Axiom Home Page Forums General Discussion Home Theater 5.1/7.1 Power Requirements

Do any of you have links to studies/ analyses that looked at how much average/ peak power is required for each speaker in a 5.1 or 7.1 system as a function of what type of sound is being played?

I have been looking and I haven't found anything. I suspect that I haven't yet looked hard enough.

I am curious about the "real" power requirements for surround speakers.

Hi Rat,

No, I've not found any documented measurements on this subject, but there are so many variables (speaker sensitivity), listening level tastes, source material, etc.

For example, for classical or acoustical recordings where the surrounds are just supplying ambient information, the power requirements for the surround channels would be rather low--a few watts per channel would likely be enough--but for movie soundtracks where the "crack" of a thunderstorm was hard-mixed to the right surround, you might need 75 or 100 watts on a brief transient.

And then you have to consider DVD-A or SACD playback and mixing, where with some rock/pop recordings, the artist and mixing engineer place you in the middle of the band. In that case, high power requirements would be essential, depending again on the nature of the musics and its dynamic characteristics.

Then there's room size, etc. But it would be fun to put an oscilloscope on the outputs to every channel and watch for clipping of the waveform using a crappy low-powered A/V receiver, a well-designed one with better power supplies, a multichannel separate power amp, and monoblocs.

A mammoth project, but if someone's doing a PhD thesis or a paper for the AES, a worthy one.

In the interim, I'd say buy as much power as you can afford!

Regards,

Thanks Alan.

On a different board, we had a most interesting discussion about RMS and FTC audio power ratings, and what they really mean. As part of the discussion, several links to FTC discussion, announcements and rulings were posted. It appears that the FTC 2 channel ruling was a result of manufacturers "comments" that all channels simultaneously was not a realistic specification for the listening audience and placed an "unreasonable" requirement on their power supplies.

Implicit in this "ruling," at least to me, is the notion that the manufacturers supplied a body of "evidence" to the FTC that supported their claim. I have been looking for some of this "evidence" just out of curiosity, but have not been able to find anything.

If anyone has a link or two, I would like to learn more.

Here is an interesting article that starts to answer the question.

power

Here is another article that gives some of the history.

receiver history

Good Reads, Rat.

Bernard, this discussion from Apogee on power supply requirements touches on your question about overall average power use, but not in great detail. Incidentally, as we discussed before, we should be careful not to use the term "RMS" as if it was an actual power rating rather than just the mathematical process used to arrive at average,as distinguished from peak, power.

On the FTC rationale, as the notice that I cited in the earlier thread indicated, the two channel simultaneously standard arose in the 1970s when the reg was being formulated. There were very few amplifiers with more than two channels and there was no serious thought about requiring more than two to be measured simultaneously at full power, so no "evidence" on this point was ever considered. As the FTC notice(which hasn't resulted in a determination)showed, other requirements were first considered about five years ago when multi-channel amps had become much more common.

Thanks John. That was a good article.

I had aready drawn the conclusion that for audio gear that you must look at both the peak and "RMS" power requirements. I think that you may draw some incorrect conclusions if you focus on one versus the other for multiple channel amplifiers that use the same power supply.

That 40% number is an interesting metric. I'm going to read that article again and study it a little more.

John: I've read the article a couple more times. It comes very close to answering the questions.

I'm wondering if the 1/8 power rating is just not enough for some types of music. That may be why some notice differences when they go to separate amps.

It has been a long time since I can remember getting MORE than I paid for. I suspect that when you pay less than $1000 for a receiver, you should expect some limitations!!

Rat,

Exactly. The 1/8th power rating isn't enough for some types of multichannel music and lots of soundtracks. But it's unlikely lots of listeners in average rooms would detect brief clipping on movie soundtracks where there is a lot going on. You could certainly hear the clipping with a test signal, but studies years ago pointed out that amplifier distortion using music as a source has to get really nasty--into whole percentages--before it becomes audible. With a lab test signal, it becomes audible at much lower levels.

The dilution of power output ratings for multichannel receivers was simply market-driven. If a manufacturer had to include power supplies for a 7.1 channel A/V receiver that would meet the original FTC power output specs for stereo receivers, it could not be manufacturered or sold at an affordable price.

In many scenarios, the current power-rating requirements, while misleading for consumers, are a reasonable compromise to making 7.1-channel receivers affordable to a large market. But there are real limitations in terms of power output. Many brands simply won't drive lower impedances (4-ohm speakers) at all and of those that will, you can't put them into huge rooms and expect them to produce high SPLs without overheating or clipping or shutting down.

Regards,