Power - RMS Vs. Continuous Vs. Program Vs. Peak

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Power - RMS Vs. Continuous Vs. Program Vs. Peak

Postby JR » Thu Mar 10, 2005 7:35 pm

Ok folks. We may have lightly touched on this subject in another thread or two, but I can't say just where.

I was reading an article from 2003 entitled “Why there is no such thing as 'RMS watts' or 'watts RMS' and never has been”

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

For some time I have been trying to get the straight skinny on power standards for amplifiers so that I could properly match them with speakers.

So far, the only thing I have learned about power standards is that there doesn't appear to be any. It seems that they vary from one manufacturer to the next.

One article I read some time ago said that "RMS" was approximately one half "Program" (or was it "Continuous"?) which was again approximately one half "Peak". It went on to say that my amp's "Continuous" (or was it "Program"?) should be at least equal to my speaker's "Average" power rating for the same impedance. (I think).

I'm starting to feel like Vinny Bobarino here. "I'm so confuuused!!!"

Any thoughts on this? I know some of you who engineer sound must have some technical knowledge worthy of educating the rest of us.

Or maybe it's just me who remains clueless.
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Postby dokai » Thu Mar 10, 2005 9:00 pm

If I remember things correctly, "RMS" stands for "Root Mean Square", which is an actual, official engineering term. It's .707 times the "Continuous" power rating, if memory serves correctly. None of the other terms are officially defined anywhere that I'm aware of, although that's not a certainty.

Generally accepted definitions state that "Program" or "Music" is twice "Continuous", and "Peak" is twice "Program". Therefore, a speaker that's capable of cleanly using 250 watts continuous should also be able to use 500 watts Program and 1000 watts Peak. This is based on the assumption that music contains louder and quieter signals that will average out to be about half of what they would be if it was a continuous signal in a narrow band of frequencies. It's also assumed that most speakers can also absorb, without exploding, very short "transient" signals (measured in milliseconds!) that would quickly fry them if the signal was continuous, leading to the "peak" specification.

The same yardstick applies to amps - an amp capable of cleanly producing 125 watts of a specific frequency should be able to cleanly produce 250 watts of music and 500 watts of extremely brief transients.

Focus on "Continuous". It's the only rating that really means anything. Just make sure that the manufacturer is stating the power across the entire audible frequency range (20hz-20khz) along with reasonable distortion. It doesn't do any good buy a 200w/ch amp if it can only cleanly produce that much power across a very narrow frequency range or starts distorting before it reaches half the rated power.

See? Clear as mud. :D

In a related issue, keep in mind that it takes twice as much amp power to produce a very slight increase in sound level, so the difference between a 200w/ch amp and a 300w/ch amp is almost nada. That's why the efficiency rating of the speakers you're using are so important. I can frequently get louder with a small amp and an efficient pair of speakers than someone else can get with a huge amp and less efficient speakers. And while there's always a premium to be paid for an amp with more power, speaker efficiency isn't something you usually have to pay extra for. Lastly, speaker placement, specifically the relationship between the speakers and each other, or the speakers and the room boundaries, can make more of a difference in your sound level than doubling your amplifier power. "Boundary reinforcement" and "acoustic coupling" can make a huge difference in how much sound gets produced.
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Postby JR » Thu Mar 10, 2005 10:35 pm

dokai wrote:See? Clear as mud. :D
Yup! No, actually quite interesting. Thank You for your input.

dokai wrote:"Boundary reinforcement" and "acoustic coupling" can make a huge difference in how much sound gets produced.

Boundary reinforcement? acoustic coupling? Now this sounds like a good topic for another thread.

Maybe someone would like to address that topic here in the university???
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Postby dokai » Thu Mar 10, 2005 11:45 pm

It's been a while, but this will be close enough even if I'm not exactly right...

