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Thread: facts about speaker power ratings

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    facts about speaker power ratings

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    hmm........
    --------------------------------------------------------------------------------

    Okay, I hear this one WAY too much and it has never made any sense to me, so I would like to clarify a bit..............

    Then are two different types of measurements when it comes to Power supplied by an amplifier and Power absorbed (or handled) by a speaker.

    1. MAX - this is the Maximum power the speaker will EVER handle, or the amp will EVER supply.

    2. RMS - this is the average power and is a measurment used in the industry MUCH more. Since many electrical compnents run on AC voltage (and an amps output is an AC output) the output is going to oscillate between it peak (MAX) positive and negative outputs. But if you averahe this over time you get numbers that are more "steady".

    The 120V supplied by a US standard power outlet is an RMS measurement. The MAX or peak to peak voltage that you REALLY get is more like 170V. Now when have you EVER heard someone talk about having to get a toaster that can hanlde twice as much power as your outlet can supply. I'd bet you haven't..

    Usually a reputable company will give output of its amplifier or the handling of its speakers in RMS terms. The place where you run into trouble is when companies with lower quality products decide to make thier products sound better by using MAX ratings instead of RMS ratings. They will advertise thier amp as putting out 14 watts when that is really the MAX value and the actual RMS rating is 10W. The consumer sees that and says "wow, that is a good deal on a 14 watt amp, thats much better than that 10W marshall amp"

    The same goes for speakers... some companies will only advertise thier power handling in MAX values, so you buy a speaker cabinet for your amp that says it will handle 100W, but it really can only handle 70W RMS. (you see this sort of things A LOT more in PA cabinets, though)

    The moral is the story is to BE CAREFUL, and study the products that your are looking to buy. Sometimes you will have to actually read the product manual or get the specs from the companies website. Then you begin to realize that that incredibally cheap PA power amp that can put out 1500W bridged at 2 ohms, is really only putting out 75W RMS at 8ohms, and is WAY underpowered!! And THAT is why it is so cheap.

    Remember to match the proper values!!! If someone is advertising thier product in MAX values, match it with a speaker of the appropriate MAX value. If someone advertises thier product on RMS values be sure you match the RMS values of the other components.

    Here are some helpful equations.....

    Max to RMS = Max/ square root of 2
    RMS to Max = RMS * square root of 2

    Square root of 2 = about 1.4 (notice is is still not quite 2)

    Now don't believe the people who will automatically tell you that you need to get speakers rated for twice the power output of your amp!! When you are dealing with classic companies like Fender, Marshall, Celestion, the are all using RMS values. A Vox AC 30 originally shipped with two 15W speakers (a 30W equivalent) and how many people have you ever heard of that have thier speakers in the AC30 constantly blow up- well you don't!!

    Just remember to match RMS to RMS, and MAX to MAX, and do a little homework. The old twice power speaker handling to power amp output rule is a good "safe" rule, but is often unneccesary. If it were absolutely true than you could never use a 4x12 cab loaded with specialty speakers like greenbacks and you definately see people running these all the time along thier 100W amps, so Never fear!! Just be smart about it and stop listening to salesmen

    THE END

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    In some cases, and if you are extremely careful, you can get great results by running speakers with a LOWER power handling than the amp. For instance, my Peavey Classic 30 tube amp outputs 30 watts RMS. The stock Blue Marvel speaker is I believe 50 watts RMS (maybe 60). What did I load it with? A 25 watts RMS Celestion Greenback! This results in speaker distortion occuring earlier than normal, but at my volume levels this usually isn't a problem, and the sound benefits of the Greenback far outweigh the lower volume. If I need more volume, then I just use my extension cab with another Greenback and drive the poweramp harder.

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    In most cases it is better/safer to run a power amp with more power than your speakers can handle. Many more speakers are blown due to underpowered amps, not overpowering your speakers.

    I don't have time to write a complete reply, but much of the info in Gus' post is incorrect. I'm not flaming or trying to be a jerk, but it's as bad or worse than much of the bad info he is trying to help people see through.

    It's a good topic, and should be discussed.

