of us, long before being introduced to the concept of power in electrical
circuits, learn about it in terms of mechanical power and/or sound power.
It takes some doing to abandon the esoteric nature of power and
be trained to grasp the scientific and mathematical aspects of power
in all its forms. When the driving source is steady state or a pure
sinewave, life is relatively simple, but such is more often than not
an exception to the system being studied. Here is a nice, short treatise
on the concept of sound power that will augment your earlier-learned
knowledge of music power rating.
October 1960 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
Electronics World articles.
See all the available
Electronics World articles.
Music Power Rating - Help or Hindrance?By Norman H. Crowhurst
author's provocative view is that this amplifier rating is a step in
the right direction, however it may not tell the whole story about hi-fi
The question that triggered this article is, "Does
the new Music Power Rating help tell the quality of amplifier ? If so,
how?" The answer to this question will be covered later, but mainly
some concepts, notions, and viewpoints need clarifying first. There
will be some explaining to do - why the particular Music Power Output
definition, as used by the IHFM (Institute of Hi Fidelity Manufacturers)
standard for rating amplifiers, was chosen, as well as what it can tell
For a long while, the accepted methods of rating or specifying
amplifier performance have been recognized as inadequate for assessing
the relative merit of different products. This has been correctly related
to the fact that music, or other program material, is different in many
respects from the pure-tone sine waves used for testing amplifiers.
These two standards differ in two respects: all test tones,
whether one or two sine waves, or a square wave, are applied steadily,
while the tones in music are constantly changing, often rapidly. Apart
from their transient nature, musical waveforms are much more complicated
than anything used for testing, even when the tones are steady.
Fig. 1. - Comparison of voltage and power waveforms for
(A, above) simple sine wave and (B, below) musical tone consisting
of a fundamental with considerable third harmonic.
Engineers in the high-fidelity industry have,
for some time, been concerned by this discrepancy. Some have tried to
do something about it and some have even dared to try and make an amplifier
whose first job was to sound good, even if its specifications didn't
read quite as impressively! But these engineers soon ran into opposition.
Some people either are not able to listen critically or else
don't believe their ears. They want something in print that they can
show around, proving that the product they have is better. We have no
argument with this attitude. The difficulty is, what figures can we
provide that really prove something? The notion of music power output
originated with engineers: it should be a way of rating amplifiers which
will more nearly evaluate how "loud" they will sound, than does the
conventional power output rating.
One problem is that steady
power tests force the amplifier to deliver its maximum power continuously.
Musical program material calls for maximum power for only short periods
of time. Why not see what an amplifier will produce when the demand
is not continuous?
If you put just one or two cycles of a frequency
through an amplifier you can see its performance during such a short
burst but you cannot measure it because the meters won't have time to
"get up to" whatever the reading is. To overcome this, the method specified
by the IHFM involves the maintenance (artificially), from an external
source, of the voltages that sag under continuous power for long enough
to get the reading.
This sounds good, when it is explained to
you, otherwise it might appear to be "cheating a bit." Next, you want
to know what the "answers" mean. And that is a good question. Until
most audiophiles and hi-fi equipment makers use the music power rating,
the more familiar continuous output power will also be given. So your
question is: which is better, an amplifier that has both ratings the
same, or nearly the same, or one for which the music power is appreciably
higher than the continuous power?
Pursuing this question reveals the fact that an amplifier with good
power-supply regulation will have ratings which are closer together
while one whose power supply provides poorer regulation will show a
greater difference between continuous and music power ratings. Surely
the amplifier with the better regulated power supply is the better amplifier?
Put it this way: do you want quality watts per dollar or are you looking
for the highest power irrespective of cost?
Fig. 2. - The effect of combining two tones of different frequency
on the relationship between the peak and average power.
One amplifier may
have both ratings at 25 watts. Another unit, by spending part of what
the first spent on the power supply on other features, may give 25 watts
continuous but also manages 35 watts music power as well. The second
amplifier will undoubtedly provide greater clean music volume - other
things being equal - than the first unit.
But we have no objection
to a third amplifier that gives 35 watts measured both ways, if the
manufacturer can make it competitively. It is an even better amplifier
than the second one, but whether it is worth paying the 30% to 40% higher
price is up to the buyer to decide.
This is, roughly, the way
the new music power rating works out. It is a step forward. but some
engineers are still no satisfied. While it does come nearer to some
aspects of how an amplifier performs on music, it still misses some
Another Look at Waveforms
What is the true relationship
between the power in a musical program and the maximum power as measured
by a single tone? This is the question the new music power rating does
not really touch because both ratings, by definition, measure average
power. Average music power can have a number of different connotations,
while the average power of a sine-wave output is unambiguous.
