RMS Power
Nick Chalk
Square Power when discussing, for example, power
amplifiers.
RMS current and voltage I'm happy with. Mean
power, no problem. But why square it before and
take the root after the mean?
Is this a case of sloppy terminology, or is there
a different measurement technique for RMS power?
Nick.
--
Nick Chalk ........................ Radio Designer
Confidence is failing to understand the problem.
Stewart Pinkerton
>I'm a little confused at the use of Root Mean
>Square Power when discussing, for example, power
>amplifiers.
>
>RMS current and voltage I'm happy with. Mean
>power, no problem. But why square it before and
>take the root after the mean?
>
>Is this a case of sloppy terminology, or is there
>a different measurement technique for RMS power?
It's sloppy terminology. There is of course no such thing as RMS
power, the proper terms are mean, average or continuous power.
In audio, it's usually taken as continuous power in the sense that the
amp has good enough supply rails and heatsinks to supply that amount
of power for several minutes.
--
Stewart Pinkerton | Music is art, audio is engineering
Ian Grant
the amplifier which in reality should be stated as X watts @ Y (k)Hz
into Z ohms, the use of other references could also be used (including
the dB but that really throws the marketing men into a frenzy), this
anomaly seams to have stuck, including with myself until I started
thinking about it. Oh and don't forget to specify how long it can source
this power for, common specifications are 8 hours or continuous.
--
Ian Grant
I G Electronics
England
Pages at http://www.igelect.force9.co.uk
--
Nick Chalk wrote in message <8gpflr$tf$1...@nickchalk.enterprise.net>...
>I'm a little confused at the use of Root Mean
>Square Power when discussing, for example, power
>amplifiers.
>
>RMS current and voltage I'm happy with. Mean
>power, no problem. But why square it before and
>take the root after the mean?
>
>Is this a case of sloppy terminology, or is there
>a different measurement technique for RMS power?
>
Les Wolstenholme
Nick Chalk <nick...@enterprise.net> wrote in article
<8gpflr$tf$1...@nickchalk.enterprise.net>...
> I'm a little confused at the use of Root Mean
> Square Power when discussing, for example, power
> amplifiers.
>
> RMS current and voltage I'm happy with. Mean
> power, no problem. But why square it before and
> take the root after the mean?
>
> Is this a case of sloppy terminology, or is there
> a different measurement technique for RMS power?
In the absence of any coherent reply, the following:
The initials RMS stand for Root Mean Square, indicating that
it is the square root of the average of the squares over the cycle.
For a sinusoidal waveform, it is calculated by the formula:
V- Pk to Pk divided by two then multiplied by 0.707
For example: 20volts Pk to Pk would be divided by two
to leave ten volts and when multiplied by 0.707 will
result in and RMS voltage of 7.07.
The resultant can then be manipulated just as can a DC
voltage in that P = E x I where P is expressed in Watts,
E in Volts and I in Amperes.
The work, eg. heating effect, of which RMS power is
capable, is exactly equal to a corresponding DC power.
Hence Vrms squared then divided by the load resistance
equals Prms.
Definition: (R.M.A. standard SE-101-A) states:
"Amplifier power is the maximum RMS power output
(including distortion) at rated distortion which the
amplifier will deliver into its rated load under normal
operating conditions."
Source: Radio Designers Handbook,
auth. F. Langford-Smith.
pub. Iliffe Books
I hope that clarifies any previous misunderstanding.
Regards,
Les W.....>>
Avondale Audio
Stewart Pinkerton
>
>
>Nick Chalk <nick...@enterprise.net> wrote in article
><8gpflr$tf$1...@nickchalk.enterprise.net>...
>> I'm a little confused at the use of Root Mean
>> Square Power when discussing, for example, power
>> amplifiers.
>>
>> RMS current and voltage I'm happy with. Mean
>> power, no problem. But why square it before and
>> take the root after the mean?
>>
>> Is this a case of sloppy terminology, or is there
>> a different measurement technique for RMS power?
>
>
>
>In the absence of any coherent reply, the following:
incorrect reply..........
