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Vertexx
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Confusion on RMS ratings.

Hi everyone. I'm a new member of the group
I have a few questions that I hope folk here can help me with.

I'm now in my 60's (OLD) but have been involved in audio since my early teens...
I remember when an amplifier rated at 100watts RMS per channel (stereo) into 8ohm speakers was considered extremely loud, and in fact mine brought many a rude comment from neighbours up and down the entire street!
Now I see units rated at 1000watts are for sale in many and various shops for so cheap it's,well, unbelievable
These ratings by the way are in watts RMS NOT peak power output or some other weird computation.

I recently went to a audio store to listen to one of these units..1100 watts RMS it boldly stated. Five speakers apparently handled this RMS output, I guess dividing the power between them with possibly electronic crossovers.

I mentioned to the salesman that I was used of listening to 200watts max and was told that "these days" that amount of power would not be loud enough in any "normal" house.
Upon turning up the volume to its sad distorting max I mentioned that if the unit was outputting 50 watts RMS I would be surprised indeed.

So, may I humbly ask you here: Has the term RMS been re-rated ?

Wattage RMS can be easily calculated by squaring the voltage across the speaker terminals and dividing it by the speaker impedance. Thus with an oscilloscope or RMS meter, a audio signal generator/ white/pink noise generator, it is quite possible to measure the output in RMS before distortion begins.

But how is "RMS" power output measured these days ?

Are these units really delivering there advertised output and it's my poor aged ears that are the problem?

Does the term RMS still have any valid meaning ?

If it does, how do these guys get away with such lies when there is supposed to be truth in advertising legislation ?
Is it just that we have become so slack/ ignorant or accepting of these claims that we don't care anymore.

It's really quite sad to me.
Cheers to all.
Vertexx.

judicata
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Re: Confusion on RMS ratings.

I'm not a techie when it comes to things like this, but there are several on here and I'm sure they'll reply soon.

But I can say that the dealer was full of it. There are fantastic, high-end amps sold anywhere from 30wpc (two speakers into 8 ohms) to 200 watts on up. And some high-end great-sounding tube power amps are even lower than 30wpc (you need to match them with the right speaker).

My amp is rated at 75 (from the manufacturer), and it'll go plenty loud (louder than I have ever cranked it).

mrlowry
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Re: Confusion on RMS ratings.

The wattage rating method used for audio products isn't standardized. The only way that wattage can be used in comparing two products is when dealing with models from the same manufacturer. Is a 300 watt Bryston twice as powerful as the 150 watt Bryston? Yes it is. Is a 300 watt amp from "X" twice as powerful as a 150 watt amp from "Y? " It might be, but it's far from certain. This is because of the variables in rating wattage. Below are just a few:

1. The Impedance used to load the amplifier's outputs during testing, rated in Ohms.
2. The frequency range being driven. Some manufacturers drive 20Hz to 20,000Hz because that is what many accept to be the human hearing range. Some cheap receivers are driven at only ONE frequency, 1kHz being the norm.
3. How many channels are driven AT THE SAME time. Just because a receiver or amp has 5 or7 channels doesn't mean that they were all driven during the test. Nearly all receivers only drive one channel during testing.
4. How long was the test. Many times an amplifiers circuit can deliver very high wattage but the power supply and heat sinks won't allow it to continue doing so for long.
5. How much "Total Harmonic Distortion" was deemed acceptable during the testing. More distortion allowed means more watts on paper but distortion is also usually the cause of damage to speakers.

For mass market goods the wattage is largely determined by the marketing department and then the engineering department does the algebra to figure out which variables give them the desired answer. The Federal Trade Commission's major requirement is that the rating method be disclosed to the public.

Jan Vigne
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Re: Confusion on RMS ratings.


Quote:
Is it just that we have become so slack/ ignorant or accepting of these claims that we don't care anymore.

Yep, pretty much.

The sort of amp you were probably listening to was measured at a specific frequency - 1kHz - and to put the "spec" on the box it didn't have to do this for very long or into anything other than a load resistor. (There were other specs included in the owner's manual that would be more consistent with how you remember amplifier ratings, though these are almost always fudged and indecipherable too.) The "max" power was distributed between all the speakers in the system which probably included a subwoofer with its own small power amplifier. If one channel could drive "X" watts, then all channels could do the same when all channels were driven, no matter what reality might say about that facts.

I would guess there was no T.H.D. spec attached to this "R.M.S." rating so the distortion was very likely allowed to run at about 10% T.H.D. to get that amount of "watts". Possibly the "amplifier" was all on one chip - which could be good or could be bad, mostly it's bad. Connected only to the speakers that come with a "shelf" type system, the chip is just another way to build something cheap and disposable.