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    so is it ok..

    so is it ok if I run 120 watts peak, 100 wrms through my marantzes? they say reccomended amp power be betwen 10wrms and 165wrms per channel, but Im wondering, is that their true RMS value, 165 watts rms, or is that pretty close to peak? the model is SP1200, and I have refoamed the woofers ( wich are holding up beautifuly, and I have one refoam under my belt! btw, I didnt do it the reccomended way because I wanted to avoid coil rub,so I put the foam behind the cone and glued it, it seems to do fine, and that glue isnt going anywhere soon! the foams are warranted for 7 years of lasting too!) I have had a sound guy at church that originaly had these tell me that he thought they would probably handel 300 watts on a peak , with the old x2rms to get peak power, but would the coils fry? Also, when you are doing 300 watts peak, what would your most likley RMS rating be, wouldnt it be higher than 165 watts? one more thing, these are "vintage" from 1978, and I know back then a lot of high end stuff was very conservativly rated, so im just wondering 3 things: 1. is my ca857 Fisher Studio Standard (100 wpch RMS, 120 wpch peak ( or at leas thats what the peak lights on it read up to , and go up there quite freqently when listening )) going to hurt the transistors, I have the amp cooled with twin Antec pc fans , each putting out 35 cfm directly on the heat sink mounted on the back. 2. is there amp dist that I dont know about when the amp is RMSing at 120 into the speakers, is that why my dust caps get warm after a while of hard listening? and 3. what can I truly run into these without hurting them, or melting the voice coil due to amp dist from running too much power out of an amp, and what is the true peak power?? sorry for all the long questions ( phew!) would a 200 watt per channel amp hurt these?


    thanks so much for any help, TMFK~
    meowers are cheaper to replace than woofers..

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    hmmm...........

    Boingoman - I would definately be intersted in hearing your take on all of this.

    I completeley understand that any of the "good" speakers out there are probally underrated as far as the power they can handle. You can probally get away with more than I am letting on, but I'd hate to see people buy junk and blow it up -- then again they would probally think twice about buying something that is quality the second time around.....

    The info I posted was based on my current knowledge of electrical circuits, but the fascinating thing is that many things are counter-intuitive like the mentioning you made of speaker blown by underpowering them - please, continue when you have more time, I'm all ears............

    I just hate for people to have to spend more money on bigger speakers so that they don't blow them up, when they really aren;t in danger of that anyway!!

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    Quote Originally Posted by gusfinley
    The info I posted was based on my current knowledge of electrical circuits, but the fascinating thing is that many things are counter-intuitive like the mentioning you made of speaker blown by underpowering them - please, continue when you have more time, I'm all ears............

    I just hate for people to have to spend more money on bigger speakers so that they don't blow them up, when they really aren;t in danger of that anyway!!
    As I understand it, the reason underpowered amplifiers are a problem is because when you are using an underpowered amp, you are more likely to have the signal from the amp clip because you turned it up higher to try and reach the same volume or whatever reason). The problem with this is that if you take a 100w amp and drive it into full signal clip, it is actually putting out 200w. This also explains why some people (and a lot of cheap amp companies) recommend using speakers with power handling that is double that of the amp. That way, the driver will not be overpowered even in the event of full clip.

    Now if you use a higher powered amp, not only does the amp not have to work as hard, but it is also capable of providing very adequate power without the chance of clipping. Should clipping actually occur, you will be in a world of trouble with some damaged speakers, but if you have any clue about what you're doing and watch your poweramp levels then you should be just fine.

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    I've touched on this subject a few times with regards to how I rated the JBL "F" series, but maybe I didn't make it very clear.

    Most "hi-fi" speaker ratings are designed around the premise that the speaker system will be playing integrated music (like orchestras and bands), covering a wide range of notes played all at once. For that, a simple RMS rating works nicely, and, as others have pointed out, a lot of manufacturers will fudge and use "peak power" and "instantaneous peak power" which can result in wattage figures in excess of 4 times the RMS value.

    But musical instruments are different, especially guitar, which only has a limited range of about 80Hz to around 6kHz. And there are two types of signal coming out of a guitar: chords, and single note solos. So how do you figure out the wattage ratings for those two conditions?

    For chords, usually the RMS rating works fine, but one note at a time solos can usually go twice that power rating (unless the notes are really low, where the speaker has to work a lot harder).

