Ever try to explain to someone the difference in RMS (root mean square), Peak, Peak to Peak and Dynamic power? :eek:

Do you understand these terms and their differences in realtion to a sine wave? :confused:

Just checking. :D

As my wife would say - GEEK ALERT !:nerd:

I think I do ... but for the benefit of everyone, why not provide some definitions ...

Generally, RMS is the average continuous power output an amplifier is capable of producing consistently over extended lengths of time. RMS power contrasts to Peak power (sometimes refered to as Dynamic power), which is used for brief moments in order to recreate sudden, high-energy sounds (transients). RMS power can be produced continuously for the length of time the amplifier is in operation. It is the most important of the two power ratings (RMS versus peak) when researching an amplifier.

For a sine wave, the RMS value is 0.707 times the Peak value, or 0.354 times the Peak-to-Peak value.

http://wardsweb.org/audiokarma/RMSGRAPH.gif

So a amp 100 watts RMS is about 141 watts Peak and 282 watts Peak-to-Peak.

Don't even think about the relevancy of PMPO (peak music peak out)which is all done at full volume.

Yeah, just grinds me to see those crafty eBay sellers advertising that HT junk as "4000 watts PMPO" or some shit like that. What a bunch of crap. Read the fine print and it states 35 wpc rms.

What kills me is that these guys all have decent feedback. What gives? This stuff has got to be junk.

Anyway, good job Ward. Nice explanation.

Yea, nice discription and pic Ward;

Sometimes it is useful to explane that AC RMS voltage is the equivelent power as DC voltage of the same value. If powering a heating load the RMS and DC of the same voltage value would create the same BTU heat output.

Interesting though, the AC peak value measurement has real purposes, one which would be in properly rating the insulation voltage rating for wire. An AC voltage circuit would require wire insulation rated greater then it's higher peak value not it's RMS value.

In my opinion people shouldn't put all the emphasis they do on power(wattage) ratings any way. (of course...I usually have just a little different way of looking at things.

All to often people look at the wattage rating of an amplifier...The way I see it...people should try and get a better grasp of the relation between their amplifier and their speakers. Just how much power do I want? Just how much power do I need? How loud do I listen to music? What peaks are my speakers capable of?

One of the best sounding receivers I had owned was a 35watt Vector Research...what a great sounding amp!

I always try and view my speakers and amp as one component...because they're really working together more than any other two components in the chain.

So, how many Watts do you need? Only you and your speakers can answer that question.

Anyone remember those pathetic under-dash "equalizer/booster" units with their 250-watt power ratings? I went into a stereo shop once and laughed in the salesman's face when he couldn't explain how you get 250 watts from one of these things when they had a 5-amp fuse in the 13.8v supply and couldn't possibly have had an efficiency above about 64%.

A good rule that I've found works well with solid-state home audio is to divide the ac-input watts by 4 to get a good approximation of the per-channel rms output.

Here's another interesting fact ... doubling the rms output power of a system only produces 3dB increase in SPL.

For those who are not familiar with the dB and its meaning, it's a way of expressing a ratio of two voltages, currents or power dissipations. dB for voltage = 20log(V1/V2) while dB for power = 10log(P1/P2) since Power = Voltage times Current.

Let's consider two circuits ... one with 80V across 8 ohms and the other with 40V across 8 ohms. Circuit A has a current of 10 amps (80V/8 ohms) and power dissipation in the load is 800W (80Vx10amps). Circuit B has a current of 5 amps (40V/8 ohms) and power dissipation is 200W (40Vx5amps)... dBV=20log(80/40)=+6dB, dBI=20log(10/5)=+6dB and dBP=10log(800/400)=+3dB.

My wife also would say GEEK ALERT if she were reading this.

John

Typo alert ...

In the dB example, power for A was 800W and for B was 200W so dBP was 10log(800/200)=+3dB

John

Oops again ... 10log(800/400)=+6dB

Let's also compare a 100W amplifier vs a 50W amplifier, 10log(100/50)=+3dB

John

It's 7:40 on a Sunday morning and I guess I'm not fully awake yet... 10log(800/200)=+6dB FINALLY

Lefty nailed the definition of RMS. His post is worth another look.

Wardsweb's original question was asking for an explanation of the various terms in relation to a sine wave.

If you picture a sine wave in your mind, it starts at zero, moves up to the peak, down through zero to the bottom, and then back to zero.

The "peak" is the distance from the zero line to the top of the curve. The 120 VAC from the wall socket is the "peak" voltage. Zero to the top of the sine wave is 120 volts.

"Peak-to-peak" is the distance from the top of the top curve to the bottom of the bottom curve. Any use of peak-to-peak measurements directed at the consumer are intended purely for hype and obfuscation.

RMS is the AREA under one half (i.e. the curve above the zero line) of the sine wave curve. If the peak is at 1.0, then the RMS value is 0.707.

It's been waaay too many years since my EE classes, so I apologize if the Alzheimer's is kicking in!