I would agree the "salesperson" didn't have a clue. It is the speaker's electrical sensitivity that typically determines how loud the system gets, not the wattage of the amplifier. If the speaker can manage 100dB with the equivalent of only one watt input, then most amplifiers will play pretty loud even if they don't play all that good.

cyclebrain
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Re: Confusion on RMS ratings.

Power ratings got pretty out of hand for a while.
Then the FTC got involved and created a standard.
This standard while not perfect, did do a pretty good job of leveling the playing field. Didn't always relate to real world audio as much as it related to testing, but was still a reference point. Car audio and surround sound systems somehow were exempt from these requirements.
You are looking at 1100W amps? Where are you shopping?

Welshsox
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Re: Confusion on RMS ratings.

Hi

I can clarify the mathematics here.

1 - The 1000 watt system that you mention is probably similar to the 5000 wtt boom boxes you see at best buy. These are based on some strange formula of peak music power calculated over a very small period of time and then multipled, in actual fact they have nowhere near the power talked about.

2 - RMS has nothing to do initially with the wattage, it is just nothing more than the sqaure root of the peak to peak voltage.

3 - When someone rates an amplifier as having say 100 watts RMS music power they are saying that the amplifier can delievr sufficient power to maintain a high enough voltage level into a 100 watt load. The calculation is simple power = voltage x voltage/load. To be strictly accurate the load should be an impedance measurement and not resistive but for most purposes these are interchangeable.

The thing is that all these ratings are relative, they also depend on the duty cycle of the signal and the required THD, you find most THD's are mesured at 1% or less load which is meaningless. To have a true amplifier rating it would be able to maintain 100% duty accross its full load at full rated power, i bet none of the hifi amps ever do this.

Alan

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Re: Confusion on RMS ratings.


Quote:
Hi

I can clarify the mathematics here.

1 - The 1000 watt system that you mention is probably similar to the 5000 wtt boom boxes you see at best buy. These are based on some strange formula of peak music power calculated over a very small period of time and then multipled, in actual fact they have nowhere near the power talked about.

2 - RMS has nothing to do initially with the wattage, it is just nothing more than the sqaure root of the peak to peak voltage.

3 - When someone rates an amplifier as having say 100 watts RMS music power they are saying that the amplifier can delievr sufficient power to maintain a high enough voltage level into a 100 watt load. The calculation is simple power = voltage x voltage/load. To be strictly accurate the load should be an impedance measurement and not resistive but for most purposes these are interchangeable.

The thing is that all these ratings are relative, they also depend on the duty cycle of the signal and the required THD, you find most THD's are mesured at 1% or less load which is meaningless. To have a true amplifier rating it would be able to maintain 100% duty accross its full load at full rated power, i bet none of the hifi amps ever do this.

Alan

Despite your attempt I don't think that you clarified the math.
RMS does have a relation to wattage. It is the equivelant AC power reletive to its DC E X I = P.
There is no such thing as a 100 watt load. The power dissipated by a load is a factor of the power applied to it.
FTC power ratings require a duty cycle of 100%, a continuous signal. A worst case situation and not realistic.

Jan Vigne
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Re: Confusion on RMS ratings.


Quote:
FTC power ratings require a duty cycle of 100%, a continuous signal. A worst case situation and not realistic.

The point to remember here is the FTC regulations apply only to the specifications typically provided in the owner's manual or in "advertising". What goes on the box or what goes on the sticker on the front of the component has nothing to do with FTC regulations. There's seldom even a THD spec when these wattage numbers are quoted. If there is a THD spec and a wattage provided on the box, they are not linked as one which would be in keeping with the FTC regulations. There's no FTC 1/3 power burn in for the wattage specs on the box.

I don't remember how the regulations were written back in the 70's when supposedly "Peak Music Power" ratings and IEF(?) power ratings disappeared. But I believe any "portable" component doesn't have to deal with the same FTC regulations as a piece of consumer home audio.

ncdrawl
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Re: Confusion on RMS ratings.

The rules are more lax for a home theater amplifier or receiver than for a stereo amplifier.

A stereo amplifier has to be rated with both channels operating at the same power. But a multichannel home theater amplifier only has to have the other channels on - they don't have to be operating at the same power as the channel being measured. So if it's a 5-channel amplifier, for the power test they can run one channel at 220 watts and the others are just barely even on. Then they multiply that 220 times the number of channels (5 in this case) and get 1100 watts, which no way does the little receiver have enough power supply or heat-sinking to actually do in real life.

NAD is one company that doesn't measure their multi-channel amps so optimistically; they measure them with all channels on. And there are probably other reputable brands that also do this.