>The initials RMS stand for Root Mean Square, indicating that
>it is the square root of the average of the squares over the cycle.
>
>For a sinusoidal waveform, it is calculated by the formula:
>V- Pk to Pk divided by two then multiplied by 0.707
>
>For example: 20volts Pk to Pk would be divided by two
>to leave ten volts and when multiplied by 0.707 will
>result in and RMS voltage of 7.07.
>
>The resultant can then be manipulated just as can a DC
>voltage in that P = E x I where P is expressed in Watts,
>E in Volts and I in Amperes.
>
>The work, eg. heating effect, of which RMS power is
>capable, is exactly equal to a corresponding DC power.
>
>Hence Vrms squared then divided by the load resistance
>equals Prms.
Exactly correct derivation, given a resistive load, and what do you
get when you square a root function? Not another root function, simply
a mean - mean power in this case.
>Definition: (R.M.A. standard SE-101-A) states:
>
>"Amplifier power is the maximum RMS power output
>(including distortion) at rated distortion which the
>amplifier will deliver into its rated load under normal
>operating conditions."
>
>Source: Radio Designers Handbook,
>auth. F. Langford-Smith.
>pub. Iliffe Books
>
>I hope that clarifies any previous misunderstanding.
The misunderstanding lies with anyone who believes that rms power is a
mathematical possibility! Mean power is obtained by multiplying rms
voltage by rms current. The result is most certainly *not* a
root-mean-square function. While in widespread use (even in EE
textbooks!) 'rms power' is simply sloppy terminology, as any competent
electrical engineer is well aware.
Dave Plowman
Nick Chalk <nick...@enterprise.net> wrote:
> I'm a little confused at the use of Root Mean
> Square Power when discussing, for example, power
> amplifiers.
Although not strictly correct, I reckon it came into use to differentiate
from 'peak' power ratings that some manufactures quoted with no real
reference for boasting purposes in advertising. A bit like BHP figures for
cars that caused both Rolls-Royce and Aston Martin to stop quoting figures
in the '60s or so, when there were no real goalposts.
--
* I'm planning to be spontaneous tomorrow *
Dave Plowman dave....@argonet.co.uk London SW 12
RIP Acorn
Les Wolstenholme
> The misunderstanding lies with anyone who believes that rms power is a
> mathematical possibility! Mean power is obtained by multiplying rms
> voltage by rms current. The result is most certainly *not* a
> root-mean-square function. While in widespread use (even in EE
> textbooks!) 'rms power' is simply sloppy terminology, as any competent
> electrical engineer is well aware.
Only for the pedantic among us dear boy, only for the pedantic....
Meanwhile, back to the engineering.......
Les W.....>>
Stewart Pinkerton
>> The misunderstanding lies with anyone who believes that rms power is a
>> mathematical possibility! Mean power is obtained by multiplying rms
>> voltage by rms current. The result is most certainly *not* a
>> root-mean-square function. While in widespread use (even in EE
>> textbooks!) 'rms power' is simply sloppy terminology, as any competent
>> electrical engineer is well aware.
>
>Only for the pedantic among us dear boy, only for the pedantic....
In point of fact, old bean, the true pedant would simply quote
parrot-fashion from a standard textbook, as you did. The working
engineer actually *understands* the process..................
>Meanwhile, back to the engineering.......
That would indeed be a refreshing change! :-)
Les Wolstenholme
Don't stick your fork in me buster, I'm only quoting
a well regarded textbook. Go take it out on Iliffe books
or Prof. Langford - Smith. While you're at it, sit down
and write to the A.E.S and the I.E.E. indicating their
blatant errors.
Don't forget the many tomes on our bookshelves, their
authors are guilty as anyone of perpetuating this crime
against mathematical correctness and are ripe for
adjustment of their thinking.
Come to think of it, why stop there..?? Every amplifier
manufacturer I can think of uses the term Watts rms to
describe the output power of their products.
These errant and sloppy firms need teaching a lesson.
Dan D'Agostino is so obviously labouring under some
misapprehension in this matter and I'm sure he'd be
grateful to receive your superior wisdom.
