What does RMS mean?
Mike Rocha
I understand amps fairly well but ther are things that are mentioned
all the time in Bass Player that just don't sink in. When thy talk about
power ratings on amps/speakers they sometimes give a rating in, for
example, "300watts RMS". What does this mean? And I vaugley understand
ohms. 2,4,8 ohms? I understand that 8 ohms gives better sound and
clarity and power but it needs a lot more power to do this. Explain
PLEASE.
thanks.
mike
Lofty & Linda
When thy talk about
>power ratings on amps/speakers they sometimes give a rating in, for
>example, "300watts RMS". What does this mean?
RMS is an accronym for Root Mean Squared. When dealing with any form of
alternating current generally the peak power rating is not an accurate
reflection of the average power the device is putting out. So using a scary
formula ( x*$&^^&/ Cheese) it averages out to a truer power rating. This is
techey stuff don't worry too much about it. (more below)
And I vaugley understand
>ohms. 2,4,8 ohms? I understand that 8 ohms gives better sound and
>clarity and power but it needs a lot more power to do this. Explain
Worry about this one a lot!!!!! Ohms are Impedance. (The AC version of
resistance) No one rating is better than another. BUT THE AMPLIFIER
IMPEDANCE AND THE SPEAKER IMPEDANCE MUST MATCH. If they don't then you're
either not running the speaker at optimum power or you'll fry your new and
highly treasured amp. If you're running more than one speaker cab off the
same amp then the impedance changes according to if they're hooked up one
way or another. If you're not sure ask
Hope this helps, Loft
Chadwick B. Owen
Exactly what RMS stands for slips my mind at the moment, but I'm
sure someone a lot more technically knowledgeable than myself
will speak up here. Anyway, the basic idea is that the RMS
wattage is the "true" power of the amplifier, as opposed to
"peak" watts, which some companies do occasionally quote in
advertisements. The peak wattage usually represents how many
watts the amp will produce at a musical peak...say, a really hard
slap on the ol' low B, or in other applications, the thump of a
kickdrum. The peak wattage for an amp is usually in the
neighborhood of _double_ the RMS wattage...sometimes even more.
If somebody advertises an amp with a wattage figure _without_ the
RMS tag behind it, watch out. They're usually trying to impress
you with a big number so they can sell you an underpowered amp.
I'll leave the ohms for someone who can explain 'em better than I
can (which shouldn't be hard...)
Flatline
/\/\____________
Chadwick B. Owen
Aha! Found it! RMS stands for "Root Mean Square".
There's an excellent article by Steve Rabe, founder of SWR, on
this type of subject. Check it out at the SWR website at
www.swreng.com/plugplay.html
Flatline
/\/\________
ShadowBass
cl...@burton.com (RAJOR) wrote:
>OK, I will. I just got a Fender Twin chassis that I plan on restoring. If
>on the back of the amp it reads "Total Load 4 ohms" and I want two 12"
>speaker, what impeadance speaker should I get?
>
>george
Depends how you plan on setting them up... If you plan on wetting them up in
parallel, get two 8-ohm speakers. If you're going series, use two 2-ohm
speakers. Of course, you could always get a cabinet with two 12" speakers,
with an input rated at 4-ohms...
drago...@fuzzbuster.net
I didn't see the answer to the original poster's question, so let me
see if I remember my analog electricity classes...
RMS is the root-mean-square value of a wave. RMS values are the peak
current (or voltage) values divided by the square root of two. This
means that the RMS value is 70.7% of the peak value. Written the
opposite way, peak values are 141.4% of RMS values. In the real world
that means a 300 watt amp produces peaks of 425 watts when it's wide
open. Keep this in mind when purchasing speakers. I've seen alot of
speakers (in the hi-fi world) advertised by thier peak values when you
really need to know the RMS values. I hope my analog teacher would be
proud of me (I sure hated him).
Dean
Dave
Mike Rocha wrote:
> I understand amps fairly well but ther are things that are mentioned
> all the time in Bass Player that just don't sink in. When thy talk
> about
> power ratings on amps/speakers they sometimes give a rating in, for
> example, "300watts RMS". What does this mean? And I vaugley understand
>
> ohms. 2,4,8 ohms? I understand that 8 ohms gives better sound and
> clarity and power but it needs a lot more power to do this. Explain
> PLEASE.
> thanks.