First, acoustic coupling:

If you have a single transducer producing a frequency, or band of frequencies, in a room, you'll get X amount of sound pressure level (spl) for a given amount of power. Double the power and you get X+3db more spl. (FYI, three decibels, or 3db, is about the smallest change that you're likely to notice, similar to turning up the volume on the tv to hear the news a little better.) Now, instead of doubling the power from the amp into one transducer, leave the amp setting alone and double the number of transducers operating in the same frequency band by adding a second speaker, and you achieve the same thing, i.e., X+3db of spl. This assumes that the transducers aren't cancelling each other out due to phase reversal or poor placement, or some other limiting factor.

So now you have two speakers producing the same signal, thereby increasing the volume level in the room by 3db. Now take the two speakers and put them right next to each other, and you'll get another 3db increase in spl. This is because the sounds "couple" or reinforce each other once the speakers are close to each other. (I don't remember the exact formula for determining how close they have to be - something like one half the wavelength of the frequency they're reproducing, or something like that. Just put them next to each other and let the physics students worry about the details.)

Summary: You achieve a 3db spl increase by adding a second speaker AND another 3db by putting it right next to the original speaker.

This is most useful for subwoofers, which are frequently run in mono, since low frequencies aren't directional enough to benefit from stereo reproduction. You also wouldn't want to screw up your stereo image by putting your L and R mains right next to each other, and if you did, since they're not emitting the exact same signal, they'd only reinforce the frequencies they had in common, which might not be very musical.

Boundary reinforcement:

If you suspend a transducer in mid air, or in an anechoic chamber, and measure it, you get the most accurate, or least "colored" measurements of what it REALLY does because it's not interacting with the room. Take the same transducer and put it on the floor, and you'll get a 3db gain in spl without using any more power from your amp. Push it back against the wall while leaving it on the floor, and it now has TWO boundaries to reinforce it's output: the floor and the wall, which causes another 3db increase. Slide it into the corner of the room, and it now has two walls and a floor reinforcing the output, for a total of 9db gain in spl. Again, this mostly applies to subs because of the wavelengths involved. Besides, you wouldn't want your L and R mains on the floor because the higher frequencies are too directional - the dancer's ankles would be getting the best sound, not their ears. But these techniques work GREAT with subwoofers, and since it's the low frequencies that use up most of the power, these techniques help where it's most needed.

Summary: Each boundary reinforces the output by 3db. Two walls and a floor = 9db increase.

Remember, to achieve a 9db increase in spl by adjusting the volume on your amp while it's driving a single speaker, you'd have to pump out EIGHT TIMES the amount of power you started with.

Hope this is useful.

By the way, I know there are some sound engineers out there. Would somebody please verify what I just put up? I know I'm probably close enough, but I'd appreciate a review by someone that does this for a living. :D
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Postby JR » Fri Mar 11, 2005 12:11 am

Wow! I think I actually understood that. It all has to do with speaker position. So if I place the top speakers in corners I could take advantage of boundry reinforcement to increase my SPL (X2), and if I doubled the tops in each corner, I could actually increase my SPL again (X2) through acoustic coupling. For a total increase of (X4) SPL, or 12db?
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Postby dokai » Fri Mar 11, 2005 1:31 am

You have the right idea, but remember, because of the frequencies involved, these techniques work best for subs. Putting a top cabinet in a corner would increase the spl a little, but not nearly as much as it would a sub cabinet. It would also probably adversely effect the sound quality of a top cab. Doubling the cabinets to get an acoustic coupling is also possible with top cabinets, but then you have to avoid a thing called "comb filtering", which is where the frequencies interfere with each other. Subs aren't as prone to comb filtering as top cabs. With doubled mains (four cabs, two per side), turn the paired cabinets away from each other so that the coverage from the horns don't overlap. I try to picture the output of the speaker like a beam from a flashlight. The idea is to get a wider beam covering more room area, with minimal overlap along the common side. Another configuration that has worked for me is to stack the pairs on top of each other, i.e., L1 above L2 on one side, R1 above R2 on the other side.

