RMS (Root Mean Squared) Power
RMS measurement is a measure of "power density". It is
0.707 of Peak-To-Peak sine-wave readings so that the roughly 30%
deducted can be used to fill "voids" in the sine-wave power envelope of
a given sine-wave.
So what does RMS have to do with evaluating an amplifier's
performance? Actually an RMS rating has very little meaning in terms of
actual dynamic performance of an amplifier, although it has some value
when comparing different amplifiers from different manufacturers. If all
manufacturers quote their RMS values honestly it is possible to
determine which machines are more powerful under RMS test conditions.
Unfortunately many manufacturers can get very creative in their
measurements, and so all RMS ratings have to be taken at less than face
value just in case .......
To obtain an RMS rating, a given amplifier is placed in a
test jig, attached to a fixed load of a specified impedance, and a
signal at a given frequency is increasingly applied until the amplifier
cannot produce any more power without "clipping" the sine-wave it is
trying to produce. At this point a voltage reading is taken with
an instrument calibrated to accurately indicate RMS voltage, or the
waveform can be measured with a calibrated oscilloscope and a
mathematical calculation is undertaken to convert its Peak-To-Peak
reading to RMS. Using another mathematical formula, the voltage,
in conjunction with the fixed load, produces the RMS rating.
What has been obtained is an indication of what power the
amplifier can produce continuously at a given frequency, to a static,
usually non-inductive load, connected to other test equipment. As
the reader might already suspect, this test is far removed from any
real-world practicality as far as reproducing music is concerned.
Dynamic Audio Power (DAP)
Dynamic Audio Power is just that. It is a measure of what
an amplifier can do when placed in a situation where it is producing
actual music, into a dynamic load (a loudspeaker). It is an
estimate of how an amplifier can handle any number of frequencies
simultaneously into a shifting impedance, as any loudspeaker can have a
number of impedances depending on frequency applied. It is a
measure of what reserve power is available to produce a smashing low
bass note while at the same time some very subtle high frequency
content. In short, it is a measure of what an amp can do in the
real world, with real people listening to it, not how it can communicate
with a lot of test equipment in a laboratory.
Dynamic Audio Power usually appears to be greater than RMS in
any given amplifier, by a factor of two (twice as much). This is
because the amplifier is not totally consumed trying to produce a static
frequency continuously as in RMS, so that when a particularly short term
heavy load is placed on it (lets say a powerful bass note), the amp has
almost all its reserve at hand to do that job alone, and so can handle
the momentary task with relative ease. This explains why
amplifiers that are rated rather low in RMS terms have the ability
to produce audio results way more than the low figures would tend to
So - which rating is more important? Actually both have
their place in evaluating an amplifier. The RMS value can be a useful standard by which all
manufactured amplifiers' power can be compared, if all manufacturers
would only tell the truth, or test under exactly the same conditions.
For instance any, poorly crafted amplifier may be able to produce
sizable RMS wattage at 1000 Hz but do terribly at 30 Hz (where the
wattage really counts), so the manufacturer will naturally quote the RMS
at 1000 Hz. We always quote our RMS power ratings at about 30 Hz
(unless otherwise stated), because if the power output is great there,
it can only be even better everywhere else.
Dynamic Power is very useful because it predicts just what
the listener will hear in real-world conditions. An amplifier is
performing an amazing task, when trying to reproduce thousands of
interacting waveforms, and their harmonics, while driving a loudspeaker
that is in every sense of the word, a "moving target".