The Whys and Wherefores of Watts
January 1957 Popular Electronics
of the visitors to RF Cafe are either engineers, technicians, or
hobbyists who deal with watts in terms of electrical power. This
article from the January 1957 edition of Popular Science deals primarily
with watts in terms of acoustic power, but it also addresses how
obtaining acoustic watts relates to electrical watts. Audiophiles
will appreciate the table of speaker watts needed based on your
room volume as well as rules of thumb for selecting the amplifier
power required to deliver that sound effectively.
January 1957 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
The Whys and
Wherefores of Watts
By Leonard Feldman
Stop scratching your head over
which amplifier to get; here's the answer to your power output requirements
authorities maintain that the minimum acceptable power-handling
capacity of a hi-fi amplifier shall be at least 25 watts."
"Our research shows that the purchase of an amplifier having
a power rating of over 10 watts is a waste of money."
leading speaker manufacturer has recommended that power amplifiers
for use with his speaker should be rated at 30 watts or better."
"Five watts of audio fed to an 'efficient' loudspeaker is
more than the human ear can stand."
If you're trying
to decide which high-fidelity amplifier to buy, you've probably
run across conflicting comments like those above. But before you
can hope to find the amplifier that best suits your needs, you should
know what this "power" and "watts" talk is all about.
the sound we hear, whether natural or reproduced, is caused by a
movement of air. High-pitched sounds mean that the air is vibrating
at a fast rate. Low-pitched sounds are caused by air moving at a
slower rate. It takes power to move this air, just as surely as
it takes power to move an automobile. The more power you apply to
the driveshaft of a car, the faster it will accelerate. Similarly,
the harder you push the air in making sound, the louder will be
the sensation to the listener.
DBs and Power.
The decibel or "db" is used as a measure of sound and power because
it indicates the way our ears behave when subjected to sound vibrations,
or moving air. When you double the power applied to move the air
in making a given sound, the sound doesn't seem twice as loud, but
only slightly louder. We call this a change of 3 db. On the other
hand, 10 db represents a power change of 10 to 1. In other words,
actual power change, measured in watts, (just like the light-giving
power of an electric lamp), is much greater than the equivalent
change in decibels. The decibel method of measurement more nearly
approximates the way our hearing system responds to changes in sound
It's obvious that the more power an amplifier can feed to a loudspeaker,
the louder is the sound that can be produced by the loudspeaker.
However, because of the way we hear sound, doubling the power of
an amplifier will not make its maximum sound output seem twice as
loud, but only slightly louder. To choose an amplifier suitable
to the needs of the listening room, we need to consider several
to estimate your power amplifier requirements.
above shows acoustic wattage required
for various listening levels in different-sized rooms. This
table, used in conjunction with instructions listed above, will
help you choose correct amplifier for your hi-fi.
bass, even slightly, may require double
the power-handling capacity of the amplifier. This little known
-but important - fact should be considered when you choose an
amplifier for a hi-fi system.
of loudspeaker is factor in determining
power rating of amplifier to be used.
1. How much power is there behind real live music?
2. How loudly would you like music played in your living room?
3. How loudly will other members of your family let you play
music? (The second and third questions are usually separated by
about 10 db!)
4. What speaker are you planning to use with the
5. Will you ever want to add additional speakers in
other rooms? (Don't answer this one too hastily!)
6. How large
is your listening area and how is it furnished?
Listening Power. Before we even tackle the first question,
there's an important point that needs clearing up. The top rating
of an amplifier does not necessarily indicate how it will sound
at lower levels. Take a good-quality 20-watt amplifier and place
it alongside a 10-watt amplifier. Send in just enough signal to
develop, say, 1/2 watt-and it is unlikely that you will hear any
difference between the two. The difference only shows when a very
loud sound of 15-watts power must be fed to the speaker. That's
when the first amplifier will handle it, but the second one will
overload and cause annoying distortion.
A good analogy is
that of a 100- and a 200-horsepower automobile creeping along in
metropolitan traffic. Since neither car can be "opened up" to its
full power, both travel neck and neck, performing equally well.
HOW TO ESTIMATE YOUR POWER AMPLIFIER REQUIREMENTS
1. Decide on the loudest average listening
level you will ever require.
