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RMS...wut exactly does it mean

Discussion in 'Amps and Cabs [BG]' started by stanger503, Apr 15, 2004.


  1. wut does rms mean or whatever. i dont understand it at all???help please :confused:
     
  2. Mario Lewis

    Mario Lewis

    Jul 6, 2001
    Clinton, MD
    RMS (according to my uneducated guess) is the acronym for Root Mean Squared. In laymens term, it is 75% of what the cab can handle.

    Now, this is strictly the stree version. I'm sure Bob Lee will dispute this. And I hope he does. Heck, I'd like to know more myself!!
     
  3. Really Misleading Stat

    no really its just a "Standard" for wattage ratings lets say one company might rate their products with a peak power rating to trick you lets say they said their amp is 800 watts well thatsd probly around 400 watts RMS most companies go by rms but you gotta be carefull and know what your getting. some companys still bend rms a little bit but not too much.
     
  4. Mario Lewis

    Mario Lewis

    Jul 6, 2001
    Clinton, MD

    You're right, I was way off, here's Bass Player's definition (copied verbatim from their site)

    "The power handling of an enclosure can be expressed by three different figures: continuous, program, and peak. Continuous power (often called RMS, or root mean square), refers to the average amount of steady, uninterrupted signal a speaker can take. This is the rating you should look for--not the program rating, which is determined by subjecting the speakers to variable power levels. (Program ratings are often double that of the continuous rating.) Even more inflated is the peak rating; it's the maximum amount of short-term, high-powered peaks a driver can endure. "
     
  5. ok thanks. more info could be used too. im still a lil bit confused. im just gonna get a little practice jam amp. maybe from johnson??? :eek: . so would like 47.5 rms be good enough or wut???
     
  6. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Define "practice/jam". For bass players, 50 watts RMS is enough to sit around the house and practice. It's not nearly enough to be heard in a band practice situation, especially if you're competing against a loud drummer and a Marshall stack. If the latter is what you had in mind, think about 300 watts "minimum".

    About the power ratings, there are two common ways to blow a bass speaker: a) driving it with way too much power for a very short time, and b) driving it with a little too much power for a long time. Option (a) has to do with the massive amount of peak power that bass transients can deliver to a speaker (think about slapping that low B string very loud, then think about the speaker cone trying to jump out of its frame as a result - that's "peak power", and it has to do with the limitations on the "excursion" of the speaker cone, or how far it will travel before the surround tears or comes unglued and the cone goes flying across the stage).

    Option (b) has more to do with RMS power. 1 watt RMS will create the same amount of heat in a resistor as 1 watt of DC power, if the signal (or voltage) is applied "continuously". If you apply more than this amount of power to the speaker, the coil will overheat and the speaker will fry. The key concept is "continuous", which rarely happens in real life bass playing situations. To measure this type of power, they take a signal generator and tune it to a specific frequency (usually 1 kHz, which is pretty meaningless for bass players), and apply that signal to the amp "continuously".

    Usually the peak power rating of an amp (or speaker) is two or three times its RMS power rating. But, there is no simple formula that relates the two. The two ways of measuring (and rating) power are completely independent, they're like apples and oranges. You can have a low-RMS and high-peak speaker (like those thrusters in the car audio systems), or you can have a high-RMS and low-peak speaker (like the drivers in the original SVT cabs).

    For bass players, both rating systems are important. If you have a 5-string bass, you'll definitely be interested in the peak power that your amp will try to deliver to your speakers when you slap that low B. And if you're running a thousand watts on an outdoor stage in the middle of summer, you'll probably be interested in the RMS power handling capabilities of your speakers.
     
  7. I'm not sure if this is too much information, but here goes...
    The reason we have this problem with ratings is because we are working with alternating currents, e.g. 50 Hz or 60 Hz mains, and whatever notes you play on your bass.

