Audio Power Trip
By
Pat Brown
How consumption, production and transmission can affect your sound system
Click here for Audio power trip Part 2
Few subjects generate more confusion in the audio world than power. There
is a very good reason for this - it’s a confusing subject, and one
that can easily fool our intuition. Most of us are on a power trip - our
attitude is that “more is better.” We want bigger amplifiers
and more “powerful” loudspeakers so that our sound systems
will be louder.
In fact, power ratings are often the main (or only) criteria considered
regarding amplifiers and loudspeakers by equipment buyers. In this series
we will consider the role of power in sound systems, hopefully without
diminishing or overemphasizing its importance. Power ratings are only
one piece of a larger puzzle. Trying to characterize amplifiers or loudspeakers
only by power ratings is akin to trying to completely characterize a person
by a photograph. There is always much more than meets the eye.
The italicized words in this article are the key words. When you
reach one, pause and think about what it means in the context in which
it is used. These are the key concepts in understanding power flow and
its implications. The careful consideration of these words by many thinkers
over the last few hundred years have taken us beyond Newtonian physics
and into the quantum world. Don’t let the real-world explanations
that follow diminish the depth of the subject matter. Like most audio
subjects, you can take this as far as you like.
To form our understanding about power, let’s initially forget about
sound systems (with the exception of an occasional reference) and consider
power in light of other ways that we use it in daily life. We will begin
with some basics. Power is both generated and consumed.
From the perspective of generation - more is better. We always
want to have more power available than what we need.
From the perspective of consumption - less is better. If a task
can be accomplished using less power, we save money since power generation
usually costs money. Power is wasted if it is not doing something useful.
In sound systems, amplifiers and loudspeakers are both consumers and
generators of power.
The amplifier consumes power from the electrical service and generates
power to drive the loudspeaker. The loudspeaker consumes power from the
amplifier and generates sound power into the room. Rating methods are
used to describe both power generation and consumption (both are in watts).
Great care must be taken to be sure which one a power rating is describing,
since a larger rating may be better for a power generator, but a smaller
rating may be better for a power consumer.
UNIVERSAL POWER PRINCIPLES
No, this isn’t a self-help infomercial – it’s a discussion
of some of the properties that affect the flow of power. Power principles
are analogous in electrical and mechanical systems. Mechanical examples
are more prevalent in our everyday lives, so it is easier to look at power
from that vantage point. We begin with energy. In fact, everything
began with energy. Energy sources include power plants, automobiles, locomotives,
bombs, animals and humans, and less-obvious sources such as plants, water,
wind and even garbage.
All energy sources must get their energy from somewhere else, making the
“big picture” question a religious one (we won’t go
there). Most of the energy on planet Earth comes from the sun - the ultimate
power source in our physical sphere of existence. Let’s get the
terminology in place. Energy is measured in joules. It can just
sit captive (potential energy) or it can be put into motion (kinetic energy),
be it water turning a turbine or a husband taking out the trash.
Work is the result of using a force to move something over a distance,
so work is equal to force times distance. Power is the rate of
doing work. So, for power to be generated, work must be done for a period
of time. If the time span is reduced, so is the generated power. Remember
that when you look at specs such as “instantaneous peak power.”
That’s an important point, and we will come back to it. Power can
be rated in watts, with one watt being equal to one joule per second.
YOU AS A POWER GENERATOR
A good way to get a better feel for power generation and consumption is
to consider a generator that we all possess - our bodies. We humans
consume energy in the form of food, store energy in the form of fat, and
then burn it up in the course of day-to-day living. We convert energy
from one form to another. It is a process that we continually experience
but rarely measure, other than the occasional reluctant glance at the
bathroom scales, which provides a composite view of intake vs. output
since the last glance. Let’s attempt to measure power by hopping
on an exercise bike - one that reads in watts, but could just as well
read in calories, cycles/min, heart rate, or even body temperature.
There are many ways to measure power, and most methods are just estimates.
Start pedaling and watch the display. If you work pretty hard you can
get it up to 100 joules per second (100 watts). So as you pedal, you are
generating 100 watts continuous average power as you pump the pedals.
If you do this for one hour, you have generated 100 watt-hours. If your
bike were hooked to the power grid you could sell your hour’s effort
to the utility company for about ten cents, assuming that they can convert
all of the generated power into electricity.
After observing your efforts for a period of time, we could ascribe
a rating that indicates how well you perform as a generator. The rating
would indicate how much power you can produce on a continuous basis. The
rating could be in watts, but there are other possibilities. We could
compare you to a horse and specify your abilities in horsepower. One horsepower
equals 746 watts, which would put your power rating at about 0.075 horsepower.
So it would take about 10 people to generate as much power as one horse,
which is why a good horse (or the modern equivalent - a tractor) might
be considered of greater value than a person, at least when it comes to
farm work.
A person who could maintain 100 watts of power generation longer than
another would be considered a more powerful source. If we exclude the
time metric, then a meaningful comparison between two power sources would
not be possible. If no time metric is stated, then the assumption is that
the source can sustain its rated power indefinitely (not likely for our
human generator). Remember this when you shop for amplifiers!
THE ELECTRIC BILL
The utility company generates power by burning things, turning things,
flowing stuff, or causing chain reactions - and we consume it. Power is
paid for by the kilowatt-hour, and the prudent owner or renter tries to
get as much benefit as possible out of the least amount
of consumption. None of us boast about how much power we use, but rather
about how little power that we can get by on.
The loudspeaker must be considered on this basis. Don’t just tell
me what it uses, tell me what it produces! Shopping for loudspeakers
by looking for the one with the highest power rating might be like shopping
for cars using only the miles-per-gallon rating, and then picking the
one that gets the worst mileage!