    A synth, playing a very low square wave, can easily destroy a speaker at well below it's nominal power rating. It's a big problem for musical instrument speaker manufacturers. Bottom line answer:

    It depends. (It depends on the character of the signal you're putting into the amp.)

    Sorry, I wish there was a more definitive answer, but there really isn't. For some music, you can run a 500 watt amp into a 30 watt speaker all day long without problems, while you may blow a 100 watt speaker with a 20 watt amp.

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    For chords, usually the RMS rating works fine, but one note at a time solos can usually go twice that power rating (unless the notes are really low, where the speaker has to work a lot harder).
    Sorry for the dumb question. Why are chords lower on power? I supposed it was the other way.

    Ah, and one more thing. How does it work for electric bass?

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    The power is dissapated across a wider band of frequencies. For equal volume, a speaker has to double its travel for every octave it goes lower. If a speaker moves X distance for a given note, it has to move 2X to play an octave lower note at the same level.

    Getting enough level at the bottom end has always been a problem. The solutions are either a tuned cabinet, horn loading, or multiple speakers.

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    Gus-

    Again, this is a good topic, and you gave some good advice, mainly know the specs and what they mean. Many cheap mfrs. leave important info out, or cloud the numbers.

    I'm a former salesman, and know that the best thing for a buyer to do is be informed. Many salesman just don't have the knowledge, and like shady mfrs, some won't tell you they don't know.

    Also, just the idea of using some common sense. As Harvey pointed out, there isn't one answer for matching speakers and amps.

    And as you pointed out, there is a difference between choosing for PA systems and guitar amps. There are also differences between guitar amps and PA amps.

    You pointed out the misleading tactic of listing a speaker's peak power handling, and people thinking that their new speakers can handle that all day long. I actually find this more in the home stereo field rather than MI PA boxes, unless they are real bottom feeders like Nady. But it is definitely out there.

    The lucky thing is that in the home studio, using a home stereo, or club-level gigging like most of us do, we never approach the limits of our gear, so it is rarely an issue. (Except for DJs and bass players. , or if you have a couple and gotta "crank it" for a minute )

    As far as selecting speakers, JBL and EAW both have white papers on this subject. The up shot is this:

    1. When using a system where the speakers will be used at or near their average/program power, with dynamic material, such as speech or live music, use an amp that is more powerful than the speakers. This allows transient peaks to be reproduced without clipping the amp, resulting in speaker damage. A power factor of two times is recommended. If the speakers start to distort, turn it down or get more speakers.
    So this is for PA systems, some guitar/bass rigs, and studio monitors, and listening to well-recorded very dynamic music, like some classical with extreme dynamic range.
    p.s.- 4X30W greenbacks can easily handle 100W, and most people never put anywhere near that into them anyway.

    2. When reproducing heavily compressed material, such as much pre-recorded music, where the speaker will be driven near or below it's program level, it is acceptable to use an amp rated at the speaker's maximum rating, as peaks will be below the speaker's maximum rating, and the amp will not clip.
    They are talking about thing like your average home stereo, and much modern music.


    3. In situations where low-level reproduction of compressed material is required, it is acceptable to use an amp rated below the speaker, provided peaks are minimal and the amp is not driven into clipping.
    This is things like paging systems in offices, and muzak systems.

    These last two also explain why you can use small speakers with a big amp, or one rated the same, like your vox ac-30 example. Guitars are very dynamic, but most of us never get to the point where clipping is an issue, or where peaks prove to be dangerous to the speakers. The average level we play at is too low, and guitar amps have their own compression/ limiting built in.(more later). It is hard to make a guitar amp clip in a bad way.

    Both JBL and EAW say that these are rules of thumb, and the most important factor is a knowledgeable operator.

    The main thing is common sense. If you have a 100W peak speaker running flat out in a club, and try to boost the snare up a little more, it's gonna blow. If you try to squeeze "a little more juice" out of your subs with a too small amp running flat out, they are going to blow.

    And for sure, some companies use the most favorable numbers they can to market their products.

    You asked about underpowering speakers. I can't sleep, so I rambled off all this stuff.

