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WHAT IS RMS ?

Discussion in 'Amps and Cabs [BG]' started by Abluesbassist, May 28, 2003.


  1. hey guys alittle confused on Rms watts vs Program watts. i have two cabs one is 600 watts rms and the other is 400watts rms does that mean i can run a 1000 watts through both?
     
  2. Christopher

    Christopher

    Apr 28, 2000
    New York, NY
    Root mean square. Its a rough measure of the "average" power output, but it's more realistic than an average because it gives less weight to outlier values.
     
  3. BruceWane

    BruceWane

    Oct 31, 2002
    Houston, TX
    Assuming both cabs are the same impedance, each cab would recieve 500 watts, which is OK. You can go over the RMS rating by up to 50% and still be pretty safe. If you're very careful about avoiding clipping, you can go even further. Going over by 100 watts is not a problem.

    I wouldn't even pay attention to the "program watts" rating. It has very little to do with real world conditions, it just looks a lot more impressive.
     
  4. GRoberts

    GRoberts Supporting Member

    Jan 7, 2003
    Tucson, AZ USA
    PHP:
    Root mean squareIts a rough measure of the "average" power outputbut it's more realistic than an average because it gives less weight to outlier values.
    I was told that the RMS rating is the amount of power an amplifier can deliver without producing any distortion. It is in essence an excellent metric indicating how much "clean" power an amplifier can deliver.

    Peak Power is typcially a higher number, but the rating also includes driving a signal into distortion. Sure you can do it, but clipping (distortion) is bad for speakers not to mentione serenity. <grin> Once you're into distortion, (i.e. clipping), speaker damage is possible. Not good.

    As always, I could be wrong, your mileage may vary, Factory and dealer incentives may apply.
     
  5. GRoberts

    GRoberts Supporting Member

    Jan 7, 2003
    Tucson, AZ USA
    PHP:
    Root mean squareIts a rough measure of the "average" power outputbut it's more realistic than an average because it gives less weight to outlier values.
    I was told that the RMS rating is the amount of power an amplifier can deliver without producing any distortion. It is in essence an excellent metric indicating how much "clean" power an amplifier can deliver.

    Peak Power is typically a higher number, but the rating also includes driving a signal into distortion. Sure you can do it, but clipping (distortion) is bad for speakers not to mentione serenity. <grin> Once you're into distortion, (i.e. clipping), speaker damage is possible. Not good.

    As always, I could be wrong, your mileage may vary, Factory and dealer incentives may apply.
     
  6. jive1

    jive1 Moderator Staff Member Supporting Member Commercial User

    Jan 16, 2003
    Alexandria,VA
    Owner/Retailer: Jive Sound
    When we were kids, we referred to RMS as "Real Motherf#@king $h!t" because we thought it was the best indicator of the amount of usable power an amp/cab could use.
     
  7. Actually there is no such thing as "RMS Power"==what I mean is that this is a misnomer, but that's what everybody calls it, so the name stuck. Here's what gives: take an amp, hook it up to a signal generator that produces a nice clean sine wave at 1000 Hz. Now hook the output of the amp up to a dummy load--a 4 ohm resistor for example, and monitor the output waveform on an oscilloscope. Or if you have the bucks, a distortion analyzer.

    Anyhow, turn the amp up until the output of the amp distorts. With an o-scope, I can see the peaks of the sine wave start to flatten out. That's clipping, and just before that point the amp is producing its maximum power. Now the scope can't directly measure power, but it can measure the peak-to=peak voltage of the sine wave. The RMS value is half of this value times half of the square root of two, or .707 times half the p-to-p voltage. I also measure it with a true-RMS AC voltmeter.

    This is the RMS voltage at clipping (or at a predetermined distortion level, if you're using a distortion analyzer). Since we know the resistance of the dummy load, we use the formula W= (V)squared divided by R
    to give us "RMS power"

    Simple, huh?
     
  8. Good explanation. It frosts me that manufacturers use the term RMS power.... I guess that makes me a nerd...;)

    So-called RMS power refers to the continuous average power (long term) the cab can handle without thermal damage occurring. Peak power is what the cab can take in short bursts (like less than a second or so). Program power is not well defined and different manufacturers use different criteria in calculating it, but it's generally used as a guide to the reccommended amplifier power to get optimum performance out of the cab.
     
  9. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    I agree with Bill and Mark that "RMS power" is a misnomer for "continuous average power."

    The continuous average power rating of a loudspeaker system is mostly a thermal rating that describes how much power can go into it without overheating the voice coils. Loudspeakers are very inefficient; they are similar to incandescent light bulbs in that nearly all of the power put into them turns into heat instead of into usable energy. There are a few different methods of determining continuous average power ratings for speakers; see this Web page for a good description of them: http://www.doctorproaudio.com/doctor/temas/powerhandling.htm. Despite the different methods, the results they would produce for the same speaker would usually be fairly similar.

    "Program" power ratings are a speaker manufacturer's roundabout way of recommending a particular amplifier power point for the loudspeaker system. The idea is that because music is dynamic, with peaks and transients and other variations in level, you can safely use an amp capable of producing more power than the loudspeaker's continuous power rating, up to a point; you want that extra power so that the peaks won't get clipped. Thus, if you choose an appropriate amp for the loudspeaker, you can reasonably figure that as long as you keep the amp out of significant clipping, the average power put into the speaker driver(s)won't exceed its/their continuous power rating. "Program" power ratings are usually about double the loudspeaker's continuous rating. If you can get an amp that's within maybe ±25% of the program power rating, that's close enough to ideal.
     
  10. NJL

    NJL

    Apr 12, 2002
    San Antonio
    Bob,

    Do you remember the EF Hutton commercials in the early 80's? "When EF Hutton talks, everyone listens" - shows a person with one of thier hands behind an ear listening closely"

    Here at talkbass, "When Bob Lee talks, everyone better listen!"

    You always have very informative posts! Thanks for all you do!

    Peace :)
     
  11. EAGary

    EAGary Registered User

    May 27, 2002
    Peak power, peak to peak power, average power, continuous power, and program power are all legitimate units of measurement. However, it is important to understand the differences. Some audio companies fluff up wattage ratings to exagerate the specs. So everyone is on equal playing ground, the industry tends to use RMS as the standard. I like to think of RMS power as being the equivalent DC heating ability of an AC signal. Effectively, the area under the pure sine wave curve(s) is equal to the DC heating ability (this is where the .707 multiplier comes in). Like Bob Lee says, a lot of energy is wasted in heat when driving a loudspeaker load. However, more watts equals more area under the curve, so more RMS means more available power to the load.

    At EA, we spec both the RMS and peak power of our products just to keep things in perspective. For instance, the iAmp800 will produce about a 48V RMS signal across a 2 ohm load which translates to about 1100 watts of real audio (or DC heating ability). The peak power is over 2000 watts, although this peak rating is more of a transient nature, and takes into account the time constant of the power supply.

    Bottom line, stick with RMS when making comparisons.

    Gary Gibilisco, Euphonic Audio, Inc.