It was the transient nature of music power that dictated the new
rating. But even a steady tone, such as that played by a single instrument,
can have quite a different average power from that of a sine wave, in
terms of what an amplifier can handle. See Fig. 1.
to the handling capacity of an amplifier is the instantaneous peak power
it can deliver. The peak power of a 10-watt sine wave is 20 watts. But
a musical tone with 20 watts peak may very well have an average power
of less than 5 watts - even as a single continuous tone. That is really
the first basis for differences.
It affects the rating question
in two ways. First, the lower "waveform factor" means the regulation
will not hurt even a sustained tone's power as much as with the test
sine waves (unless the sustained tone is sinusoidal, approaches a square
shape, or has a "waveform factor" such that its average value is higher
than a sine wave - Editor.) Second, it alters the picture as to what
"average power" itself really means.
Next point: music consists
of many tones played at any single instant - most of the time. Each
tone has a different frequency. At a frequency corresponding to this
difference, the peaks of two tones will coincide. At each such coincidence,
it is the voltage, not the power, that adds to determine the total peak.
If peak voltage doubles, peak power is quadrupled, as shown in Fig.
Suppose each of three tones has an average power of .5 watt and a peak
power of 2 watts. If the impedance is 8 ohms, each peak will be 4 volts.
Three of them will reach occasional "spikes" of 12 volts (Fig. 3). This
is a power of 18 watts, although the average is only 1.5 watts. If a
pedal tone is also present, it may need a further 5-watts average with
perhaps IS-watts peak, also 12 volts. Now we need 24 volts, which is
72-watts peak, for just a pedal tone and trichord, totaling an average
power of 6.5 watts. See Fig. 4.
Fig. 3. - Voltage and power relations that exist in an audio
amplifier when three different tones are combined.
The more complex the music,
the greater the basic factor between average and peak power. You may
have noticed that orchestral music at a certain nominal output power
does not seem as loud as, say, a jazz combo.
This because the
ear recognizes the sum of all the average powers. To make the reckoning
easy, let's assume each instrument contributes 0.5 watt average
power and needs 2 watts or 4 volts peak.
A four-piece combo
will give 2 watts average, but needs 16 volts or 32 watts peak power.
A 40-piece orchestra will give 20 watts average, but needs 160 volts
or 3200 watts peak power! Nobody has that kind of power, so assume we
play with the same margin of safety against possible distortion: the
four-piece combo can be played at a realistic level of 2 watts average
from a 16-watt amplifier (32 watts peak) ; but the orchestra will have
to be turned down to only one-fifth of a watt total average, to stay
within 32 watts peak.
This explains why a much bigger ampliifier
is needed to handle good orchestra music, even though the sound doesn't
seem any louder - if as loud. It also explains why no single music power
rating, based on an average, can ever be completely practical.
With so many variables, the only point of common reference between
amplifiers and the music they handle is peak power. This is what determines
when the amplifier starts to distort the music. The average power that
corresponds to this peak power depends entirely on the music, not the
amplifier. The "waveform factor" may react on the power-supply regulation
to modify the peak capability according to what the average power is.
But even the worst power-supply regulation usually makes little difference
on this, with most kinds of music.
There was a move, some years
ago, to rate amplifiers by peak power. Proposed with honest intentions,
it was based on similar reasoning but the simple two-to-one relationship
for test sine waves led to its serious misuse.
Give a Dog a
The intention was not to just double the number already
on the amplifier nameplate, but this is what many companies did. The
peak power intended was what the amplifier could handle on peaks, representative
of the relative duration encountered in music. But insufficient explanation,
plus the urge by some just to use bigger numbers, led to confusion .
Some firms measured peak power the way the IHFM now defines
music power. It was really a short-term average power. Others double
the number they already had, based on the mathematical sine-wave relationship.
Yet others doubled the short-term average power, giving the instantaneous
peak value of such a wave - and the highest number of all.
last figure was really the most logical and, if everyone used it, would
be the most informative regarding amplifier performance because it does
relate directly to the musical program the amplifier can handle.
But the fact that so many methods of rating were used, led many
to believe that the "good old standard" watts were "honest watts", with
the obvious implication that the others were cheating - "just doubling
the numbers to make the amplifier look bigger!"
This was understandable.
If the new number really told something extra about the amplifier's
performance, it was useful. But most often it was obtained by the arithmetical
operation of multiplying by two. This had no value, except to further
confuse the already confused consumer.
Experiences like this
are difficult to live down. On the committee that discussed the new
IHFM music power rating, several engineers favored the use of a peak
music power rating. This would double the figure as presently defined.
But they remembered the hangover of adverse criticism from the last
attempt and compromised by using the present definition.
