"Roderick Stewart" wrote in message
om...
In article , Bill Evans wrote:
Back in the late 1970's and early 1980's when I was purchasing quality
stereo amplification equipment for various projects, I became quite
familiar
with the techniques used by manufacturers to overstate the power output
of
their amplifiers - specifically statements presenting total instantaneous
power, peak power, or other such number-inflating techniques. I had
thought
that the practice of requiring power output in RMS to be specified at a
specified distortion level had put an end to that type of power inflation
technique and provided serious buyers with an appropriate yardstick for
comparison.

RMS values of the current and voltage are used to calculate the mean (or
average) value of the power, because these values of voltage and current
are
equivalent to the DC values that would supply energy at the same rate, so
"mean
power level" is the quantity that should really be specified.

The power output, like any varying quantity, will have an RMS value, but
it
doesn't relate to any significant physical quantity so it isn't
particularly
meaningful or useful to know it.

I suspect that the common avoidance of the correct terms "mean power" or
"average power" is simply down to the fact that advertisers don't like the
sound of them, whereas "RMS power" sounds much more technical and obscure,
even
though it's wrong.

Even "maximum continuous mean power" wouldn't always tell the whole story,
as
it doesn't always represent the output level that puts the greatest strain
on
the output devices of the amplifier itself.

Rod.


I've also seen the reason for the apparently incredible output levels of the
TescoSonic style home cinema systems, being a lot simpler than all of the
clever explanations above. It's becoming increasingly common for all of the
channels of these systems, to be quoted together, with some clever wording
that makes it sound like they are being quoted seperately. Thus, a six
channel system that appears to do an incredible 120 of whatever flavour of
watts you choose, per channel, is actually 120 watts spread across the six
channels. Inside, it just has a few low quality 20 watt car audio ICs, and a
power supply just about man enough to run them.

You'd weep if you saw inside some of this apparently high specced crap that
I have to fix ...

The biggest offenders for misquoting, have got to be the computer speaker /
amplifier systems, that often claim such nonsense as 120 watts RMS super sub
woofer, and contain a speaker that you would be ashamed to design into a
transistor radio.

Arfa