Can anyone explain the "RMS" watt rating on an amp?
I know that the output is much lower, but can anyone explain wat RMS is?
Thank you

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Well, if I remember correctly from my GCSE physics when I was 15 (waaaaaaay back) RMS stands for Root Mean Square and as you correctly stated, RMS Wattage is lower than the "normal" Wattage quoted on any appliance.

The term RMS is most commonly found when discussing Voltages but since power (Wattage) is directly proportional to potential difference (Voltage) the term can also be used when describing Wattage if RMS Voltage is being used in the calculations.

The RMS value comes about because the normal value quoted on any electrical conductor is what the Voltage (or Wattage in this case) would be in a perfect world. But we all know that the world isn't perfect and that's where the RMS value comes in to help us out as it provides us with an estimate of the reduced Voltage.

My memory is a bit fuzzy but I'm pretty sure that RMS is worked out by measuring the Voltage of an appliance (the wattage can then be found from simple equations) at fixed intervals over a fixed period of time. If the Voltmeter is sensitive enough, you will get a different value each time.

Now, when the measurements have been made, you square them individually, add these squared values together, divide this total by how many measurements you made and then find the square root of this resulting number. [It's this process that the name RMS derives from: RMS = the square ROOT of the MEAN average of SQUARED values] This gives the RMS value and is LOWER than the value quoted on the appliance due to energy being lost within the electrical system.

Energy (in the form of a potential difference, ie, Voltage) can be lost in a number of ways among which is eddy currents in the conducting material. Transformers suffer from this a lot, with the energy being lost mostly through sound energy. This is why you can sometimes hear a buzzing sound from large transformers.

Energy can also be lost through impurities in the material that constitutes the conducting material, for example, non-conducting atoms (such as carbon) present in an iron conductor. Any non-conducting atoms will impede the flow of current since they will not contribute electrons to the electron flow. This has the effect of lowering the Voltage since Voltage is directly proportional to the current flowing in a conductor.

Anyway, the main thing to know is that RMS is lower than the quoted value.

As I said my memory's a little fuzzy but I hope this explains what RMS is and why it exists.

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alright
now, does that have anything to do with the amps output? (ie a 100 watt head) how does that or does that wattage have a common ground with the RMS watts??????

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The way i learned it is that if you have a 250 watt amp, 190 watt RMS, then you should be able to play at the full 250 watss for a short time but you run the risk of blowing it. at 190 you be the peak power you could play for long periods of time witout blowing the amp, or massivley screwing shit up. I had a 40 watt amp and turned it up all the way, gradualy not from the start. And i played for about an hour or so before it strated vibrating and shit. I ended up blowing the speaker. Im pretty sure this is true but whatever.

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The output will tend to be lower than the 100W stated as 100W is an upper bound that is never reached. However the amps output does not solely depend on it's Wattage (RMS or otherwise).

Wattage is another word for Power, which is the rate of change of work done with respect to time (moving up to A-level physics now!). Basically, this means that the wattage rating is a measure of how much work your amp can be doing at any particular moment.

So, for example, if you have two 12" speakers with a Wattage rating of 100W the sound output will be lower than two 10" speakers with a Wattage rating of 100W.

This is because it takes more work to get a 12" speaker to vibtrate than a 10" speaker and therefore the 10" speakers will vibrate more vigorously, producing higher volume/output.

The relationship between RMS and normal Wattage is something similar to what Copsevomit98 described above. If you crank you amp right up to it's maximum settings, in the beginning your amp will have an actual Wattage near to the normal Wattage rating (although slightly below it).

However, if you maintain that setting, the energy losses within the electrical system will begin to increase (eddy currents within the conducting wires become more prevailent, for example) and you amp's output will begin to drop to the RMS level.

This is all very well theoretically, but most electrical systems don't like being put under intense or prolonged stress and will sometimes fail if the stresses they suffer become too great. This is where my (somewhat fuzzy!) knowledge ends, so if you would like to know the specific reasons why they fail you'll have to find somebody who did electrical engineering at university.

It's a little hard to explain but I hope this helps.

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Yes that does help, that you very much, thank you to all who have helped,
its a tiny bit fuzzy, but i get the just of it now

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No problem, glad to help out.

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This is probably one of the only topics I can actually help people out in.