    1. A power amp's listed maximum power output is not the max it will ever put out. It is far from it, in a lot of cases. The max output of a power amp is usually listed in relation to %THD, or total harmonic distortion. So an amp will be rated at say 260W per side into 8ohms @.01% harmonic distortion. This is the rating for my Crown PB2. The actual level the amp can put out is much higher.
    When comparing power amps, the %THD is one of the things to look at. This type of distortion is described as the difference between the input and output signals. Basically, if you put a sine wave in, at what point does it change, and by how much? The higher the THD, generally the worse the amp sounds.
    Two amps:
    1. 200W into 8 ohms @ .01% THD
    2. 200W into 8 ohms @ .1% THD
    The second amp will actually have a much lower power rating at the same %THD as the first.
    The second amp will put out 200W, but will sound worse.
    Leaving out the %THD is indeed a marketing tactic, to make a product look better.

    My PB2 can actually put out 6-700W spurts, but at unusable and speaker-frying distortion levels. And not the good guitar-amp type distortion. Discounting crappy designs which have high THD to begin with, THD rises as output level rises. So the amp with the higher THD above should put out lower THD at lower levels. As the amp gets clipped (see below) the THD rises to extremes of level. So an amp with a higher level of THD listed is either a poor design to begin with, or the mfr. is listing the max power much closer to the point where THD can become harmful, ie clipping.

    An amplifier takes an input and amplifies it. So a 1 volt sine wave in might be a 10 volt sine wave out. 2V in might be 20V out. This goes on until what are called the "rails" are reached. The rails are simply the maximum voltage the amp can put out with the THD specified. After that, clipping and distortion as the input increases and the amp struggles to match it with more output.

    As the input voltage increases, the output starts to "clip" when the output voltage limits are reached. In the case of a sine wave input, the output waveform starts to get flattened at the top and bottom. The nice round top and bottom get "clipped" off, as the voltage limits are reached. It starts to resemble a square wave. Most amps and speakers canhandle square waves, to a point. But this square wave generates many higher frequency harmonics, which come out of the amp at very high power levels. Remember THD? Very high at this point. A 1khz tone fed into a power amp to the point of severe clipping can generate harmonics well up past 20Khz, and at three or four times the amp's rated output. Goodbye tweeters.
    The fundamental plays a part as well. A 1khz tone, clipped enough, resembles a square wave. It also resembles DC. So a severely clipped sine wave essentially looks like + and - DC voltage at the rail limits.
    For that PB2, 260W= 45.6V. The voltage rails actually go higher, but that is close enough. So the speaker is being fed +/- 45.6 VDC, plus all sorts of high power high freq harmonics. Dead speaker really fast. Dead woofer, dead tweeter, maybe dead crossover.

    This is the danger of underpowering a speaker. A ten watt amp driven into severe clipping can take out a speaker designed to handle 100W.

    That being said, it is hard to get a guitar amp to clip like this. A guitar simply cannot deliver enough sustained output to drive the amp to severe clipping for long enough to cause damage. Clipping at the input results in preamp distortion, which the amp is designed to do, and is not related to output clipping distortion.

    So, this is where the idea of overpowering speakers comes from. Headroom to avoid clipping.



    2. A speaker peak rating of 100W doesn't mean it can only handle 70W RMS. Peak and RMS aren't really related that way in speaker specs. With speakers, all power ratings are RMS. But it has to do with input voltage to the speaker, not the output power or power handling.

    RMS is used because it is the DC equivalent of an AC voltage, in terms of power, resistance, heat dissipation, etc. The formulas for DC are easier to use, as AC is reactive and changing with power and freq. A speaker may be rated at 8 ohms. But really, it might be 6 ohms at 1k, 3.6 ohms at 150hz, and 9.7 ohms at 2.5k, at 50W input. These values might change at 100W.

    So we fudge and use RMS voltage values amd nominal impedances, so we can use nice neat DC formulas. And it all pretty much works out.

    Power=voltage squared/resistance

    100W=Vsquared/8 ohms
    100W= 28.3V into 8ohms

    So an 8ohm speaker rating of 100W peak means "This 8 ohm speaker can handle a max of 28.3V RMS for a very short time".

    As Harvey pointed out, peak, continuous, and program ratings don't follow a formula. They are results of testing.

    Sample speaker specs:

    100W RMS continuous
    200W program
    375W RMS peak

    Continuous is pink noise for a given length of time, hopefully specced by the mfr, but usually eight hours at least. Program is a bit nebulous, as it varies in level and frequency to mimic speech and music. Peak refers to momentary transients.

