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RMS?

Discussion in 'Amps and Cabs [BG]' started by Brendan, Nov 8, 2000.


  1. Brendan

    Brendan

    Jun 18, 2000
    Austin, TX
    I'm thinking about getting a cabinet for when I get my GK head later on, and all of them say something like "800 Watts program, 400RMS" What is "program" and "RMS". I already searched in the Amp archives, and couldn't find it. Help me!!
     
  2. phil_chew

    phil_chew

    Mar 22, 2000
    Asia
    RMS means "root mean square". Somebody else can explain the scientific/mathematical definition. I'll just tell you that a rating in watts RMS indicates the true power of the amp. Watts in "Program" or "continuous program" rating probably just means the peak power that the cab can take and is not a true rating. Steve Rabb of SWR would explain that "with a cabinet rated at 100 watts RMS, you can play 16th-notes on your 100 watt amp at full volume (under clipping) constantly for extended periods".

    What is more important is the impedance (ohm) rating. Some cabs are 8 ohms, some are 4 ohms. If your amp can only take a minimum of 4 ohms and you want to use two cabinets, you have to make sure that both cabs are 8 ohm cabs so that the total impedance is 4 ohms (impedance divided by no. of cabs).
     
  3. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Since audio signals are oscillating waves, they vary in power over time.

    For instance, a 1000 Hz sine wave (oscillating 1000 times a second) goes from zero voltage at time 0 to maximum POSITIVE voltage at 1/4000 of a second to zero voltage again at 1/2000 of a second to maximum NEGATIVE voltage at 3/4000 of a second to zero voltage again at 1/1000 of a second.

    RMS is an averaging of the voltage over the entire wave cycle and is defined for sine waves as 0.707 (1/2 the square root of two, thus the "root mean square" name) of the maximum voltage of the wave.
     
  4. Oysterman

    Oysterman

    Mar 30, 2000
    Sweden
    Great, Brian, only that we're not talking volts here but watts... ;) They are proportional (P = UI) however, so watt (pun :D) you're saying goes for effect as well.
     
  5. The RMS value of a periodic electrical waveform is the equivalent DC value as far as power is concerned. This is due to the fact that Power is proportional to the square of the voltage.

    Wall voltage in the US is 120VAC RMS, so you could put 120VDC across a light bulb and it would glow the exact same brightness as if it were plugged into the wall.

    Here's the formula for wall voltage in USA:

    v(t)= (169) * sin (377)(t), where 377 = 60 Hz * 2 * PI for radian frequency.
    169 is the peak voltage, 169/(squareroot of 2) = 169/1.414 = 120 V RMS

    For different waveforms, the RMS value is different. For a sine wave, RMS = peak/1.414. For a 50% duty cycle square wave with no DC, RMS = peak value. And so on......

    Pretty cool.

    Chris

     
  6. Dave S...

    Dave S...

    Oct 13, 2000
    It seems that an important aspect of power ratings is the actual signal that's being amplified...whether it's just a single frequency (eg: 1khz sine wave) or 'pink noise' (basically, 20hz to 20khz--all at the same time and the same volume for a steady signal...)

    I always understood 'program' as intended to emulate a real world situation. Full frequency ranges, but in pulses (like drums!!!) to allow the amp/ speaker time to recover before the next dose.

    I also understood that 'program' ratings tend to be little higher than RMS rating (depending, of course, on how the mfg rated the power--sine wave or pink noise---lots of room for discrepancy!)

    Remember also, that wattage is basically an expression of 'how much heat can the voice coil take before it melts' and doesn't necessarily have any effect on volume, tone or any of that.

    Also--I have no electronics degree...I'm just a bass player who has tried to get an informal education on this stuff to spend my dollars wisely...I'm NOT an expert

    Best Wishes!