Even more sins have come to light, the major radio
manufacturers have adopted this slatternly attitude to
measurement and stand in need of your correction.
Kenwood and Yaesu are two of the firms guilty of
this diabolical conspiracy.
More crucial, our universities are teaching this patently
untrue method of measurement and the truly outraged
should be lobbying his MP at once.
This matter is so serious that we appeal to you to put
pen to paper at the earliest opportunity and address this
travesty of scientific measurement as a priority.
So, your Grace ( grovelling apologies, I had no idea )
on with the green eyeshade, head down, saddle up
and go for it…..we're right behind you.
Serious mode now for this bit….honest.
The term rms power has passed into common use within
this industry. Whether it be mathematically precise (and
please understand that this is far from being an exact
science), is of little consequence to the amp maker or
indeed the punter. What is important is that the term is
a yardstick which is easily understood and readily
measured. I suppose use of the term 'fridge' is
grammatically incorrect but again is freely used and
understood.
Any manufacturer adopting say, dBW would be seen
as 'smartipants' and as we all well know, nobody loves
a smartipants.
In the same way, bhp has crept into common usage.
Owners and makers alike understand it and millions of cars
are sold under this power description. In the absence of
anything clearer, it has to suffice.
Back to wacky mode now.
Is his Worthiness suggesting that we adopt some obscure
function which is so much unwieldy gobbledegook, just
to appear 'correct.'
Peak amps into a coathanger for instance or mean voltage
into a pair of ferrets in parallel…??
Perhaps some manufacturer will recognise his true
greatness and acknowledge his contribution to the
industry by applying his name to amp power.
May I humbly suggest Pinkies Per Parsec for example,
it has a nice ring. I'm sure the boys and girls can think
of other appropriate terms.
Now, would that please your Grace..??
That's it, I've said me bit, back under me rock..
Tarra,
Les
Stewart Pinkerton
>Don't stick your fork in me buster, I'm only quoting
>a well regarded textbook.
Shame that you can't actually just do the simple math.
>Come to think of it, why stop there..?? Every amplifier
>manufacturer I can think of uses the term Watts rms to
>describe the output power of their products.
>These errant and sloppy firms need teaching a lesson.
Really? Specify *one* respected manufacturer who uses Watts rms. And
provide references to actual documents.
>Dan D'Agostino is so obviously labouring under some
>misapprehension in this matter and I'm sure he'd be
>grateful to receive your superior wisdom.
I see no mention of 'Watts rms' in any of my Krell brochures, and as
it happens I use a Krell myself. Getting a bit desperate here aren't
we, old boy?
>So, your Grace ( grovelling apologies, I had no idea )
Wrong again, but rapidly becoming SOP for you, Les.
>Serious mode now for this bitů.honest.
Do you actually have any idea what the 'h' word means?
>The term rms power has passed into common use within
>this industry.
What industry? Ripoff high-end audio, or industrial electronics?
> Whether it be mathematically precise (and
>please understand that this is far from being an exact
>science), is of little consequence to the amp maker or
>indeed the punter. What is important is that the term is
>a yardstick which is easily understood and readily
>measured. I suppose use of the term 'fridge' is
>grammatically incorrect but again is freely used and
>understood.
I think we're all agreed on what is meant by fridge, tannoy and
hoover, but just what do you think the average punter understands is
meant by 'rms power'?
>Any manufacturer adopting say, dBW would be seen
>as 'smartipants' and as we all well know, nobody loves
>a smartipants.
Most of the pro-audio manufacturers use this, but that's a different
market where you actually have to *prove* the product.
>In the same way, bhp has crept into common usage.
>Owners and makers alike understand it and millions of cars
>are sold under this power description. In the absence of
>anything clearer, it has to suffice.
Excuse me? DIN bhp is *exactly* defined, as are the now common PS and
kW ratings for engine power. At least try to drag yourself into the
latter half of the 20th century, even if the 21st is clearly beyond
you.
>Back to wacky mode now.
Oh, I must have missed the part where you left it.......
>Is his Worthiness suggesting that we adopt some obscure
>function which is so much unwieldy gobbledegook, just
>to appear 'correct.'
