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
January 1957 Popular Electronics
[Table of Contents]People old and young enjoy waxing
nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October
1954 through April 1985. All copyrights (if any) are hereby
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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."
shows that the purchase of an amplifier having a power rating of over
10 watts is a waste of money."
"A 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.
All 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 intensity.
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 factors:
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.
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 system?
5. Will you ever want to add additional
speakers in other rooms? (Don't answer this one too hastily!)
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
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.
3. "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 enough.
Determine the acoustic power in watts from the chart below.
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.
8. Allow 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 in power.
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 extremes.
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,
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
Home vs. 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.
All 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!)
Furthermore, 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
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
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. An orchestra,
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.