Rms

davidein

I hope this is not a stupid question but I would like to know what the difference is between a 125 w RMS channel and a 125 w channel.

Or maybe what is RMS?

TroyD

RMS, the term I believe is Root Mean Square. I could be wrong though. Anyway, essentially it means that, in this case the amp is capable of producing 125 watts per channel continuously.

It's not a stupid question.

BDT

davidein

is there a way of calculating RMS W. from normal W.?

the reason I ask is because I read in a review that my amp is 125 w/channel but 180 RMS w/channel

Chief CW4

RMS is Root Mean Square, and is a term applied to AC voltage as well as power. The RMS voltage you are most familiar with is the 110 VAC. In a pure sine wave, part of the time there is no voltage as the signal passes through zero volts after its positive and negative half cycles, part of the time it is at maximum voltage, either positve or negative. RMS is used to express an equivalent voltage not zero, not peak, not average but which indicates the work that this voltage can do while flowing through a load.

Remember that, in a sine wave, the average voltage is zero, being positive half the time and negative half the time. RMS is close to the average voltage of one half of the sine wave. Without getting into trig, it is 0.707 times the peak voltage of the wave (from zero to the top of the "hill" of one half cycle). This just happens to be the sine of 45 degrees. Taking 110 VAC, we can find the peak voltage by multiplying 110 by 1.41414 to get 155 volts. This is the peak level as seen on an oscilloscope. To convert that to RMS, 155 * .707= 110 volt RMS. Think of it as the DC battery voltage needed to do the same work as a 155 volt peak AC source.

How does that apply to speakers and amps? The above applies exactly only when a clean sine wave signal is being amplified; distortion and complex waves cloud the picture somewhat, as RMS is based on the geometry of a sine wave and its mathmatical relation to a circle stretched out in time. A square wave gives an "RMS" equal to the peak, and a sawtooth wave has a RMS equivalent lower down than a sine wave. In music or speech, complex waveforms result from overtones, mixing together of tones, etc. The result is more difficult to predict as compared to a sine wave. Thus two standards have evolved; the RMS power and "music power". RMS power is the RMS voltage squared divided by the resistance of the load, or RMS volts times resulting current in amps(amperes) pushed through amplifier output/speakers equal power. (from Ohm's Law: power = (amps times volts) =(amps squared time resistance in ohms) = (volts squared over resistance). This reveals the continuous power output capacity of an amplifier or input allowed to a speaker for long duration tones (from say a flute, which has an almost sine wave like waveform) or a lab bench audio oscillator. By the way, if you have a good low end amp, you can run a 60 Hz (cycle) signal into it and run an AC radio from it by using its power cord instead of a speaker; I won't think about what it would do to you PA though, and the impedance may be a bit high.

The RMS power spec tells you the steady current allowed into a fixed impedance load (speaker), the heat disapation characteristics of a device, is an indicator of how robust the power supply and heat sinks/output devices are, etc. You will note that RMS ratings are shown on good quality equipment, as it is the tougher spec and is a real indicator of quality, along with distortion specs, etc. A special note for low freq. signals and hardware (woofers and subwoofers). Very low frequencies are harder on amps because the waveform is so very long (lower freq = longer wavelengths) and begin to look like DC, as the wave stays at a high value for a longer time. Consider a 1 KHz tone; It is only one millisecond long (positive through negative, the whole "S" curve), and the time spent at the peak (or very close to it - the peak itself is actually nearly instantaneous) is longer. As this is 1.4 time the RMS level of a sine wave, the low freq components must endure the near peak stress for longer than mid range or tweeters. Over the whole sine wave it still is the RMS level voltage that does the work. Remember that the low freq wave is also at/near zero longer, where all parts can take a rest, if they were not blown up by the extended near-peak currents and volts. This relationship between "rest time" and peak work time to show effective work time/level is what RMS expresses.