The best thing to do is to try a few different configurations and see how it changes the sound. Depending on the speakers involved, and the room with which they're interacting, you sometimes stumble across things that work better than expected. Sometimes the opposite, too, of course. ;-)
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Re: Power - RMS Vs. Continuous Vs. Program Vs. Peak

Postby TheBartman47 » Fri Mar 11, 2005 1:38 pm

Power ratings for amps and speakers mean two different things. When talking about an amplifier's power rating, that is basically it's peak rating, or at least very near it's peak rating taking into account headroom and published specs on distortion ratings at said power levels. So, if an amp claims to be 1,000 Watts, then that's it. It may be able to peak up to about 1,200 watts, but at that level it will be quite distorted and can't be really claimed as "available power", so the published spec of 1,000 watts is what you need to read.

For speakers, it's a whole different ball game. The RMS is actually a term for .707 of the PEAK of a sine wave, which means the average power of that sine wave. So, technically Continuous, Program, and Peak power are all RMS numbers of those ratings. However, many speaker makers use RMS and Continuous interchangeably even though the terminology is not correct. Anyway, if the speaker manufacturer uses the standard accepted test noise which contains +6dB peaks from the mean power level, then this one signal determines all three power ratings. If a speaker handles 500 Watts continuous, then it will handle 1,000 watts program, and 2,000 Watts peak. How do you decide which amp to go with this speaker? For most users the choice should be to match the program rating. This will allow the amp to produce an undistorted continuous signal at 500 Watts and safely pass up to 1,000 Watt peaks. Most users are not skilled enough to use a 2,000 Watt amp to maximize their speaker's efficiency by allowing a full +6dB of peak passability (which would be 2,000 Watts in this example).

So, what do you do if a speaker doesn't publish Continuous, Program, and Peak ratings but only gives one rating and doesn't tell you what it is? Usually you can guess that it will be the program rating, or if it says it is the RMS rating, then double that number for your amp size selection. However, most of the time when a speaker doesn't give you much power rating information, you probably don't want that speaker anyway because it is usually a tip-off that it is not pro-gear.
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Re: Power - RMS Vs. Continuous Vs. Program Vs. Peak

Postby JR » Fri Mar 11, 2005 3:50 pm

TheBartman47 wrote:How do you decide which amp to go with this speaker? For most users the choice should be to match the program rating.
...Most users are not skilled enough to use a 2,000 Watt amp to maximize their speaker's efficiency by allowing a full +6dB of peak passability (which would be 2,000 Watts in this example).


On the first point, good, I believe I've done that with a QSC 2450 750w/ch @4ohm and PV-215 700w@4ohm (1400 peak)

On the second point, how does one achieve better speaker efficiency? It appears form your comment that I could use a larger amp to do so, but I'm also getting the hint that I could overdrive the speakers as well. What techniques would I apply to achieve the greater efficiency without causing damage?
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Re: Power - RMS Vs. Continuous Vs. Program Vs. Peak

Postby TheBartman47 » Fri Mar 11, 2005 7:59 pm

For absolute maximum amplifier fidelity, speaker clarity, and optimum headroom, you technically need 10dB of headroom! However, for most people it is incredibly impractical to get a 5,000 Watt amp to drive a 500 Watt speaker. A well seasoned professional sound system operator can do this, but most can not. Most would fry the speakers in a second. Keeping the amps power rating at near the speaker's program rating offers a limited level of speaker protection from overpowering. However, most speakers fail because of clipped signals or over-excursion from too much sub-bass frequencies at high power levels. If you get an amp rated significantly more powerful than the speaker's power rating, then you cure the problem of the amp clipping before reaching the speaker's full power, but this "cure" can be worse than the "disease" to begin with since now you are very vulnerable to overpowering the speakers and burning up the voice coils. Also, this only "cures" the amp from clipping itself. It will still faithfully pass a signal that was clipped in other circuitry further up the chain such as in the mixer's pre-amp section.