We all know experientially that if we move something from point A to point
B there is always a force present that will object to the movement and
oppose it. In a mechanical system, one such force is friction. Friction
converts some of the applied power into heat (another form of energy).
The analogous parameter in an electrical circuit is resistance.
Resistance opposes the flow of current through a component or conductor,
and dissipates some of the applied power in the form of heat. Resistance
forms a load on the power source - something which it must overcome.
Reducing the resistance causes more power to flow due to the lower
opposition, and we say that the power source is under a greater
load.
This is why two loudspeakers in parallel cause an amplifier to produce
more overall power than it would into a single loudspeaker (but
usually less power per loudspeaker). A bucket with two holes in
it loses its water twice as fast as a bucket with single hole in it. There
is less resistance to the water leaving the bucket, and hence more flow.
Power consumption is all about heat. If there is no heat, there is no
power consumption. The factor that determines a loudspeaker’s power
rating is its ability to dissipate heat. An overpowered loudspeaker is
one that is getting too hot. Note that there are other ways to devour
loudspeakers besides toasting them (over-excursion for example), but these
are not necessarily power issues.
THE LIGHT BULB DECEPTION
One reason for the confusion surrounding power in audio systems can be
attributed to the light bulb. Light bulbs are rated in watts, and we all
draw a correlation between the wattage rating and the brightness. This
produces a “more power, more light” mentality, which many
intuitively apply to sound.
But if you read the package closely, the real parameter of interest regarding
the light generating properties of a light bulb is its luminosity
– the lumens generated by the applied power. The power rating indicates
how much power is dissipated in the process of generating the rated number
of lumens. Both numbers must be considered to evaluate the bulb’s
performance. If I can get more lumens and consume less power, I have a
better bulb - assuming that I have not compromised some other key parameter,
such as longevity.
Like the light bulb, a loudspeaker has a wattage rating that indicates
how much power it consumes continuously as it does its job (its job being
to produce “X” amount of sound). But the real parameter of
interest is the amount of sound power that results from the consumed
electrical power. In fact, most of the applied power gets converted
into heat rather than sound.
The sound power, like the electrical power, is rated in watts. A perfectly
efficient loudspeaker would convert all of the applied electrical power
into sound, with no resultant heat, so one watt of electrical power would
yield one watt of acoustic power. In reality, the conversion rate is much
lower, typically less than 25 percent for compression drivers and less
than 10 percent for cone loudspeakers (ironically similar to the light
bulb’s efficiency, with similar heat production!). It’s a
good thing that we can’t touch those voice coils during use.
The ratio between the applied electrical power and the radiated
sound power is the loudspeaker’s efficiency, which indicates
the percentage of electrical power that is converted into sound power.
We can already see that it may make more sense to increase a loudspeaker’s
efficiency than to increase its power handling. If less is wasted, we
don’t need as much from the source. And some means of increasing
the power handling of a loudspeaker actually reduce its efficiency,
yielding a higher power rating but less sound production!
So it is entirely possible that a loudspeaker with a lower power
rating is actually a better transducer than one with a higher rating.
The only way to know is to consider the efficiency, which is something
that is often neglected by equipment buyers.
BACK TO SOUND SYSTEMS
Now, in review let’s bring it back to sound systems. The power generators
in a sound system are the amplifiers and loudspeakers. Notice that I didn’t
say power amplifiers. Power isn’t amplified, it is generated.
An amplifier that is rated at 100 watts continuous can do just that -
generate 100 watts continuously just like the dude on the exercise bike.
And while it must be measured using a load, the available power is actually
independent of the load.
A greater or lesser load does not change how much power is available
from a source. An automobile rated at 240 horsepower has the same available
power whether it is coasting down hill or pulling a trailer. You are just
more likely to max it out pulling the trailer. A light bulb rated at 1000
lumens can do so in either full sunlight or complete darkness.
Amplifier specifications do not usually describe available power, but
rather how much power can be generated into a specific load, such
as an 8-ohm loudspeaker. This number is always less than the available
power from the amplifier to allow for stable operation, longevity and
high fidelity. Although it is often overlooked, the power that the amplifier
consumes from the electrical service is also important. Some amplifier
types consume less power (and run cooler) than others in the course of
providing their rated power. Others may serve us well as a space heater
while generating a few watts for our hi-fi system.
LOUDSPEAKER RATINGS 101
The electrical power rating for a loudspeaker is a measure of consumption.
Loudspeakers consume electrical power and convert it into heat and sound
via mechanical motion. Since the sound is the part we are interested in,
the main parameter of interest should be how much sound we get for the
applied power, not how much power we can apply. The ideal loudspeaker
would generate the desired sound level consuming no electrical
power. But because this is impossible, we must rate the loudspeaker’s
ability to dissipate the waste.
The electrical power rating of a loudspeaker is a measure of waste removal,
not a measure of sound production. A higher wattage rating is a good thing
only if was achieved by a method that doesn’t reduce the
efficiency. Like the light bulb’s luminosity, the loudspeaker’s
sound power is the most important specification.
And as important as it is, you won’t often find efficiency ratings
on a specification sheet. Instead you will find sensitivity ratings
– numbers which describe the sound levels that result from confining
the sound power to smaller areas (directivity) and increasing the power
transfer to the air (horns, baffles, etc.).
Hopefully I have you thinking about some of the relationships and terminology
regarding power generation and consumption. In our next installment we
will dig a little deeper into the intricacies of power flow with “uncomplicating
the complex impedance.”
Pat Brown, with his wife Brenda, heads up Syn-Aud- Con, leading audio
training sessions around the world. For more info, go to www.synaudcon.com.