Speaker Watt Conversion

George Redmon · Post by **George Redmon** » 4 Jun 2007 6:52 pm

Is there a rule of thumb, or a conversion chart, that will tell me how many RMS watts my speakers are? For instance i have two speakers each rated at 450 Watts Continuous. How do i find out what RMS wattage they are? Yes i searched the forum. Jack Stoner had some great post on converting amplifier watts to speaker matching, and SPL ratings of each speaker make. But i couldn't find any chart, or rule, for converting from Continuous, or peak power or anything else to RMS...why so many different rating systems????

Jonathan Cullifer · Post by **Jonathan Cullifer** » 4 Jun 2007 7:32 pm

http://www.hifi-writer.com/he/misc/rmspower.htm

There is no such thing as RMS power. It's average power. A continuous power rating on a speaker means that it can handle a certain amount of average power indefinitely. It can handle more power, but for a shorter amount of time. There's no simple conversion between the formulas.

Donny Hinson · Post by **Donny Hinson** » 5 Jun 2007 3:30 am

Are these just speakers, or speaker cabinets (with multiple speakers)? What brand are they?

Generally, if you drive speakers near their power limit, you can expect failures after awhile...that just goes with the territory. It's really hard to come up with meaningful data unless you know how the speaker is going to be used, as program material varies greatly with the type of instrument and the way that it's played. Laboratory tests, while giving meaningful engineering guidelines, seldom represent real-world application.

But let's just say you knew what the RMS rating of the speakers really was. Okay, then how would you know what (RMS power) your amp was actually putting out? If you're in the lab, and using continuous tones, that's one thing, but that's not the way we use speakers, is it?

The whole thing becomes rather academic.

Mike Wheeler · Post by **Mike Wheeler** » 5 Jun 2007 4:59 am

George, here's a previous discussion on power ratings that might help.
Click Here

George Redmon · Post by **George Redmon** » 5 Jun 2007 6:30 am

Thank you guys for all your help, as always you came through. I was asking about speakers, not speaker cabs. The reason i was asking is because i will be posting them for sale. But anyway i contacted the manufacture Celestion. And they sent me this link, and told me these are rated at 450 Watt Continuous, 300 Watts RMS. But it sure does get confusing alright.

http://professional.celestion.com/pro/p ... TN1530.pdf

Donny Hinson · Post by **Donny Hinson** » 5 Jun 2007 8:05 am

You're right, George, it does get confusing. The thing to remember is that ratings (be they "peak", "RMS", or "continuous power") are all different, and common sense should prevail when you're using speakers. In other words, if it sounds too loud...it probably is. A 100-watt speaker may seem quite safe with a 75 watt amp, but it all depends on how they're driven. Some amps, when driven into overload conditions, will put out considerably more than their "rated" power, as the rated power is often the max output with a fairly low distortion level and a certain input level. Once you get into distortion (whether created by the amp itself, or an outboard device, i.e. "fuzzbox"), things escalate very rapidly.

As an practical example, Lloyd Green could probably use a 100-watt speaker with a 200-watt amp forever, without worrying about blowing the speaker, due to his soft, smooth playing style. On the other hand, Robert Randolph might destroy a 300-watt speaker using an amp rated at 200 or 250 watts, based on his playing style.

I speak from experience, as I have "cooked" an EV "SRO" speaker (rated about 150 watts RMS) with a B/F Twin Reverb (which was rated at less than half that).

Mike Wheeler · Post by **Mike Wheeler** » 5 Jun 2007 8:49 am

The problem that we run into with speaker ratings is that those ratings are based on a clean, undistorted waveform. When distortion is introduced, a significant amount of heating occurs very rapidly in the voice coil. Heat is the worst enemy of a speaker. When it builds up high enough, the coil form coatings soften, or even melt, causing the coil to deform or come loose.

I once saw a guy playing a solo thru a Twin with JBLs, maxed out, and driving it with a preamp and treble booster. Everything was going well for about 60 seconds, then the voice coil came blasting out of the grill cloth into the audience, and the amp went up in smoke! The audience went crazy, cheering as if it was part of the show.

The point is, when a speaker is driven with a hard distortion, the waveform of the signal approaches that of a square wave. This can cause extreme heat buildup in the voice coil and can result in a failure.

George Redmon · Post by **George Redmon** » 5 Jun 2007 11:09 am

I take it that the "Distortion" from an over drive or distortion pedal doesn't cause this "type" of distortion?

Donny Hinson · Post by **Donny Hinson** » 5 Jun 2007 11:34 am

Wrong, George. Distortion, wherever it's generated, has the same effect when it gets to the speaker.

Mike Wheeler · Post by **Mike Wheeler** » 5 Jun 2007 11:47 am

George, as far as the speaker is concerned, the source of the distortion doesn't matter, but, the level at which it's driven with that signal does...what I'm refering to is what the speaker "sees". The shape of the waveform could be caused by overdriving the amp, or an effect pedal, or whatever else. The distorted tones are the result of a pure sine wave's shape being modified. The exact shape of a given distortion sound's waveform can vary quite a bit, but the bottom line is that the waveform takes on some of the characteristics of a square wave.

Where a sine wave's top and bottom peaks are rounded, the square wave's are flat. That flat area is what causes heat in the speaker voice coil.

Using a distorted signal, if you push a given speaker to a modest loudness level (modest for that speaker, that is), you shouldn't have a problem. But when that same speaker is pushed to it's power limit, it won't be able to disipate all that extra heat fast enough. That's where trouble can happen.

I hope that answered your question. I'm trying to be as non-technical as possible.

George Redmon · Post by **George Redmon** » 5 Jun 2007 12:37 pm

I figured heat would raise caine with a speaker..since they say so much about venting, and heat fins and that sort of thing...thank you all for trying to help with this confusing rating non sense.