For your situation, you have 750 Watts going into a speaker with a program rating of 700 Watts. This is a very good match. As far as further protection, mainly train your ears to know what distortion sounds like and don't let any "red" lights come on on your mixer or amp. When you first hear distortion, it's time to cut back.
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Postby CJ Greiner » Fri Mar 11, 2005 8:43 pm

One of my favorite accessories has a great discussion on how to properly set up your mixer and amplifier for proper gain structure so that the clipping indicators on your mixer will match the clip indicators on your mixer:

DBX DriveRack PA wrote:The DriveRack PA offers a wide range of tools for sound system design and setup. These tools
can make your system more efficient and better sounding, but to get the best possible sound it
is important to use these tools properly. In the DriveRack PA we have included a Wizard setup
tool to help in system setup. If you use the Wizard to set up your DriveRack PA it will automatically
set the limiters for some amplifier selections. If your amplifiers are not available in
the Wizard, you should choose the Custom setting. The following section explains how to maximize
system gain and how to use the limiters to protect your amplifiers from clipping.

In traditional system design, the output of your console would be routed to a system EQ, a compressor,
and a crossover with output level control. From the crossover, there may be additional
filters that are employed to improve the response of your speakers. There may also be limiters
set up to keep your amplifiers from going into clipping and protect your speakers from
the hazards of a clipped signal. Your amplifiers play a vital role in system setup, because they
are last item in the chain before your speakers and offer the greatest amount of gain (that is
their job after all). If your amplifiers are incorrectly setup you will not be using your system to
its fullest potential and could be harming your speakers.

One thing that is critical to system setup is maximizing gain structure. Gain structure refers to
aligning the gain of each device so that they all clip at the same point, and the noise floor of
the entire system is at its absolute minimum. Quite often PA systems are setup with the amplifier
input controls turned all the way up in the incorrect assumption that this is the only way
to get the maximum output level. Amplifiers are fixed gain devices, turning down the amplifier
input attenuators does not change the potential output of the amplifier; it only requires more
input voltage to get full output power. Many amplifiers will clip with an input level greater than
+6 dBu when the input attenuators are turned all the way up. Most mixing consoles can deliver
over +18 dBu of output level before clipping. This means that with your amps tuned all the
way up you are sacrificing 12 dB of headroom, resulting in poorer noise performance and the
potential of system clipping. By adjusting the amplifier controls properly, you can maximize
your system performance.


A way to set up your amplifiers for maximum gain structure is to use the clip indicators of the
console and amplifiers themselves. Disconnect the output of the amplifier from the speakers.
Run a continuous signal (pink noise or sine waves – many test CDs are available that have these
types of signals) through your console. Turn up the output of the console until it begins clipping.
If there is no clip indicator on the console then use the output meters; most reputable
console manufacturers use red LEDs at the top of the meters to show the onset of clipping.
Once the console is clipping, back the output gain down slightly until the clip indicator turns
off. Run this signal through the DriveRack PA and into the amplifiers with the crossovers and
output gain section in the DriveRack PA set for your particular speakers. Make sure that the
output limiters are turned off. Run this signal into the amplifiers and turn up the input attenuators
until the amplifier clip indicator begins to turn on. Turn the attenuators down slightly, so
the clip indicator no longer is on. You have just maximized the gain through your system. This
amplifier setting should give you maximum gain without clipping, another way of say this is
that when the output of your console is clipping you will also be at the clip point of your amplifiers.

