Re: RMS

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "35045 by way of Terry Fritz <twftesla-at-uswest-dot-net>" <free0076-at-flinders.edu.au>



On Wed, 10 Jan 2001, Tesla list wrote:

> Original poster: "Albert Hassick by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <uncadoc-at-juno-dot-com>
> 
> Hi, Terry, Members.   I am still trying to figure out why many list
> members say that RMS ratings have no value.  But if RMS ratings have no
> significance, then why are they used so widely in audio equipment and
> transducers? I always considered the RMS to be the safe 'clean' output

RMS values of voltage and current do have significance to us, however
average power is used in preference to RMS power since RMS power doesn't
mean anything to us.

> rating of power amps.  Any more power output than RMS will result in
> excess heat/clipping/and distortion in the waveform that the human ear
> can readily discern. And you know that you cannot run audio equipment (or
> any thing else for that matter)at 'continuous ratings'  for long because
> the equipment invariably begins break up at some point in time.  A case
> in point,  I have several big old Electrovoice 15" SRO loudspeakers in my
> "Leslie" twins, which are nothing more than a real big voice coil and
> magnet that can more efficiently pump air volume.  They are rated at RMS,
> which to a keyboard musician like myself would be the "clean" power
> output of the transducer rated at about .6.  Then there is another rating
> for the same transducer that rates continuous power at about .707. Then
> last but not least we have the ultimate peak power output of the
> transducer which in this case is 300 watts or a rating at peak of 1.0. 
> Then they give a 600 watt 'peak to peak' rating. This also encompasses

I think that whoever is giving these figures is doing something strange
with the numbers like taking the peak power and doubling it or whatever.
As for the whole rating system you are quoting, it seems to be quite weird
and defy the definitions of peak, average, peak to peak and RMS.

> the entire begining and end of one complete cycle of the 'sine waveform'
> .  And I seem to remember vaguely from tech school about the values of
> Sine,Cosine etc. tangents on the slide rule that were applied as
> constants to a given electrical component at a true value as to the
> amount of work that the component could give at varying levels of 
> torque, load and failure rate that as a model could be calculated.  I can
> try to scan and post some interesting scope readings that Electrovoice
> made available showing the graphs that delineate the effect of the
> different power levels and their effect on the copper voice coil at RMS,
> continuous, and peak ratings.  I still think RMS ratings have relevance. 

That would probably clear the whole thing up for us - thanks!

I doubt that RMS ratings have relevance to us at least. There may be a
quirk of the operation of amps and speakers that I'm not aware of, but in
general we use average power not RMS power.

> If not, then why do all manufacturers of audio output amps and
> transducers even bother to use the RMS value when rating their equipment?
> Is it just a moot selling point?   Anyways, I think these ratings could

In the case of a pure sinewave driving a resistor, the RMS power value is
higher than the resistive one. Perhaps they test their amps with a
resistive load and then quote RMS power to get a bigger number.

For a sinewave voltage delivering 1 Watt peak to a resistor, its average
value is 0.5 W and this is the value that we like to use since it tells us
how fast it will het got. However you could always calculate the RMS power
which is 0.707 W... Makes the power source sound a little better doesn't
it?

> be similarly applied to say, neon transformers and Tesla secondary coils.

In our case there is no point talking about RMS power since it doesn't
tell us much. We always use average power. Most people around the world
use average power in electrical/electronics, or peak power if they are
interested in pulses.

>  AL.


Have fun,
Darren Freeman