Clay

Well, a manufacturer can still crib all this any way they wish by specifying the output in a narrow band of frequencies, say 1kHz only; at a rediculous distortion percentage, sometimes as high as 10% with only one channel driven; or into a lower than normal impedance load.

In the late 60's or 70's the gov stepped in and required everyone to specify things. This they do, but in the case of hucksters, they do as I illustrate above and make it a very lenient specification.

-Ed

Has anyone ever figured out exactly what PMPO means? I have never obtained an exact answer, best I could offer is Peak Momentary Power Output.

As far as how much power to buy, I look at it this way, in receivers the more power you buy the more features you also buy, something to consider. If you are just using it for CD listening or something simple it is one thing but at least in the vintage gear you may have ended up with a better tuner for the cash outlay.

Consumer electronics gear power ratings had gotten so out of hand by 1974 or so, that the FTC stepped in and defined a series of specs that all manufacturers had to follow in an attempt to level the playing field. By this time, even that bastion of all things consumer, Consumer Reports, had weighed in that there was really no way to confirm the specs of all those 250wpc Electrophonic compact stereos that Montgomery Wards and Woolworths were selling.

So, the specs turned out to be a real bear to meet. There was a pre-conditioning test where you ran the amp at 2/3's power for some period of time (think it was an hour), which made the amp get *really* hot and then you could officially measure the power output into 1Khz and from 20-20KHz and then you could establish the distortion metric.

Turned out that many amps, including some well-respected amps, couldn't pass the pre-conditioning test and their offical, gov't-mandated specs caused them to not appear cost-competitive any longer. Of course, all that Morse/Electrophonic junk quickly dropped all the high-wattage stickers they sported.

BTW, the FTC mandated RMS measurements only for power output specs.

Today, I see many clueless individuals quoting much the same thing but don't give it any mind as I believe they're only hurting themselves. PT Barnum proved the essential thing about consumers being born every minute - ok, so that's a stretch but you get the idea.

Cheers,

David

The "peak" is the distance from the zero line to the top of the curve. The 120 VAC from the wall socket is the "peak" voltage. Zero to the top of the sine wave is 120 volts.




Yep RMS, .707 which is what meters read, then theres .637 which is average efficieny or something but we dont use that.

Your ac is actually peaking at around 162 volts 115-120 is the RMS. :)

see. college and booklearnin never hurt anyone

So, how many Watts do you need? Only you and your speakers can answer that question.

Absolutely. I have a 200-total-watt Aiwa bookshelf stereo which is actually too much for my small apartment (I never run the volume up past 3, 4 or maybe 5). I got this system mainly to replace an older Zenith 4-mode modular unit (and to fit my space; my apartment isn't big enough for a large, elaborate stereo rig), but looking back, and noting that the Zenith was only 5 wpc, maybe I could have gotten along just fine here with it and an external CD player (the system did have an aux position on the function selector). Add to that the fact that my Zenith stereo had an analog FM tuner (the AM was fair to poor, especially at night) which probably would run rings around the digital tuner in my bookshelf system, and I now wonder why I ever got rid of the Zenith. Heck, the only thing really wrong with it was that it was out of date (it had an 8-track player/recorder, whereas all modern systems have CD players/changers and the newest ones, like Aiwa's BMZ-K1 and K2, have USB connectivity for use with a PC).

Oh well. As I said, the Zenith system was far too big, physically, for my apartment, although the system worked well at my former home, a 3-bedroom house in a Cleveland suburb, the 17 years I had the rig--and it would have been just fine for here, being only 5 wpc. However, I must say, even running my Aiwa system far below its maximum output (I absolutely cannot play this beast anywhere near maximum, for obvious reasons), it still sounds great. As the title character of the 1980s TV series "Hunter" used to say, "Works for me."

Guys, for clear, non-academic discussions of power ratings and distortion figures and how they are (mis)figured, try these articles on Audioholics

http://www.audioholics.com/techtips/audioprinciples/amplifiers/receiverpower.php

http://www.audioholics.com/techtips/audioprinciples/amplifiers/allchannelsdriven.php

Among the best I've ever read on the subject!

Thanks for the links - interesting, provocative articles. REALLY enjoyed the snake oil cable ones...no flames, tho, please!

Personally, I don't agree with the criticism about the all channels driven testing methodology - it gives a stable basis for comparison, and designing for all channels driven testing ensures that the power supply design will be adequate, if not overdesigned, for the most demanding listening situations. Of course I may be biased - I'm the sort of person that buys more horsepower in his cars than is required to maintain a steady 65mph :naughty: (why do you need to go faster, its just illegal?!)

Biggie size for me, please!

A good rule that I've found works well with solid-state home audio is to divide the ac-input watts by 4 to get a good approximation of the per-channel rms output.

'Divide by four' is a good rule of thumb for any claim about any thing.

That rule works as well for my own system, which is rated at 200 total watts. Divide 200 by 4 and you come up with 50 watts per channel (the system has four 50-watt amplifiers, two of which are used for surround).

Well, for starters - there is no such animal as 'RMS watts', even though that terminology has run rampant through the audio industry.

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'.

Cheers,
Russ