Once you have found the clip point of your amplifiers, you can mark this position and turn the
amplifiers back up to the point where they are clipping. You can now use the output limiters
in the DriveRack PA to protect the amplifier from clipping no matter what you do at the console.
With the amplifiers clipping, now go to the Limiter page of the DriveRack PA and turn
the limiter for each output band on. Make sure that the threshold is all the way up to +20dB.
Now slowly reduce the threshold until the clip indicator of the amplifier turns off. You have
successfully set the output limiter up and the amplifier will not be able to go into clipping no
matter what the input is. You should now proceed to the next output band and go through the
same process. If you are using a sine wave to create clipping in your amplifier you should
make sure that the sine wave is within the frequency range of the output band that your are
working with. Once all the limiters are set up you can now turn the amplifiers back down to
the gain maximized position. The Wizard will automatically set your limiters based on your
choice of amplifier. Because of component variances within the amplifier circuitry, you may
need to adjust the limiter settings by a few dB after running the Wizard to ensure that they are
set correctly.

Once all the limiters are set up and the gain is maximized, reduce the output of your console
and reconnect your speakers. Now for the fun part, sit back and send your favorite music
through the system. If your system is not loud enough, you may want to consider an amplifier
or amplifiers with greater output power. Most loudspeaker manufacturers recommend an
amplifier that can provide 1.5 to 2 times the rated RMS power of the speaker. If this setting is
too loud when your speakers are reconnected you can turn down the amplifier input attenuators
even more.


Image

Specific info available at:
http://www.driverack.com/PA.htm
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Postby TheBartman47 » Sat Mar 12, 2005 12:40 am

DBX DriveRack PA wrote:...Quite often PA systems are setup with the amplifier input controls turned all the way up in the incorrect assumption that this is the only way to get the maximum output level. Amplifiers are fixed gain devices, turning down the amplifier input attenuators does not change the potential output of the amplifier; it only requires more input voltage to get full output power. Many amplifiers will clip with an input level greater than +6 dBu when the input attenuators are turned all the way up. Most mixing consoles can deliver over +18 dBu of output level before clipping. This means that with your amps tuned all the way up you are sacrificing 12 dB of headroom, resulting in poorer noise performance and the potential of system clipping. By adjusting the amplifier controls properly, you can maximize your system performance.


I disagree in part. Turning down the amplifiers TAKES AWAY HEADROOM! Yes, many mixer's will pass up to +18 dBu, but by no means is this the best level to operate it. In fact, this will be the most noisiest level of all. The best signal to noise ratio is when the mixer is as close to unity as possible (0 dBu) Then for about 6-9 dB of headroom, this is where you wan't your amps to max out. Also, don't forget, especially for those not using a drive rack, you most likely will be going through an EQ before hitting the amps, and most settings on EQs will be cuts, so if your amps are turned down, then the mixer may have to be well over +12 dBu in order for the amps to go full power because of cuts in the EQ. Also, most all manufacturers amplifiers have a soft-limiting circuitiry to prevent clipping, and if your turn the amps down, you throw away the headroom of this limitting circutitry.
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Postby TheBartman47 » Sat Mar 12, 2005 12:51 am

DBX DriveRack PA wrote:...Run a continuous signal (pink noise or sine waves – many test CDs are available that have these types of signals) through your console. Turn up the output of the console until it begins clipping. If there is no clip indicator on the console then use the output meters; most reputable console manufacturers use red LEDs at the top of the meters to show the onset of clipping. Once the console is clipping, back the output gain down slightly until the clip indicator turns off. Run this signal through the DriveRack PA and into the amplifiers with the crossovers and output gain section in the DriveRack PA set for your particular speakers. Make sure that the output limiters are turned off. Run this signal into the amplifiers and turn up the input attenuators until the amplifier clip indicator begins to turn on. Turn the attenuators down slightly, so the clip indicator no longer is on. You have just maximized the gain through your system. This amplifier setting should give you maximum gain without clipping, another way of say this is that when the output of your console is clipping you will also be at the clip point of your amplifiers.


Again, this is not the best signal to noise ratio of a system, and it isn't desirable to have your mixer's clip indicator to light at the same time as your amps.

