In article ,
Spehro Pefhany wrote:
On Fri, 07 Jan 2005 20:53:13 GMT, the renowned Howard Eisenhauer
wrote:
On Fri, 07 Jan 2005 05:49:13 GMT, "Leo Lichtman"
wrote:
"Howard Eisenhauer" wrote: Is that watts RMS? (clip)
^^^^^^^^^^^^
Howard, I hope you did that on purpose. "Watts RMS," a term popular in
advertising audio amplifiers, has always galled me. I can see no way for it
to have any meaning, considering what RMS stands for, and what it is used
for. :-)
On the contrary actually, I consider RMS to be the most meaningful
measurement of power, but that may be specific to the areas I use it
in.
"RMS watts", although mathematically possible to compute, would have
no useful *physical* meaning that I can think of. Usually they are a
misnomer for "true power".
RMS Watts gives the actual power to do work. Assuming the
signal is a sine wave, and assuming (for convenience) the load is a pure
resistor acting as a heater to avoid needing to deal with imaginary
current):
1) If the input power is 120 VAC (that voltage is *also* RMS, BTW.
for 120 VRMS, the peak voltage is 1.414 (sqrt(2)) times the RMS
voltage. So -- with that 120 VAC RMS, you get a peak voltage of
169.68 V
2) Now -- let's assume that the resistor is 100 Ohms, so at that
peak voltage, it would be dissipating 287.91 Watts.
3) However, it is doing so only for a tiny part of the whole
cycle. At other points, it has voltages much less than that
peak, and even less than the 120 V which the RMS value has.
4) So -- if you actually measure the heat output, and do the
necessary math, you will find that the actual average heat out
of it is the same as though it had 120 VDC applied.
5) So -- RMS is a measurement of how much work you can get out of
the signal, whatever it is doing. (In an audio system, that
task happens to be moving air in specific patterns.) How much
you can actually get out is also a function of the efficiency of
the speakers, usually with the higher quality speakers being
less efficient -- more of the power is going to controlling
cone overshoot and other forms of mechanical distortion.
As far audio goes I'd be more interested in THD, peak power/dymanic
range & whether or not the dual isolated power supplies were built
using oxygen free silver buss bars 
.
The "peak" power used in audio rating is not even that used in
my explanation above, which at least has a basis in physics. Instead,
it is what power you can get out of the amplifier for an instant before
the power supply capacitors discharge to a lower voltage. So -- your
peak power gives you an idea what a single castanet clack can produce
(hopefully without distortion) -- but no promise that a rapid series of
them will produce the same sound.
The RMS power, instead, gives you a *real* measurement of how much
power it can produce for a long sustained note. It is *defined* for a
steady-state output. It can even give you an idea whether you can run a
machine tool motor from the output.
Any attempt to run a 100 W light bulb from an amplifier rated at
100W Peak will burn out the amplifier in fairly short order. An
amplifier rated in RMS Watts will do the job -- assuming that it can
produce enough output voltage (not a guarantee even with your preferred
peak Watts).
H.
You don't want to spend $500 on a wooden knob that prevents
"microvibrations" from entering the delicate signal path? ;-)
It is beginning to sound as though that is the case. :-)
Enjoy,
DoN.
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