. But I guess it is somewhat getting out of the scope of this forum. But anyway :)

    A sine form with a RMS value (also called effective value or DC-equivalent) of 10V will produce the same power in a resistance as a DC voltage of 10V. So RMS has to do something with power but there is nothing like RMS Watt. (though understandable where it is comming from)

    So I vote for notanaggie. :)
    Sorry nonsqtr I think you are wrong in some of your definitions.
  6. What you're saying is correct except in the terminology. 'RMS Watt' is indeed a misnomer. The correct term for power calculated from RMS voltage/current is 'continuous average' power. The RMS power thing has become common parlance in the audio world and is even used by lots of people who should know better.
  7. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    RMS voltage and RMS current result in average power.

    To the original question, it makes a difference whether you're talking about amplifiers (what they can put out) or loudspeakers (what they can take in).

    "Peak" power means different things to different manufacturers, so I pay little attention to it.

    Although there are different procedures for measuring continuous average power, they at least produce more consistent results than "peak" ratings, which often suffer from marketing-relating rounding up and inflation.
  8. Somebassguy


    Nov 5, 2003
    Everett Wa
    Great explanation, I understand it now. thanks :)

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