Pots and Kettles
Fig. 4. Voltage and power levels that exist in musical phrase
in which there are 1 bass and 3 mid-range notes.
This compromise is not unreasonable when you
consider some other factors. Most important among these: what does the
power rating of a loudspeaker mean? It is intended to be the electrical
input power the speaker can handle, as delivered by the amplifier. A
10-watt speaker with 10% efficiency (and that's high) should be able
to take in 10 electrical watts and deliver 1 acoustic watt - maximum.
But it is called a 10-watt speaker.
That's not all. Few loudspeakers,
rated at 10 watts, will handle this much input power at a single sinusoidal
frequency, through the range for which they are designed - which most
good amplifiers do with ease. But the same loudspeakers are quite happy
handling the maximum musical power a 10-watt amplifier can deliver,
which is a mixture of many frequencies, with a low waveform factor.
Taken in conjunction with this fact, and other inconsistencies
we haven't space to explain here, it is not so illogical to rate amplifiers
according to the new IHFM music power definition. So let's take another
look at that question, "Does the new Music Power Rating help tell the
quality of an amplifier?"
What Good is MPR?
If you want
to use all the figures you can get hold of about every available amplifier,
you will probably try to make some deductions for which the published
information was never intended. In introducing music power rating, the
intention is to bring into use a figure for comparison that is more
realistic than the one now used.
During a transitional period,
progressive manufacturers will have to use both ratings, not to provide
more information about their own product, but so that comparison can
still be made with other products that do not yet use the new rating.
It is hoped that all firms will ultimately use music power rating -
at least as the main figure.
For simplicity, most people do
not want to digest a whole catalogue of specifications about each product.
From this standpoint, music power rating is a more informative single
figure than continuous power rating. If you are sufficiently interested,
continuous power rating, as a second figure, will convey an additional
measure of merit.
We should not compare amplifiers with the
same continuous power ratings and different music power ratings. When
both figures are given, we would compare amplifiers first by music power
rating; if two amplifiers have the same music power, we may then compare
continuous power ratings.
This is important. Viewing music power
rating as the secondary figure, we are tempted to conclude that a bigger
difference between the two numbers represents a better amplifier. Our
intuition favors the amplifier with better power-supply regulation,
so we suspect the whole notion of music power rating.
on the other foot, with music power rating as the primary figure and
continuous power rating secondary, our intuition supports the rating
inference. It is legitimate, to get reasonable quality at low cost,
to put music power ahead of continuous power, by using an inexpensive
power supply. But using a better power supply results in an amplifier
with "more solid" quality.
We would not say music power rating
completely solves the problem of providing a relative evaluation of
an amplifier, though. Properly used, it is a good step forward but there
are still many things the present method of specifying performance doesn't
tell. Let's put it this way:
Provided the amplifier does not
handle musical transients in some peculiar fashion the specification
does not show, and provided you always work it so musical peaks never
exceed twice the rated music power output, this rating does give a relative
indication of how much power amplifiers will deliver.
still have amplifiers that do strange things on certain kinds of musical
sound. None of the figures currently published shows what is likely
to happen if a momentary peak overshoots the peak music power (twice
the music power rating). One amplifier may handle these things without
"batting an eyelid," while another has electronic convulsions. That
the specs do not tell us even now. This difference can often account
for one amplifier seeming to give a lot more undistorted power than
another of the same or similar rating, even when both the ratings are
fully supported by tests.
We still have the effect that loudspeaker
loads can have on an amplifier, as compared with the "dummy" resistance
load, used for testing. The low distortion figures are always obtained
with the dummy load. Nobody publishes, even if he measures, distortion
with a loudspeaker load, because this depends, in an amplifier, on how
much reactance the loudspeaker has - and no two loudspeakers are alike.
A good amplifier may give up to twice the test distortion when
a loudspeaker is connected. A poor one may give many times as much distortion
as soon as a little reactance enters the picture.
be possible, for the benefit of those who really care, to find a standard
means of evaluating these various differences. It would be good to see
the manufacturers make a move, possibly through the IHFM, in this direction.
Meanwhile, it remains true that the specifications are a good starting
point in judging an amplifier: but the real test is how it performs
in your system.
1. Crowhurst, Norman H.: "Why Do Amplifiers Sound Different?", Radio &
Television News, March, 1957.
2. Crowhurst, Norman H.: "Some
Defects in Amplifier Performance Not Covered by Standard Specifications,
" Journal of the Audio Engineering Society, October, 1957.
Van Recklinghausen, D. R.: "Mismatch Between Power Amplifiers and Loudspeaker
" Journal of the Audio Engineering Society,
4. Crowhurst, Norman H.: "The Amplifier Distortion.
Story," Audio, April and May 1959.
Posted March 7, 2014