>
> mike
RMS is an acronym for "root mean square." AC current can be viewed in
wave form, with peaks and valleys. The power at any instant is dependent
on the point at where you are are on the wave. The observed power is
expressed as RMS, which can be determined by multiplying the peak power
(the power at the peak of the wave) by .707 (or 1 divided by the square
root of 2, rounded).
8^) Dave
Lofty & Linda
RMS values are the peak
>current (or voltage) values divided by the square root of two. This
>means that the RMS value is 70.7% of the peak value. Written the
>opposite way, peak values are 141.4% of RMS values
It's all Dutch x 2 to me :)
Mark Amundson
> (the power at the peak of the wave) by .707 (or 1 divided by the square
> root of 2, rounded). 8^) Dave
RMS does indeed mean Root Mean Square. The term is used mathmatically in
a equation to determine the effective voltage, current, or power
equivalence to a DC signal. The .707 applies only to peak sinusoidal
voltages or currents; as .5 applies to sinusoidal RMS power from peak
power.
For example, a Peavey amp I own has 300 watt RMS into 2 ohms rating.
Using P=V^2/R,
(watts law) the resulting voltage is about 24.5 volts RMS or 34.6 volts
peak. It is likely that this rating is current limited due to a 210
watts RMS at 4 ohm rating. The 4 ohm voltage swing is about 29.0 volts
RMS or 41.0 volts peak. The peak-to-peak voltage would be double the
peak voltage or 82 volts peak-to-peak. Since amplifiers drop a little
voltage across the transistors during these peaks the amplifiers power
supply has to deliver greater than 82 volts for rated power. I reality,
the amp has +/- 52 volt DC supplies to cover worst case wall socket
voltages, design tolerances, and the DDT limiting circuit headroom.
Mark Amundson,
Scott McDaniel
Mark Amundson wrote:
>
> > > power ratings on amps/speakers they sometimes give a rating in, for
> > > example, "300watts RMS". What does this mean? mike
>
> > RMS is an acronym for "root mean square." 8^) Dave
>
> RMS does indeed mean Root Mean Square. The term is used mathmatically in
> a equation to determine the effective voltage, current, or power
> equivalence to a DC signal.
>
C'mon you guys. Quit messing with him. We all know RMS means "Royal
Measuring System". A set of rules put in place to counteract the
terrible Metric System.
If your amp sez 300 watts RMS than you know it's good to go in England
or the USA, or Canada, Or Australia. Just plug it right in and watch the
sparks fly. Alternatively, without the RMS tagging, and if you are
touring in, say, Nepal, you must first use a ground lift (technical
term: Three Prong Adapter).
Hope this helps.
I am just joking, of course. I could see the bill from his insurance
company, asking me to pay for three SVTs.
Scott "SVT means 'Super Volume Thingy' " McDaniel
Ben Jacoby
Mark Amundson (mamu...@mr.net) spake thusly:
: RMS does indeed mean Root Mean Square. The term is used mathmatically in
: a equation to determine the effective voltage, current, or power
: equivalence to a DC signal. The .707 applies only to peak sinusoidal
: voltages or currents; as .5 applies to sinusoidal RMS power from peak
: power.
This is correct. The meaning is as follows: Take some waveform and first
square it. (this makes the all the negative parts positive) Next find the
average value of that squared waveform. (the word "mean" refers to a
certain kind of averaging) Finally take the square root of the value you
get. This is the RMS value and is equivalent to a DC signal giving the
same watts. The main utility of this is that you can multiply rms volts
by rms current and get rms watts which is the same heating that you would
get if you used DC currents and voltages instead.
For a bass player the main thing you need to know about the RMS rating is
that it is the amount of power the amp can put out *continously*
Other ratings you may see are "peak power" which is the value you get if
you multiply the highest voltage and current you find coming out of the amp.
This value is *much* higher than what the amp can put out continously and
is largely used as a sales gimmick to fool newbies. However, it does
have some relation to the ability of an amp to produce attack.
Another rating is "music power". This is typical of tweeters etc. It
means that since music if full of ups and downs in volume as well as a
distribution of frequencies, the amp (speaker) may produce a sound that
may be as loud as if the amp had the given rating for a short time, but
won't do it for very long. So if an amp is rated at 200 watts music
power, but 100 watts RMS, it may produce a loud crash etc. but any
sustained note at that volume will overload (fry) it. Music power
ratings are not a good guide for instrument amps where sustained
frequencies are the rule.