2. Add 20 db
to the above, for peaks in program material.
"Size up" your listening room by multiplying the length by width
by height of the room. Work to the nearest foot. That will be accurate
4. Determine the acoustic power in
watts from the chart below.
5. Find out the
efficiency of the loudspeaker you plan to use.
Divide the acoustic wattage requirements by the speaker efficiency.
7. Double this figure to aflow for overly
absorptive draperies, rugs, etc.
for any additional boosting you plan to do by means of tone controls,
bearing in mind that 3 db of bass boost represents a 2-to-1 increase
in power - 6 db, a 4-to-1 increase - and 10 db, a full 10-to-1 increase
9. If, after making the above estimations,
you find that the amplifier turns out to be too costly, revise your
top listening level DOWNWARDS and purchase the best possible loudspeaker
you can afford - for, at comfortable room volume, with all other
things being equal and the amplifier not "pushed" beyond its capabilities,
it's the quality of the loudspeaker that most determines what your
home music system will sound like.
How Loud Is Music?
The "quietest" sound anyone can hear is measured at "0 db." The
loudest sound anyone can hear before it actually becomes harmful
is about, 120 db louder than the softest sound. Anything we call
"sound" - including music - falls somewhere in between those two
To get a good idea of where a symphony orchestra
playing about 20 feet from you fits into this picture, refer to
the sound level graph on page 49. At first glance, it would appear
that a level of about 80 db is the loudest sound you can expect
to hear. However, bear in mind that these are average levels, and
that instantaneous peaks or bursts of music may actually exceed
this particular level by as much as 20 db. (Notice the curve for
the bass drum solo, for example.)
Now, if we had a simple
way of relating watts of amplifier power to decibels, we would know
how much power would be needed for the amplifier. But - we have
to consider the second and third factors.
Concert Hall. Since the purpose of an amplifier and speaker
system is to "push some air around," the question naturally arises,
"How much air?" Even traveling at the same speed, a motor scooter
could never haul the load of a two-ton truck. Similarly, a little
one-watt amplifier might make a lot of noise come out of a loudspeaker
(if both you and the speaker are in a coat closet), but you'd hardly
even hear such a system if it were installed in Carnegie Hall. The
truth is, when playing records, you're not in Carnegie Hall but
in your living room. So we've got to determine just how much air
has to be pushed around in that room and how hard it must be pushed
to satisfy your musical tastes.
of this is not meant to imply that you can't duplicate the concert
hall level of 80 db or even 100 db in your living room. All we're
saying is that you will most likely settle for a comfortable 60
db with the possibility of occasional 80-db peaks. In any case,
that's something you must decide for yourself. Having made up your
mind whether to go for full concert hall volume or not, and knowing
the size of your listening area, you can determine from the table
on page 50 just how many acoustic watts of power you'll need to
fill the room with that much sound.
Let's work out a sample
based on an average living room which measures 12' x 20' and has
an 8'2" ceiling. The volume of such a room is just under 2000 cubic
feet (width x height x depth). Suppose, despite our warnings, you
decide that at some time you will want to "crank the system wide
open" and really duplicate concert hall volume, or go for a glorious
80 db on average music, with possible 100-db peaks on the cymbal
crashes. Consulting the chart, you find that you need a mere .31
acoustic watt (less than 1/3 watt) to do the job. Seems hardly necessary
to invest in a power amplifier at all, does it? Actually, we've
been neglecting the biggest unknown of all, the loudspeaker.
Efficiency. All along we've peen talking about acoustic
power needed to do the job of making music for our ears. That means
actual power caused by the "back and forth" motion of the loudspeaker
cone itself. There isn't a speaker manufacturer we know of who will
deny that by far the most inefficient of all the components in a
hi-fi setup is the loudspeaker itself. That doesn't mean it's the
most inferior part of the system but it does mean that the speaker
puts out far fewer acoustic or usable watts than are put into it
by the power amplifier. Actually, most of the power an amplifier
feeds to a loudspeaker "goes up in heat." (In much the same way,
an electric light bulb only converts a small fraction of the electrical
"watts" fed to it from the socket into usable light. The rest is
wasted in the form of heat - a fact easily checked by touching a
bulb that has been lit for several hours!)