    Taken together, they give a pretty good idea of how much abuse the speaker can handle.

    Good companies are kind of conservative. I had a set of JBL subs that I ran at about twice their peak rating for hours at many gigs. Never a problem. Less reputable companies will push the envelope, and like you said list only peak power ratings.

    There are other things about speaker specs that can help you out.

    Frequency response:

    Anyone seen this ad?

    The amazing wonder box! Hear it roar!
    85hz-20khz.

    This is meaningless, as it leaves out variations and down points.

    85hz-20khz -10 db
    120hz-17khz +/- 3db

    This actually tells you something. The "10db down points" as they are called, pretty much outline the extreme usable range of the speaker. That is where the response falls of 10 db. It is arbitrary, but useful as at those freqs it will take 10 times the power to bring up the response to the rest of the range. A box listed at 85hz might look good on paper, but if that is the 10db down point, you gotta boost the bass and watch your amp choke to get those 85 hz.

    The +/- 3db range is where for a given power input, the response will vary by no more than 3db.

    This isn't as important for guitar speakers or raw drivers, only complete boxes. Raw speakers often have big peaks and dips, and some guitar speakers are designed that way to sound better for guitar. Putting drivers in boxes shapes their response, to an extent.

    Good companies will list the 10 db down points, and the +/-3 db range, and sometimes the 6db down points as well, where four times the power is required to even the response.

    Shady marketers will want to list the -10 db range without saying it is the 10db down.


    Sensitivity: can be very important!!

    Seen this?
    87db @1W/1M

    This means that they put 1 watt into the speaker and measure the sound pressure level (SPL) 1 meter away. This particular speaker put out 87 db SPL.

    This is called the efficiency. There is a formula to find the % efficiency, but I forget it. It's not that important, but as an aside, speakers are incredibly inefficient, anywhere from 2-25%. 25% is for ultra-modern tweeters. Most of the stuff we see is under 5% efficient. That means over 95% of the power from your amp is lost as heat in the voice coil. 5% comes out as sound. That monster ear-splitting Marshall 4X12 with the 100W DSL head? 2-3W actually make it to the cones.

    Anyway, why is efficiency important?

    Here is an example:

    Speaker 1: 87db @1W/1M
    Speaker 2: 93db @ 1W/1M

    For the same power, speaker 2 is 6 db louder. Speaker 1 needs 4W to reach 93db. Doesn't sound like a lot.

    As far as SPL goes, +3db is noticeable. +6 db is a significant volume change. +10 db is double the volume. Kind of subjective, but those are the accepted terms.

    +3 db= 2X power
    +6 db= 4X power

    To double the volume (+10db SPL) you need ten times the power.

    Let's put 1000W into each.

    Speaker 1: 1W= 87db
    10W= 97db
    100W= 107db
    1000W= 117db

    Speaker 2: 1W=93db
    10W= 103db
    100W= 113db
    1000W= 123db

    They are both pretty loud, but for speaker 1 to be as loud as speaker 2, it will need 4000W! (+6db= 4Xpower).

    Conversely, speaker 2 will be as loud with 250W as speaker 1 is with 1000W.
    (123-6= 117db, 1000W/4= 250W)

    If both of these speakers peak at 126 db, to allow for a safety factor of twice the power, the guy who owns speaker 1 will need an 8000W amp. The guy who owns speaker 2 will be able to get a 2000W amp.

    Thankfully many small speakers are in the mid to high 90s for efficiency these days.

    If two guys have matching guitar amps, the guy with the most efficient speaker will be louder.
    On the other hand, the guy with very inefficient speaker will be able to crank his amp more for the same volume, perhaps resulting in better tone.

    For guitar guys, differences of less than 6 db don't matter much.

    For PA guys, more efficient speakers can mean less money spent on amps. It can also make a difference in how much power you need to run your PA, and how much weight you have to lug.

    It used to be that more efficiency meant a crappier sounding speaker, but not so much anymore. This can still be true, as three-way cone midrange speakers tend to sound great, but are less efficient compared to two-ways.

    Anyway, I've gotten kind of off the original topic, but.....

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