Now to "music power". This is derived from that peak voltage at the top of the sine wave. We are not really dealing with sine waves in music, as I said, but the peak voltage itself is not different, just the shape of the wave beneath it. As the signals (music) usually stay at the peak only briefly (but they stay at a higher level longer as distortion and over driven rock music is played; this tends to flatten out the waveform and the loss in fidelity causes the peak to stay at the peak level longer, more like square waves). Other signals have a pointy (sawtooth) wave, and all are usually mixed in an orchestra recording. The industry takes the peak power, and if that level of brief voltage output can be generated by the amp in a linear fashion (what goes in = what goes out, except in power level) that would be the peak music power rating. Others reduce that level in a guess as to how much the effective power level is reduced in typical music waveforms. This is a more realistic rating. There is no industry standard, nor can there be, as "music" means different things to different listeners. The variation in average power between folk, acoustical, symphony and rock is very great. It can best be thought of as being the power level available to handle brief bursts of sond, but tells you little about low freq power capability, and real power disapation characteristics.

Hope that helps!

Karl

Chief CW4

A cut and paste goof in my RMS comment: a sentnce get erased. The paragraph is fixed here:

The RMS power spec tells you the steady current allowed into a fixed impedance load (speaker), the heat disapation characteristics of a device, is an indicator of how robust the power supply and heat sinks/output devices are, etc. You will note that RMS ratings are shown on good quality equipment, as it is the tougher spec and is a real indicator of quality, along with distortion specs, etc. A special note for low freq. signals and hardware (woofers and subwoofers). Very low frequencies are harder on amps because the waveform is so very long (lower freq = longer wavelengths) and begin to look like DC, as the wave stays at a high value for a longer time. Consider a 1 KHz tone; It is only one millisecond long (positive through negative, the whole "S" curve), and the time spent at the peak (or very close to it - the peak itself is actually nearly instantaneous) about 0.1 milliseconds (one tenth of a thousandth of a second). A 10 Hz wave is much, much longer. 100 times longer to be exact. The same time spent near the peak level is now one tenth of a second, about the time a typical fuse takes to blow, a transister to evaporate. THe time to heat up and to discharge power supply filter capacitors that reduce hum and help maintain a certain output voltage also is longer. As this peak/near peak level is 1.4 time the RMS level of a sine wave, the low freq components must endure the near peak stress for longer than mid range or tweeters, and at a voltage that is 40% more than the RMS. Over the whole sine wave it still is the RMS level voltage that does the work. Remember that the low freq wave is also at/near zero longer, where all parts can take a rest, if they were not blown up by the extended near-peak currents and volts. This relationship between "rest time" and peak work time to show effective work time/level is what RMS expresses.

HBombToo

That sums it up Chief! I knew our little engineering team here in MO could handle that 1...

[Deleted User]

Hello,
I certainly don't want to split hairs, but, technically speaking there's no such thing as RMS Watts. You can measure RMS Amperes and RMS Volts, but, mathematically speaking, when you multiply them together, to have power, it is no longer an RMS measurement. The correct term is continuous Watts, not RMS Watts.
Regards, Ken Swauger

TroyD

Hey, what they hell does Ken know anyway? He thinks you can't bi-wire RT1000's.:D

BDT

davidein

Thanks for the info that answered my RMS question

now can someone tell me if the Polk Audio RM6200 is

20 - 125 w/channel
or
20 - 125 RMS

http://www.polkaudio.com/home/specs.php?category=2&speaker=34

WA2KBZ

This is an RMS rating. I think Polk uses that miore quality spec. Check the spec sheet on the individual speakers to confirm (not the speaker package set summary). Means speakers can handle a SINE wave input (typically set to 1 KHz) amplified until the output voltage of 31.6 VRMS (44.7 V peak) across an 8 ohm load. Expect an RMS current of 3.95 amps into speakers, or 5.6 amps peak. Peak power is 250 W for brief peaks. By the way, size speaker wire at peak amps number to avoid voltage drop above what you consider OK during peaks.
Karl

HBombToo

I think this link may help summarize what has been stated and hope it helps.

http://www.telling-amplification.co.uk/power-ratings.html

IMHO
RMS or Continuous power... for me they are the same but I'm not going to debate the correctness of the terminology. If I say RMS Power, I am implying RMS Voltage Squared divided by resistance.

Terms, Terms and more Terms confuse the hell out of me!


With Respect
HBomb;)