not all loudspeakers have the same efficiency. They vary from "highly
efficient" units of 10% to 20% to all-time lows of considerably
less than 1%. If you've been following the latest advertisements
put out by loudspeaker manufacturers, you probably realize that
there's something of a "factional war" going on between the proponents
of the "high" efficiency units and the relatively "low" efficiency
units. Far be it from us to get into the squabble. All we want to
do is point out the fact that a speaker having an efficiency of
1% will require 100 times as much amplifier power for a given acoustic
power - or that, in our example, to get .31 acoustic watt means
hooking up such a speaker to an amplifier able to supply 31 watts
to the speaker. A speaker having an efficiency of 10% will need
10 times as many amplifier watts for a given number of acoustic
watts or, again using our example, 3.2 watts supplied by the amplifier
are all that will be needed to meet the same acoustic requirements.
It should now be apparent that unless you have a fairly
good idea of the efficiency of the loudspeaker you propose to buy,
you cannot even attempt to estimate your amplifier requirements
with any reasonable degree of accuracy. This is the single factor.
that we can't put a number on. Only the speaker manufacturer involved
can do that for you - and more and more of them, realizing the misunderstanding
that exists on this one point, are becoming less and less reluctant
to publish this information as a regular part of their sales specifications.
It's really nothing to be ashamed of! Most of us have known for
years that an automobile engine only converts about 6% of the energy
contained in a gallon of gas into usable power, but we still buy
millions of cars each year!
Multiple Speaker Systems.
If you feel that eventually a second or even a third loudspeaker
installation in another room or two is good future planning, the
formula is simple as can be. If the second room is about like the
first and you ever plan to have both speakers going at the same
time, you'll need double the power of your first calculations.
Important point: many loudspeaker installations are equipped
with so-called level controls, or pads, with which it is possible
to turn off the sound coming from a given speaker. Just because
the sound is turned off in this manner doesn't mean you're using
less power. The same amount of power is being absorbed at the speaker
terminals. I It's simply all used up as heat in the wire-wound resistor
which makes up the speaker-control. The only way an amplifier can
be used for two speakers and still require the power calculated
for only one is if you actually switch the appropriate speaker in
and out, effectively disconnecting one terminal of each speaker
not in use.
Tone Controls Use Power. If
your listening room, your personal tastes, or any of countless other
factors make you want to add a little bass boost by means of the
tone controls on your amplifier, you'd better take that fact into
account before you go shopping for watts. Remember, a bass boost
of only 3 db requires double the power-handling capacity at certain
frequencies than would be the case if all your tone controls were
set for flat response!
Power and Frequency Response.
As long as we're talking about specifications, there's one the amplifier
people could be a little more detailed about. Amplifiers, as a rule,
perform best for "middle" tones. The extremely low-pitched and high-pitched
tones are generally much harder to reproduce at high power levels.
however, has instruments at both extremes as well
as in the "middle", and you'll notice from our graph that all the
frequencies need pretty much the same power for a given volume of
sound. Therefore, it's important to know not just that a given amplifier
produces a maximum undistorted power of "5 watts" or "10 watts"
or even "50 watts," but that it can produce that much power at all
the frequencies involved in musical reproduction, or at least from
30 cps to 15,000 cps. If this were not the case, certain instruments
would be clear and undistorted at maximum volume while others, such
as the bass drum or cymbals, might be annoyingly distorted at the
same listening level.
The important thing to remember when
choosing an amplifier is that a high-fidelity system has to work
properly as a whole, and that the loudspeaker, listening room, and
personal listening preferences must be carefully considered before
you can pin a number on the amount of watts required. The right
amount of watts, however, can really "make" a hi-fi system.
1996 - 2018
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas
and reference material while performing my work as an RF system and circuit design engineer.
The Internet was still largely an unknown entity at the time and not much was available
in the form of WYSIWYG
All trademarks, copyrights, patents, and other rights of ownership to images and text
used on the RF Cafe website are hereby acknowledged.
My Hobby Website: AirplanesAndRockets.com