And lastly, sometimes you will just see an overall wattage rating meaning
the watts going in the plug. This is pretty meaningless unless you know
how efficient the amp is. It is especially meaningless in tube amps.
But often newbies get sold by a huge powercord rating. In a transistor
amp, it's a kind of indicator, but still not much to go on.
--
Benjamin Jacoby | "Some rob you with a six-gun and some with
| a fountain pen." ..........Woodie Guthrie
(SPAM GUARD! Delete the no spam letters in name to email.)
John Smith
In article <34dcce1d.0@vlinsvr>, co...@vlinsvr.vsla.edu says...
>
>I'll leave the ohms for someone who can explain 'em better than I
>can (which shouldn't be hard...)
>
>Flatline
>/\/\____________
You already know that amplifiers are rated in Watts. A watt is a term
that describes the amount of "Power" generated by the amp.
Now in any electric circuit you have several different ways to measure
what the little electrons are up to in the circuit. Voltage measured in
volts(E), Current measured in amperes(I), Resistance measured in
ohms(R), and Power measured in watts(P). These are all tied together by
something called "Ohm's Law", which is a simple formula that states:
E (voltage) = I (current) x R (resistance)
If we know any two of the values we can calculate the missing. For
example, if you hook a 10v battery to a 100 ohm resistor, the circuit
would draw 0.1 amps of current (10 = 100 x 0.1). Now the circuit is
also generating some type of power, which must be dissipated by the
resistor. Resistor are rated in watts by how much power the can
withstand. So we use another formula that states:
P (watts) = E (voltage) x I (current)
In our example, we know that the voltage is 10, and we know the current
is 0.1, so we can calculate the power to be 1 watt (1=10 x 0.1). So the
size of resistor must be able to handle at least 1 watt of power, or it
will fry.
So we know that the amount of resistance (measured in ohms) directly
influences the amount of power generated in a circuit. Now in your amp,
most of the power is generated by either large power transistors or the
tubes. Now let's assume that your amp is drawing a constant 5 amps of
current at a certain setting. By changing the above formula around we
can determine that:
Power (watts) = I (amps)^2 (squared) x R (ohms)
5^2 = 25
If you send this into a 4 ohm speaker we get 25*4=100 watts, if we send
this into an 8 ohm speaker, 25*8=200 watts. As we see we are getting
twice as much power from the 8 ohm speaker.
So, if your amp is rated 200 watts @ 8 ohms, no problem. Even using 4
ohm speakers, you are only generating 100 watts. You are not getting
the full output, but you are not going to fry anything either.
Now lets assume your amp is rated 100 watts @ 4 ohms. With the 4 ohm
speaker you are driving the amp at max, but what happens if you use 8
ohm speakers? You are now driving your 100 watt amp at 200 watts!
Things are going to get real hot inside.
Another quick point, resistance in series adds, resistance in parallel
divides. So two 8 ohm speakers wired parallel equal 4 ohms. Two 4 ohm
speakers wired in series is 8 ohms.
Hope this helps!!!
la...@ws6502.gud.siemens.co.at
jsmit...@aol.com (John Smith) writes:
>
> Now lets assume your amp is rated 100 watts @ 4 ohms. With the 4 ohm
> speaker you are driving the amp at max, but what happens if you use 8
> ohm speakers? You are now driving your 100 watt amp at 200 watts!
> Things are going to get real hot inside.
>
And when you short circuit the output you are driving your amp at zero
watts and are perfectly safe, right?
WRONG!
Your post was completely wrong for a very simple reason that 99.999%
of all the power amplifiers are not current but voltage amplifiers.
It is not the supply current, but the supply voltage that is set, and
the amplifier compares the output voltage to the input voltage rather
than the corresponding currents. Therefore, halving the output load
doubles the drawn current and doubles the power dissipated on the load
(output voltage remaining almost constant for the vast majority of
power amplifiers).
The dangerous part is that the power is being dissipated on transistors
and tubes (the active elements) as well--if you double the current
drawn by the load, so doubles the current through the active elements,
and therefore the dissipation on them as well: they might be damaged
by this (in fact, an output short circuit will almost always fry an
unprotected power amplifier).
Please read some on the power amplifier design before you post such
misleading information, especially this dangerous and potentially
expensive.
/Marino
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