I'm under the impression that it's permissible to measure the RMS wattage output using a single frequency (such as a 1 kHz sine wave), and that the measured RMS output at clipping would be much less on a broadband signal (music or pink noise). One amplifier manufacturer has said that it's common for an amplifier to clip on a broadband signal at power levels well below its RMS rating - like maybe 10% to 20% of its rated RMS power. I don't know if that's true or not, and if so I don't know how "common" it really is.

So anyway your observation might be absolutely correct, but the amp manufacturer is playing the numbers game.

Finally, it's fairly likely that the speakers are either lower in efficiency or lower in impedance (more difficult load) than their spec sheet claims, or both.

So anyway, when they tell you "1100 watts RMS", you can ask 'em if that's with all channels driven, and you can ask 'em what kind of signal was used - a sine wave, or a broadband signal - and into what kind of load (a resistor, or a simulated loudspeaker impedance).

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Re: Confusion on RMS ratings.


Quote:
So anyway, when they tell you "1100 watts RMS", you can ask 'em if that's with all channels driven, and you can ask 'em what kind of signal was used - a sine wave, or a broadband signal - and into what kind of load (a resistor, or a simulated loudspeaker impedance).


TEE HEE ! How to subdue and audio retailer in one easy lesson.

Welshsox
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Re: Confusion on RMS ratings.

Hi

I beg to differ.

If you have say a 50 ohm speaker load and you have a 100 VRMS signal this would be a 200 watt load. this formula can be transposed at will.

Your referring to hifi, in the real world there are constant loads and constant source signals. In the public address world speakers have real loads of say 6 watts, 15 watts etc. This is how amplifier power is measured. In addition these amplifiers are designed to drive 100 % duty cycle at 100% rated output sometimes for an hour contiuously. This where my belief that hifi amp wattages are totally ficticious in terms of real power.

Alan

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Re: Confusion on RMS ratings.
bertdw
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Re: Confusion on RMS ratings.

RMS is sometimes called the DC heating value. 25 watts DC will cause a load resistor to reach the same temperature as a 25 watt RMS AC signal. This means short duration peaks in power output, such as those found in music, don't affect the numbers very much. One amplifier could clip a sine wave at just over it's RMS rating, another could reproduce large peaks for a very brief time without thermally stressing the amp or the load. So you're justifiably confused, Vertexx. The ratings aren't extremely useful by themselves, without a lot of other information.

By the way, I once saw an oscilloscope measurement of a trumpet playing one continuous note - "Taaaaaa." Because of the high harmonics, the peak voltage level was 13dB above RMS. Stick that in your calculators!

Jan Vigne
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Re: Confusion on RMS ratings.


Quote:
Finally, it's fairly likely that the speakers are either lower in efficiency or lower in impedance (more difficult load) than their spec sheet claims, or both.

Actually, if this was a "system" the op heard, the speakers were very likely not low efficiency systems. They were matched to the the amplifier, and that's the problem. They were both unimpressive designs.

What the op hasn't told us is the selling price of the system he heard. That alone would probably tell us most of what we need to know.


Quote:
So anyway, when they tell you "1100 watts RMS", you can ask 'em if that's with all channels driven, and you can ask 'em what kind of signal was used - a sine wave, or a broadband signal - and into what kind of load (a resistor, or a simulated loudspeaker impedance).

That would be fun if you were the sort who pulled wings off flies and scorched ants with a magnifying glass. Why bother to talk over the head of the salesperson? He'll only curse you afterwards. Listen and then do as the op did, walk out. You probably weren't there to buy anyway.

ethanwiner
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Re: Confusion on RMS ratings.


Quote:
I'm now in my 60's (OLD) but have been involved in audio since my early teens ... I remember when an amplifier rated at 100watts RMS per channel (stereo) into 8ohm speakers was considered extremely loud


I just turned 60 so I remember those days too. RMS power still means the same it always did, but many loudspeakers are less efficient than back when we were teenagers. The first acoustic suspension speaker I remember hearing and being impressed by was the AR3a. This type of speaker can play very low frequencies with a small box size, but the trade-off is much lower efficiency. So 30 watts is enough to get big sound from an Altec Voice of the Theater, but you need five or ten times more power to get the same volume level from the small acoustic suspension type speakers many people use today.

--Ethan

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Re: Confusion on RMS ratings.


Quote:
That would be fun if you were the sort who pulled wings off flies and scorched ants with a magnifying glass.

I was, and am. actually my favorite thing to do is zap flies with a taser, and tie strings to their legs.

Jan Vigne
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Re: Confusion on RMS ratings.

There are not many small acoustic suspension speakers on the market today. The field is dominated by vented designs.