>
>Peak amps into a coathanger for instance or mean voltage
>into a pair of ferrets in parallelů??
Continuous power will do just fine, and is used by many manufacturers.
>Perhaps some manufacturer will recognise his true
>greatness and acknowledge his contribution to the
>industry by applying his name to amp power.
>
>May I humbly suggest Pinkies Per Parsec for example,
>it has a nice ring. I'm sure the boys and girls can think
>of other appropriate terms.
Why muddy the waters? We already have the correct mean power, or the
universally understood continuous power.
>Now, would that please your Grace..??
Wrong again, but we're getting used to that from you.
>That's it, I've said me bit, back under me rock..
As ever, with no useful contribution. Perhaps you might consider just
staying there?...................
Nick Chalk
Les Wolstenholme <avon...@zetnet.co.uk> wrote:
> Even more sins have come to light, the major
> radio manufacturers have adopted this slatternly
> attitude to measurement and stand in need of
> your correction. Kenwood and Yaesu are two of
> the firms guilty of this diabolical conspiracy.
That's terrible! ;-) Personally, I use dBm* to
specify RF power; I don't work on big enough
devices to bother with dBW.
* dB relative to 1mW.
> More crucial, our universities are teaching this
> patently untrue method of measurement and the
> truly outraged should be lobbying his MP at
> once.
I'm lucky, then. My analogue lecturer always
railed against RMS power.
> The term rms power has passed into common use
> within this industry. Whether it be
> mathematically precise...
I don't think the argument is with the
mathematics; you can calculate RMS power if you
wish. The problem is that RMS power has no
physical reality, as I understand it. Mean power,
on the other hand, is a measurable quantity.
> (and please understand that this is far from
> being an exact science)
Have you tried RF design? Now there's a black art!
It's an exact Engineering discipline, but when
you're pushed for time, and that PA just _has_ to
work, you resort to prodding the circuit, tweaking
components...
Dave Plowman
Tony Skitt <tsk...@dial.pipex.com> wrote:
> When did a satellite engineer ever mount his Ku-band HPA on a set of
> spikes to improve the modulation quality? Or when did a radio amateur
> ever use special directional coax to feed his 2m antenna with the lowest
> cable loss?
1. 'Cause he's an engineer. 2. That amateur has passed an exam on some of
the basic laws of electronics.
--
* 'ome is where you 'ang your @ *
Stewart Pinkerton
>I don't think RF engineering is as black an art as audio. When did a
>satellite engineer ever mount his Ku-band HPA on a set of spikes to improve
>the modulation quality? Or when did a radio amateur ever use special
>directional coax to feed his 2m antenna with the lowest cable loss?
Mayhap you misunderstand the subtle difference. RF engineering (and
especially microwave plumbing!) is a black art in so far as best
results are achieved by those able to apply some almost instinctive
tweaking over and above what the calculations suggest, thereby
producing a *measurably* better result.
Audio 'black arts' however rely on slick sales skille to gain profit
from weird and wonderful devices which have no real function
whatever...........
Dave Plowman
Stewart Pinkerton <pat...@popmail.dircon.co.uk> wrote:
> >
> >1. 'Cause he's an engineer. 2. That amateur has passed an exam on some of
> >the basic laws of electronics.
> Exsqueeze me? I hope you were being humourous, otherwise you have some
> real good explaining to do!
Present company excepted, many of those who claim to hear the difference
between gold plated mains plugs and lumps of rubber placed under their
amplifier etc seem to exhibit a remarkable lack of knowledge of the basics.
--
* When I'm not in my right mind, my left mind gets pretty crowded *
Dave Plowman
Nigel Orr <news@river-view_dot_freeserve.co.uk> wrote:
> My favourite is still the '400W' computer multimedia speakers, run
> from a 5VA PSU, with sometimes something like 1.5W rms in small print
> on the back...
Or car radios with a simple transformerless class B output claiming 10
watts.
--
* Two wrongs are only the beginning *
Ian Iveson
with this nonsense.
I trust that there is nothing capable of measuring something which makes no
sense. Ignorance is a disease and it's spreading.
regards, Ian