Continued...
Once you have found the clip point of your amplifiers, you can mark this position and turn the amplifiers back up to the point where they are clipping. You can now use the output limiters in the DriveRack PA to protect the amplifier from clipping no matter what you do at the console. With the amplifiers clipping, now go to the Limiter page of the DriveRack PA and turn the limiter for each output band on. Make sure that the threshold is all the way up to +20dB. Now slowly reduce the threshold until the clip indicator of the amplifier turns off. You have successfully set the output limiter up and the amplifier will not be able to go into clipping no matter what the input is.


This is the most dangerous piece of "advice" I've ever heard! This is leading users into a false sense of security. You can still clip a signal within the mixer, and now that the amps are turned down and even with a driverack limiter in place, it is even MORE likely that you will clip an input signal on the mixer because you keep turning it up trying to compensate for the lower input settings on your amps. No, the amp itself will not clip, but the amp WILL faithfully reproduce that clipped signal generated at the mixer's input, and that is why so many toast their speakers.
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Postby CJ Greiner » Sat Mar 12, 2005 1:58 am

Hmmm...

I'm not sure it's dangerous at all. In fact, it aligns your indications so what you see at the mixer is what you see at the amplifier. This gives you a better indication of what's going on with your system.
Here's what I mean: some DJs are used to operating "in the red" on their mixers -- that means loud, right?
Here's the thing, if you're pushing the output of your channels or main outputs to the top of the indication range on your mixer, then you probably ARE clipping in the mixer, and that's exactly what you don't want. (As you said, it's the clipping on the input that causes the speaker-damaging noise.)
There's a way to set up your system so that the mixer never clips in normal operation before the amplifier, which allows the amplifier to perform the "soft clipping" you mentioned:
If you have three colors on your mixer -- green, yellow, red -- then your normal mixer output should be somewhere in the green or yellow.
When you're hitting the red (about to clip at the mixer) -- you should have your system set up so you're hitting the clipping point of your amps. That's what the article is referring to.

Additionally, you don't lose headroom at all. Remember -- the controls on the front of the amp are input gain controls. They simply adjust how much input voltage it will take to make the amplifier produce a full-range output signal. The lower they're set, the more voltage (higher peak) is required from the mixer output to push the amplifier to full output. The full amount of output power is still there... just waiting to be used...
(Therefore, I believe the section of article quoted in your first post above is quite accurate.)

The article provides you with the ability to do two things: maximize the gain structure and therefore maximize the output of your system, and it tells you how to set up your system so that clipping occurs at the amp when your mixer goes into the red. That sounds a lot safer than a system that requires you to look down at the amplifier to realize that you're constantly clipping -- and probably not taking advantage of a full 12dB of headroom that you should have!

Those instructions work whether or not you use a DriveRack, EQ, or nothing at all in-between your mixer and amp.
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Re: Power - RMS Vs. Continuous Vs. Program Vs. Peak

Postby TheBartman47 » Mon Mar 14, 2005 2:13 am

But, you still miss the point that when a mixer (or any other device in the signal chain) is operating above 0 dBu, it is introducing additional noise because the circuitry is amplifying not only the signal, but the noise floor is rasied as well. With the mixer set to regularly operate above 0 dB, then the noise level is considerably high. You get a MUCH better signal quality by running the amps full open and keeping the mixer signal levels lower. If you are worried about clipping, then see at what level your mixer output causes your amps to go full power, then just remember which LED lights up at clipping, whether it be the first or second yellow LED or all the way up to the first red LED.

