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.
[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. As time permits, I will be glad to scan articles for you. 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."
"Our research 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
"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 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
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 system?
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
HOW TO ESTIMATE YOUR POWER AMPLIFIER REQUIREMENTS
Decide on the loudest average listening level you will ever require.
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.
4. Determine the acoustic power in watts from
the chart below.
5. Find out the efficiency of the loudspeaker you plan
6. 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.
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 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, 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.
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,
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 acoustic requirements.
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.
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!
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.