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ive never been more confused in my entire life

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this is what confuses me, what you said above sounds right but wrong at the same time.
so let me see if i get what you ment,a smaller speaker = less wattage needed to vibrate it/make it loud.
if thats what you ment, i understand how that makes sense, however, 12" speakers are normally louder and produce more volume than 10" speakers(although there are normally variables between different cabs). this is because a bigger speaker will move more air, producing more sound. something to that affect, so i'm confused now, it all makes sense it just confuses me how it all works together.

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Yeah, I guess you're right. Sort of! I was only talking about the actual speaker itself because it goes without saying that a smaller speaker will take less energy to vibrate at a certain level than a larger speaker.

There are obviously other factors to take into consideration such as, as you mentioned, the volume of air surrounding it and the material that the speaker is made of, but my knowledge of those areas of acoustics is very minimal and it's not worth me commenting on. Maybe somebody here who does can help out?

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this shit is way confusing, there are so many factors; open or closed back cabs, wattage, resonance of the cab, size of the room, acoustics, size and numbers of speakers, blah blah blah. after a while i start to feel like none of it matters as long as my amp is loud enough.

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Amen to that!

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OK, I'm a bit rusty on this stuff, but I'll try to explain it easy for everyone. RMS is basically the amount of power you would get out of an AC signal if it was DC (there's all kinds of technical power measurements and concepts like real, apparent power, etc, but I won't go into that). Electrical power is mostly dissipated as heat, and can be converted to other forms in the process . e.g. sound, but the sound energy is minimal compared to the heat energy. So your speaker voice coil has an AC signal across it. AC is voltage that is always changing, usually in the form of a sine wave, so the average value over time is zero. Thus, since watts are proportional to voltage, it's a bit hard to measure. An RMS (root mean square) wattage of a conductor with AC power through it equates to the amount of heat/power that would dissipate in a DC conductor. It is calculated by taking areas under the sine wave curves and squaring them or something like that; I forget.

100WRMS for a speaker or amp is also usually referred to as music power. i.e. the power that it can constantly handle. On the other hand, Peak Music Power (PMPO) and IHF Short Term Power Output, refer to the power level that can be output for a short time (about 1 second). Always go with the RMS

With amplifiers, they are usually only 50% efficient, meaning half the electricity power input comes out of the amplifier output. So if you have a 100W RMS amp, the power supply *should* be 200W. So that is why you usually see higher values printed on the back of a product; this is the input power that the whole product consumes, not the amplifier output. Normally "clipping" is caused when the amplifier is turned up to loud. This is due to amplifier trying to draw too much power from the supply; the power supply rails capacity is being exceeded. This causes a, say, audio signal (which consists of sinewaves) to have the peaks of the sine wave to be flattened at the top (i.e. horizontal lines = DC). These flat bits are DC (constant voltage) and can go to your speaker and damage it. A speaker voice coil should only have AC going across it, since the power of a DC signal will heat up the voice coil fast. So even if your speaker is rated at 200WRMS, and your amp is 35WRMS, you're not safe. Trying to turn the 35W amp up can ruin the 200WRMS speaker. You're less likely to ruin a 35WRMS speaker on a 200WRMS amp by turning the volume up. Some people put in speaker protectors in hifis that protect against clipping and overdriving the speaker. I'm not sure if these are used in guitar amps.

Guitar amps have confused me. I have several distortion circuits that alter the guitar waveform to produce distortion. Some add harmonics, some flatten sine waves, etc. I can't understand how guitar distortion pedal and amp manufacturers allow these DC signals to go through the speakers. It's my general feeling that speakers in guitar amps are not really hi-fi speaker, so it's OK to put rough signals through them. On a normal hi-fi I would suggest turning the volume down if it sounds harsh or distorted. Usually it's the amp clipping (as described above), causing the distortion, rather than distortion from the speaker being overdriven. Usually, you're best bet is to only turn the volume up half way (this comes back to the power supply rating). You'll usually get clipping after that point, and not much more volume anyway.

Now, if you have the bass cranked up, it's another factor. Lower frequency signals (bass) take more power to generate than higher power signals at the same Sound Pressure Level (SPL) (~volume). Thus, you're amp will start to clip at less than halfway probably. It will sound "ploppy". Bad for the speaker.