    When you have a direct current system, like a torch, power is applied continuously, but with an ac system, the power is applied, drops to zero, applied, drops to zero, etc. The mathematical fix that you use to make ac power comparable to dc power is to take the root of the mean of the square, hence RMS power.

    Finally, your ear is not linear in what it hears, so twice the power does not give you twice volume. If you are looking at two amps, and one is 45 W and the other 50 W don't base your choice one one being more powerful - you won't hear the difference.
    IMO, 10 - 30 watts lets you play along with CDs at home, 50 - 100 lets you rehearse with a band, and where you go after that depends on the band, the type of music, size of venue and the type of PA system that you have. Nonsqtr has a different figure for rehearsing, based on the music being played, but if they can't hear you, and you can't go any louder, they can always turn it down.
     
  8. The other factor that no-one has mentioned yet is that what you put the watts into also makes a difference.

    A small practice amp, like a Peavey microbass, sounds good, but only has an 8 inch speaker. To move enough air to be loud, that would have to move a long way in and out. A single 10 inch speaker is better, but not by much. A small amp driving a quality extension speaker cab will probably sound louder because the speaker will be more efficient, but then it will not be as portable.

    The threads by people that use small amps for perfomance are almost all in the acoustic/jazz/church worship areas, and if that is not what you play, you may not get the volume you need from a small combo amp.

    The bottom line is that you will probably end up with two amp/speaker systems, to cover the three scenarios, practice at home, rehearsal, performance. You need to decide which scenarios you want each one to cover, and then look for one that fits.
     
  9. HeavyDuty

    HeavyDuty Supporting Curmudgeon Staff Member Gold Supporting Member

    Jun 26, 2000
    Suburban Chicago, IL
    "Wut"?

    :meh:
     
  10. Wut Wot?
     
  11. With due deference to a moderator, perhaps I should try harder.

    I covered three topics in two posts.
    Are you saying
    (a) Yes it is too much information, and I don't need it, or
    (b) I don't understand some of it, but I want to, or
    (c) I don't understand any of it but I want to, or
    (d) I don't agree but I'm too polite to say so, or
    (e) None of the above?

    Any clues about which bit would also be useful.
     
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    RMS stands for "root mean squared," which is a method of measuring AC voltage or current (not power) in a way that allows it to be compared to a DC voltage. When you calculate power using RMS voltage and/or RMS current terms, you get average power.

    I guess that "RMS power" came from the fact that the power is customarily calculated from measuring RMS voltage across a known load resistance--as opposed to measuring power by using, say, a bomb calorimeter. ;) RF transmitter power (particularly TV) is usually measured calorimetrically, but audio generally is not.

    Technically, there is no such thing as "RMS power," but customarily it has come to mean "continuous average power with a steady sine wave." And usually it does, but not always. A few years ago I saw specs for an obscure power amp brand from Europe--Spain, I think it was--and their power testing method involved pulsing 20 milliseconds of a 1 kHz sine wave at full power every half second (this is the old IHF power measuring method). And they called it "RMS power," I would presume because they must've measured the RMS value of the output voltage during a 20 millisecond burst and calculated the power into the load from that. But that is definitely not a continuous power rating. So beware, because what you expect "RMS power" to mean is not always so.

    Here's an article I wrote for Live Sound magazine a few years ago, part of which describes "RMS power":
    http://www.qscaudio.com/support/library/papers/puzzle.pdf
     
  13. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    AES has a published procedure for measuring RMS power. I'm not sure it's a "standard", but AFAIK many manufacturers have chosen to adopt it.
     
  14. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    That's for measuring loudspeaker power handling capacity.
     
  15. HeavyDuty

    HeavyDuty Supporting Curmudgeon Staff Member Gold Supporting Member

    Jun 26, 2000
    Suburban Chicago, IL
    (e). It was a social comment on the original poster's use of English.
     
  16. Oops, my mistake.

    Perhaps I need to turn the sensitivity down a bit.