Also, this IS reducing headroom when you lower the amps. You reduce the headroom available in the mixer, because now you have to run the mixer to it's very edge to go full power. If you have an unexpectedly high input (someone drops a mic) and your mixer is already at near full output, then this will definately clip the mixer, and this clip will get reproduced by the amp. The only way to protect against this is if your limiter is on the mic before it goes into the mixer (or used on the mixer's insert for that mic channel). If you only have one point of limiting, then you want everything before that limiter to have as much headroom as possible. If you are using the limiter built in to most amps, then you will want the amp to be turned full up so that it's built-in limiter will have the greatest headroom for clamping down those potentially clipping signals. Then when that mic gets dropped, the mixer will be able to pass it unclipped because you are running it at a lower normal output level, then the amp's limiter will be able to pad that clean, though overly high, signal so it ends up not blowing the speakers.
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Re: Power - RMS Vs. Continuous Vs. Program Vs. Peak

Postby FDJA » Mon Jun 06, 2005 2:13 am

Great topic... Dokai and Bartman are both right about various things surrounding this topic and have covered them in detail. The only thing I would add is a simplistic approach to understand the wattage ratings and how they affect the typical DJ.

One thing I've noticed more speaker manufacturers doing lately, is advertising the PROGRAM rating of the speaker which makes the speaker look like it can take more juice than the competetive speaker which only advertises RMS, (even though a speaker that advertises 500 watts program is still probably only a 250-350 watt RMS speaker).

A common mistake made in sound system purchases is selecting an amp which has a maximum RMS equal to or less than the speaker's RMS... This results in less headroom, (ability to deliver enough current to keep a nice sine wave during demanding peaks). The recommended amp power is at least twice the speakers RMS at the same resistence or "ohms".

When it comes to RMS, PROGRAM and PEAK though, pay the most attention to RMS (continuous), this is the number everything else is based on... Remember that "watts" is a meaurement of heat. RMS is the actual sustained long term THERMAL limit that the speaker coil can deal with, or that the amplifier can produce without thermal fatigue. Both PROGRAM and PEAK ratings are typically an estimate or approximate thermal limit which indicates at what point the speaker coil will melt or the amplifier will fail.

One other thing I might mention, because it's so often overlooked: When multiple loudspeakers are necessary it's best to keep them symetrical and matched. For example, if running multiple loudspeakers from the same amp, they should be identical. When they are different, (disimilar brands, driver sizes, etc), it throws your amp into an electrical frenzy... An amplifier's ideal performance will occur when it's driving what appears to be one constant "load"... A load which has the identical electrical and mechanical reaction. If one loudspeaker does not respond EXACTLY as another, it limits the amplifiers ability to deliver a pure and powerful signal without distortion or clipping. I mention this because often times DJs will have EXCELLENT equipment which delivers or handles high power, but gives poor performance when combined improperly.
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Re: Power - RMS Vs. Continuous Vs. Program Vs. Peak

Postby TheBartman47 » Mon Jun 06, 2005 12:04 pm

FDJA wrote:When it comes to RMS, PROGRAM and PEAK though, pay the most attention to RMS (continuous), this is the number everything else is based on... Remember that "watts" is a meaurement of heat. RMS is the actual sustained long term THERMAL limit that the speaker coil can deal with, or that the amplifier can produce without thermal fatigue. Both PROGRAM and PEAK ratings are typically an estimate or approximate thermal limit which indicates at what point the speaker coil will melt or the amplifier will fail.

For speakers, not only thermal limit but mechanical limit as well, as in overpowering them can also cause the coils to extend beyond the magnet gap or bottom out on the back of the magnet structure. This can occur even if driven at the posted RMS rating, but fed too low a frequency for the speaker to handle.

For amps though, the peak power ratings aren't necessarily the thermal limits of the amp, but rather the amps voltage rail limits. Thermal limits are more easily exceeded because of placing too much load on the amp (trying to drive four 8 ohm speakers on one channel that is only rated down to 4 ohms). Even though you may only be running it at low volume, you could still over thermal the amp because the impedance (ohms) rating is being exceeded.

One other thing I might mention, because it's so often overlooked: When multiple loudspeakers are necessary it's best to keep them symetrical and matched. For example, if running multiple loudspeakers from the same amp, they should be identical. When they are different, (disimilar brands, driver sizes, etc), it throws your amp into an electrical frenzy... An amplifier's ideal performance will occur when it's driving what appears to be one constant "load"... A load which has the identical electrical and mechanical reaction. If one loudspeaker does not respond EXACTLY as another, it limits the amplifiers ability to deliver a pure and powerful signal without distortion or clipping. I mention this because often times DJs will have EXCELLENT equipment which delivers or handles high power, but gives poor performance when combined improperly.


