Axiom Home Page Forums General Discussion Technical Questions Can someone explain RMS power rating?

At one time, I knew this information but it has since floated out of my brain...

As I'm checking out the Black Friday sales, I start to wonder if maybe Axiom Audio/Denon ripped me off...I'm seeing these home theater-in-a-box systems that are offering 1,100W of awesome surround sound power for like $250(US)!

Now, I know for a fact that my Axiom Epic Grand Master 350/Denon AVR-1910 uses right around 90W when it is really rockin'.

Personally, I think my current system sounds pretty darn nice, but I can't imagine how much more fantasticer my system could sound if I buy the home theater-in-a-box. That's like 12 times the amount of power!!! And on top of that, it costs less than 1/10th what I paid for my current setup! And it has a built-in BluRay player! And, the WHOLE system weighs less than my Denon AVR-1910! How convenient!

Hopefully by now you have picked up on the sarcasm. But seriously, how does these tiny little home theater systems make such outragous claims about their power handling? I'm pretty sure it has something to do with RMS power, but for the life of me I don't remember what that is exactly.

Any info would be appreciated, and the simpler the better for my old brain...this was just a passing curiosity as I was browsing through ads this evening.

Well, first they add up the highest numbers they can get (what's the THD percentage on those numbers? My guess >1%), and if you add the sub's power in there, you can get nice and big numbers.

No mention anywhere about THD...all it says is that the rated power is at 4 ohm, peak. 180W into each of the fronts, center, and surrounds and a 200W subwoofer.

Ha, the center channel weighs 10 ounces. God, I love my 22 pound VP150!

I think RMS power= total (peak) power divided by the square root of two.

Not only do they usually list total power, but as ken mentioned they typically add power for all of the channels which makes no sense.

so your 90wpc * 7 channels= 630 watts + the wattage of your subwoofer. multiply all of this by the square root of two and then maybe you'll feel better about your purchase

Here is the partaining quote from this article.

Music Power
The music power of an amp is real, and is generally higher than the continuous power. It is measured by using a tone-burst generator, and is the peak power than an amp can supply for (typically) about 10ms. This is quite reasonable, but not terribly useful when it is examined carefully. Since music is very dynamic, with the peak amplitude exceeding the average by 10 to 20dB (depending on the type of music), an amplifier is never called upon to provide full power all the time (at least if clipping is avoided, which should be all the time).

If the power supply is regulated or has considerable excess power capacity, the continuous and music power ratings will be almost identical. The difference was (at one time) measured, and was called 'dynamic headroom'. Few amps have a dynamic headroom of better than 1 or 2dB, and the greater the headroom, usually the cheaper the power supply for the rated power.

An amplifier with a much greater music power than its 'RMS' power usually has a transformer and/or filter capacitor that is too small. In most cases, a 90W (RMS) / 100W (music power) amp will not sound louder than a 90W amp with a regulated supply (so RMS and music power are the same). The extra 10W represents a little under 0.5dB, which is barely audible in a comparative listening test.

LOL. I started reading this thread and was about to offer you (BWeasner) $250 for your "sub-par" Axiom/Denon setup so that you could buy one of those AWESOME HTIB systems... I would have even paid for shipping...

Then the sarcastic comment was mentioned, and I got less excited about getting a deal on the Axiom/Denon setup.

LOL, that was a good one!

Brandon, the first point is that there's no such thing as "RMS" watts, despite some use even by manufacturers. RMS, which is a special type of average calculation you're probably familiar with in your work, applies to voltage, which varies between about +/- 170 volts, with an RMS value of about 120 volts for power in N.America. A watt, however is a fixed amount of power and isn't subject to an average. A correct term for the legally required(by FTC regulations)power rating is continuous power, and it has to be measured for five continuous minutes.

Even that HTIB system has to comply with the FTC regs and show its continuous per channel FTC rating in its specs. The advertising which you saw isn't that, but appears to be adding together the meaningless peak power calculation(at 170 volts double what it is at the RMS value of 120 volts)of 180 watts per channel for the five speakers and 200 for the sub. At a typical distortion rating under 1%, the number might be on the order of 50 watts per channel.

Quite often those 'high power' HTiB systems have around 10% THD for the number spec'd out, not at all the actual useable power.

I hear/read that the Consumer Electronics Association has come up with a standard to measure audio amplifiers, and that the FTC is about to adopt and mandate (recommend?) this standard. The "Standards" section of the Wikipedia article kind of allures to it but it seems to be under some flux at the moment and the references are a little sloppy. Still, the article is worth a read to understand the issues involved.

I think the CEA standard is:
http://www.ce.org/Standards/browseByCommittee_2624.asp