Speakers also have a SPL rating. Usually db/m @ 1W. Decibels (dB) is a base 10 logarithmic scale. i.e. 20dB is 10 times louder than 10 dB, and 30dB is 10 times louder than 20 dB (so 30 dB is 10x10=100 louder than 10 dB). A speaker with a SPL rating of 94dB (which is quite good), will produce 94dB of Sound Pressure at a distance of 1 metre when 1W of power is input. SPL can be used to compare speakers. Usually larger speakers have higher SPLs, that is, they produce the higher volume sound than a smaller speaker when both have the same power input. So When you put more than 1W into a speaker, e.g. using a 50W amp, this increases the SPL output above the rating of the speaker at 1W. All these things tend to work on logarithmic scales. Take, distance from the speaker for example. If you are 3 metres away, the loudness of the speaker will be 1/3 squared = 1/9 of what it is at 1 metre away. At 6 metres away, it'd be 1/36. The 1/d squared relation is caused by the sound dissipating in a sphere pattern. The sphere's surface area is in a squared relationship to the radius. So as you move further away, the sound at any point of the sphere is a smaller fraction of the whole sphere, as the sound energy has to distribute across the surface area of the sphere.

So, in reality, as you mentioned, room size, sound reflections, etc contribute a great deal. They'll cause dips and peaks in the frequency response of the speaker in different areas of the room. You should try to position your amp so "standing waves" and cancellations of waves are minimised. That is why there'll be sweet spots for you and the amp in certain rooms. Better yet, the room should have acoustic dampening material on the walls to stop reflection. Usually stuff like curtains absorbs some of the waves and stops them reflecting.

If you want a loud amp though, I suggest the main factors to be SPL rating of the speaker, and Watts RMS of the amp. A good chunky power supply is also good if you can check that out, or make sure the product's total Watts rating on the back is about twice as much as the amp's RMS rating. It is also genrally a good rule of thumb to use a speaker and amp that are close in power rating. Quad boxes seem like a good idea, since I'm told that if you wire up 4 small speakers vs one big speaker, with both configurations using the same amp, you'll get higher SPLs/volume, but the "loudness" won't be as great. The presence of the sound fiiling the room and moving air or something I think is what they meant would be less, but the pressure on your eardrum would be greater. I'm not sure. But I do know that having more than one speaker creates noise cancellations and noise reinforcements at different points in space, due to the 2 o r more sources interfering.

If your amp is 4 ohms (amps that are designed to be hooked up to either a 4 ohm or 8 ohm speaker usually have around double the power output on 4 ohms), and you have a single speaker, I'd suggest trying this: Buy 4 cheap car speakers that are rated at about a quarter of the power of the single speaker, and hook them up in a series/parallel fashion so the overall impedance is still 4 ohms. Hook those to your amp and see how those go. Bear in mind that they should be in a cabinet; I won't even start on cabinet details, there's a shitload of books on it. Just knock up something, it doesn't have to have a back or anything, just try not to make it too small. See how that goes, you'll usually get a better frequency response, like you are running a guitar effects unit through a hifi.

I hope that explains stuff.

The "normal" energy value is found by integrating, or put another way, finding the area under the curve of the function, f(x) ("f of x"). f(x) is usually of the form f(x) = asin(bx/t) for some constants a,b and variables x,t. a is the amplitude of the sine wave and b either shortens or lengthens the frequency of it (if b=1, then the frequency is 360 degrees, ie one cycle every 360 degrees). x and t are angle and time respectively. There is no need to square this value once calculated (the Root Mean Squared term arises from the statistical method of calculating it).

The RMS value can be found by integrating the function representing the "clipped" AC current. However this can be difficult since the resulting function is non-continuous, ie the graph of the function has sharp points (cusps). These are at the points where the sine curve suddenly becomes a flat line parallel to the horizontal axis or conversely, at the points where the flat line suddenly becomes the sine curve again. This is how the non-continuity arises, since the graph is existing as two functions (a straight line and a sine curve) at the same time. It is a major stumbling block when we attempt to integrate at these points and can usually only be overcome by using advanced mathematical methods such as integrating the function in the complex plane.

However, this is an EXTREMELY cumbersome method of calculating the RMS when a good statistical approximation already exists (see my earlier post for how this is calculated).

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