# Rms??

Discussion in 'Amps and Cabs [BG]' started by bassdementia, Aug 14, 2001.

1. ### bassdementia

Jul 14, 2001
Apache Junction, AZ
my carvin PB300 is 300 watts RMS. what does the RMS stand for and what does it do?

2. ### Phat Ham

Feb 13, 2000
DC
RMS stands for root mean square. When measuring power output of amps, usually you want to look for continuous RMS, or sometimes it'll just say RMS. This is basically the amount of average power the amp can put out continuously over long periods of time. Sometimes you will see ratings for peak power. This is the amount of power an amp can put out over a short period of time, like 1/10th of a second. IMO peak ratings are relatively useless.

3. ### john turnerYou don't want to do that. Trust me.Staff Member

Mar 14, 2000
atlanta ga
root-mean-square. it's a way of measuring AC signals, like the output of your amplifier. simple magnitude measurements, like on DC, don't tell enough of the story to be useful for AC signals, since the magnitude of an AC signal changes over time.

rms measurements roll both the magnitude and time criteria together for the measurement. functionally speaking, the value is measured by taking the mathematical function that describes the signal and squaring it, then evaluating it's mean magnitude over time(mean value through 1 wavelength), and then taking the square root of the result.

many times this is measured on an amplifier by feeding it a simple sine wave and measuring the output peaks - the RMS value will be 1/(sqrt(2)) x the peak value . (that's "one over the square root of two times the peak value).

4. ### mikemulcahy

Jun 13, 2000
The Abyss
In all the different RMS figures on cab and amps, are there any time coefficients that are standard measurement. Such as 400w RMS continous (6+ hours ) etc...

Mike

5. ### Freakapotamus9

Jun 20, 2001
yeah what they said

6. ### john turnerYou don't want to do that. Trust me.Staff Member

Mar 14, 2000
atlanta ga
usually, it's implied that the amp is being excited with some steady, mono-chromatic sine wave, like 1 kHz, when the measurement is taken. it only takes one wavelength to determine the RMS value. it shouldn't vary from wavelength to wavelength.

7. ### Joris

I have to comment on that last post, JT. It matters how long you measure, at least for the continuous power rating. For true rms measurement, you're right, one complete sine will do, but a one-sine power rating is pretty useless

Things that influence the continuous power are heatsink capacity (even fan life span), power supply capacity, wiring and internal connector endurance at high temp, etc. etc.

8. ### john turnerYou don't want to do that. Trust me.Staff Member

Mar 14, 2000
atlanta ga
god, there ya go, bringing up that "real world" crap again. who cares about real world?

(attn all newbies : i am joking, back away from the flame).

9. ### CYoung

Nov 30, 2000
Gainesville, FL
Many amp companies will either quote the RMA power with a 1KHz sine wave or give it for a range (20Hz to 20KHz typically). The real-world power will differ slightly, of course, with peaks exceeding the RMS power quite easily.

I always get a kick when I see cheap computer speakers or car amps that claim to put out something like 200w into a 2" speaker. Obviously, this is the 'peak' figure and usually results in a major malfunction of some kind!

What I usually look for is the RMS power in a given frequency range, and the THD (total harmonic distortion) at FULL POWER, not 1w.

10. ### Joris

I'd like to. Not care, that is.

I once built a strobe light. Puts out about 25 kW during 1 millisecond. [thinking to self: on] "Hey, couldn't I hook that one up to my speakers?" [thinking to self: off]

Who cares?