On Mon, 10 Jan 2005 06:53:04 +0800, the renowned Old Nick
wrote:
On Fri, 07 Jan 2005 19:18:57 GMT, "Leo Lichtman"
vaguely proposed a theory
......and in reply I say!:
remove ns from my header address to reply via email
"Vaughn" wrote: I disagree. In my experience, Watts RMS is the ONLY
meaningful measurement of the power output capability of an amplifier.
^^^^^^^^^^^^^
Vaughn, lets look at this one step at a time. RMS stands for "root mean
square." If you look at the equation for power, it is I^2 R, or E^2/R. For
DC that is very simple, but for AC, it is necessary to use an average figure
for E or I that gives the correct power. The way to get that average is to
square the instantaneous voltage or current at every instant, take an
average of the squared values, and take the square root of the
average--hence: root (meaning square root) mean (same as average) square.
The wattage rating or operating point of an amplifier is already in power
units. It is perfectly OK to talk about "average power," or "peak power,"
but RMS power suggests doing something to the numbers that has already been
done. To square, average, and take the square root again would produce a
figure that has no meaning. That's my gripe.
But if you use a standardised method of measurement (sine wave) the
RMS is the same power dissipation as DC, isn't it? So you are
measuring the ability of the amp to continuously dissipate heat,
basically.
x volts (RMS) voltage across a resistor will give you x^2/R watts
dissipation (regardless of whether it's sine or not).
x amperes (RMS) through a resistor will give you x^2*R watts
dissipation (regardless of whether it's sine or not).
The reason you want the square is that with a resistor (a very useful
special case) the voltage and current are proportional. Hence,
increasing the voltage also increases the current. If you only measure
one of the two (with a resistive load) you can presume what the other
one is:
i = e/R or e= i*R.
(I use lower case because both are functions of time)
So, average power is the average of e*i = average of e^2/R
(or the average of i^2*R). That's where the squaring comes from.
Taking the square of a number that has _as part of its makeup_ a
squaring process of only one constituent of that number is not "doing
something that has already been done" at all.
There is no squaring process in the most general measurement of power.
Average power is the average value of (instantaneous voltage *
instantaneous current). (NOT the average value of voltage * average
voltage of current)
If you were to actually calculate the average RMS watts, it would NOT
be the same as the average power.
For example. 100 Watts for 1 second, off for 9. Average watts is
10W. That's exactly how much power dissipation there will be in the
load.
But "RMS watts" would be about 32W. That has no useful physical
meaning that I can imagine. It's just an erroneous term.
Best regards,
Spehro Pefhany
--
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