Jan Vigne
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Re: Confusion on RMS ratings.

When I typed that, I knew that was the response you would give. You just seem the type.

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Re: Confusion on RMS ratings.

Bert

My calculator would confirm that peak voltage was indeed higher than RMS voltage. Thats exactly what should be happening

Alan

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Re: Confusion on RMS ratings.

Yes, but I fear you missed my point. The peak value of even a sine wave is greater than its RMS value. With a peak to RMS ratio of 13dB, an amplifier which would clip at 100 watts RMS with a sine wave would only deliver 5 watts RMS to the load before clipping with the trumpet!

cyclebrain
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Re: Confusion on RMS ratings.

WOW! I don't even know how to respond to the misinformation in this thread. I think that much of the problem is with people having different definitions for the same terms.
Peak to some, means the maximum voltage level of a signal.
For those, peak to peak = 2 x peak, RMS = .707 x peak. To others here peak is refering to changes in signal levels independent of the unit value.

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Re: Confusion on RMS ratings.


Quote:
Peak to some, means the maximum voltage level of a signal.

That's exactly what peak means. The defiinition is not open to interpretation.


Quote:
RMS = .707 x peak

Only for a sine wave. This will not be true for a signal containing a narrow spike of brief duration, such as the trumpet harmonic of which I spoke.

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Re: Confusion on RMS ratings.


Quote:
I once saw an oscilloscope measurement of a trumpet playing one continuous note - "Taaaaa

A scope is not an RMS measurement unit.

Jan Vigne
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Re: Confusion on RMS ratings.


Quote:
I once saw an oscilloscope measurement of a trumpet playing one continuous note - "Taaaaa

I would think that humanly impossible. There must be a beginning and an end. Both would imply a change from "R.M.S." status.

bertdw
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Re: Confusion on RMS ratings.


Quote:
A scope is not an RMS measurement unit.

Many 'scopes have built-in voltmeters, and can perform an RMS measurement. Some digitize the signal, and can perform a spectral analysis or a distortion measurement.

bertdw
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Re: Confusion on RMS ratings.


Quote:

Quote:
I once saw an oscilloscope measurement of a trumpet playing one continuous note - "Taaaaa

I would think that humanly impossible. There must be a beginning and an end. Both would imply a change from "R.M.S." status.

Do you really not know what I'm saying, Jan? During the time the trumpet player held the note, ignoring the start and stop, ignoring the attack and decay of the note, the peak level was 13dB higher than RMS for that period of time. I mentioned this to show that peak to RMS ratios are not only affected by loud and soft parts of the music, but also by the harmonic content of an instrument's sound.

ethanwiner
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Re: Confusion on RMS ratings.


Quote:
Do you really not know what I'm saying, Jan?


Yes, really.


Quote:
ignoring the start and stop, ignoring the attack and decay of the note, the peak level was 13dB higher than RMS for that period of time.


As an interesting side note, Orban sells a device for radio stations that uses phase shift to "rotate" a waveform to achieve a smaller disparity between average and peak levels. This lets the station raise the audible level of a typical male announcer's voice without raising the peak level which would cause distortion.

--Ethan

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Bring back DUP

bertdw
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Re: Confusion on RMS ratings.

Rotate the waveform? You mean something like invert the negative half-cycles? Doesn't sound pretty!

Jan Vigne
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Re: Confusion on RMS ratings.


Quote:
I once saw an oscilloscope measurement ...

I once saw a man walking a frog! Doesn't prove much, does it?

What I'm saying is, if we ignore the start and stop, why bother? This becomes another pissing match to prove nothing. It has nothing to do with music no matter what the harmonics are providing. If it has nothing to do with music, why belabor the issue just to say you "once" saw something?

I doubt this is what the op was referring to when he asked about the fate of R.M.S. specs.

However, if you must piss on Ethan to satisfy yourself, be my guest.

ethanwiner
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Re: Confusion on RMS ratings.


Quote:
Rotate the waveform? You mean something like invert the negative half-cycles? Doesn't sound pretty!


LOL, no, the Phase Rotator is an all-pass filter:


Quote:
Phase rotator
The phase rotator is a chain of allpass filters (typically four poles, all at 200Hz) whose group delay is very non-constant as a function of frequency. Many voice waveforms (particularly male voices) exhibit as much as 6dB asymmetry. The phase rotator makes voice waveforms more symmetrical and can sometimes reduce the peak-to-average ratio of voice by 3-4dB. Because this processing is linear (it adds no new frequencies to the spectrum, so it doesn
bertdw
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Re: Confusion on RMS ratings.

Interesting, thanks. But I don't think I'll buy one for my hi-fi.

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