Excellent point. The reason is that speakers are NOT exactly 8 ohms (or 4 ohms depending upon models) at all frequencies. In fact, the impedance will be all over the place on a chart, but the majority of the impedance should be near 8 ohms (or 4) for most frequencies. When mixing unalike cabinets, the frequency response will be different and the impedances for each cabinet will be higher or lower in one than the other at varying points. And the cabinet with the lowest impedance at a given frequency will receive more power than the other at that frequency. This results in weird coloration of the sound. It would be like having an EQ on your speaker that had random frequencies turned up or down. Also, if using an amp with maximum potential power for the two combined speaker ratings, then one or both could be overpowered because at many points they won't be sharing the power equally. Suppose both speakers are rated 500 watts and you have a 1000 watt amp, then each speaker will get 500 watts if they were identical. But with mixed speakers, you can easily see how this could be fatal to the speaker because at varying frequencies, the power sharing won't be equal and so at some points one could get 600 watts while the other only 400 watts.
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Postby dave lundon » Mon Dec 19, 2005 11:38 am

A rough formula fo r RMS power is to take the maximum output of one peak and multiply it by .707 so a 500 watt max out put would run continuoes roughly 353 watts.

Older amps had no volume controls and they were always run wide open for increased headroom.
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Postby TheBartman47 » Mon Dec 19, 2005 1:04 pm

dave lundon wrote:A rough formula fo r RMS power is to take the maximum output of one peak and multiply it by .707 so a 500 watt max out put would run continuoes roughly 353 watts.

Older amps had no volume controls and they were always run wide open for increased headroom.


Not quite. The power rating on an amplifier is already in RMS terms. So, a 500 Watt amp actually has peaks that are much higher. And for power, it isn't simply .707, but you need to calculate the power by voltages if using the root-mean-square method (the inverse of the square root of 2 is approximately .707)

So, let's crunch some numbers and see what happens...
P=Power in Watts, I=Current in Ampeers, and V=Voltage in Volts.

I=V/R
V=I*R
P=I*V

To get P in terms of V and R, substitute the I in the power equation with it's equivalent (V/R), thus making the new formula P=(V/R)*V or V^2/R (V squared divided by R)

Now, if an amp was supposed to be 500 Watts peak (not RMS) into an 8 ohm load, then this would mean it has to be at a peak voltage of V.

Let's solve for V...
Multiplying by R on both sides of the equation results in P*R=V^2, then take the square root of both sides and you get V=sqrt(P*R). Now plug in 500W and 8 ohms and V=63.25V.
Now, calculate the RMS value of that peak voltage assuming it is a sine wave V RMS=V/sqrt(2), so V RMS = 44.72V.

Now that we have the RMS voltage value, let's see what the RMS power would be...

P=V^2/R so, P = 2000/8 = 250 Watts. So, as you can see, the power difference of a sine wave's Peak and RMS value is NOT a factor of .707, but in fact a difference of 3dB. (So, if your amp was truely rated 500 Watts peak, it's only 250 Watts RMS, but this is irrelavent because amps are rated RMS to begin with.) This is why an amp can produce TWICE it's rated RMS value if driven by a pure square wave (extreemly clipped signal) and thus results in driver's being blown big time. If you have an amp rated 500 Watts, and feeding a speaker rated at 250 Watts continuous, 500 Watts program, and 1,000 Watts peak, which is a very good match by the way, if fed a seveerly clipped signal, the amp will actualy be putting out 1,000 Watts, and since a clipped signal is DC, this is actually a continuous signal, so the net result is 1,000 Watts continuous into a speaker rated only 250 Watts continuous. Meltdown is inevitable.
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