Overpowering, Underpowering, Distortion, Clipping, and Everything in Between

- Image: 3D Sound Wave Rendering from FXshare.com

One of the most fundamentally misunderstood concepts in audio is what
happens when we apply power to a speaker. We understand that power is
what makes the magic happen, but so few of us "get" what that power
does. For example, you've probably heard about someone who was
"underpowering" their speaker; what on earth does this mean? You've
probably heard this before and if you haven't it will probably be a
wake-up call: if underpowering really existed, you would be
"underpowering" your speaker every time you turned down the volume.
This is one of many ideas that continues to falsely permeate the audio
industry and, together, we can put an end to it.

So where do these misunderstandings come from? Well, the power concept
is inevitably encountered in two situations: 1) You are looking for a
certain level of output, and 2) You have a speaker that doesn't work
anymore. These two concepts are completely related: if you seek a
certain level of output that requires more power than the driver can
handle, then you're looking at scenario #2.

*Definitions*
To understand the idea behind power, let's define a few things first,
shall we?

/Overpowering/ - Overpowering should be simple to understand: a driver
is being "overpowered" any time it has been damaged. If the driver has
not been damaged, have you really "over" powered it? Logic should say
no. In fact, this is probably a term you can stop using in the future.

/Underpowering/ - As I mentioned above, this is a completely useless
and non-sensical term. If you don't have the output you want to hear
from a speaker, say as much. Again, we should stop using this term.

/Distortion/ - Distortion can come from a lot of sources. In the
simplest sense, distortion is when the output signal differs from the
input signal. For the purpose of this article, we will be looking
primarily at distortion generated by the amplifier and passed to a
speaker. As a secondary form of distortion, we will briefly talk about
driving a speaker into distortion, by which I mean that the speaker has
been driven beyond Xmax. Distortion from your speaker is a whole other
ball of wax and we will only briefly stop here today.

/Clipping/ - Here we will strictly stick to the type of clipping that is
passed from your amplifier to your speaker, and is created by setting
the input sensitivity (read: gain) too high when considering the input
voltage from your source unit (ie. headunit, preamp, etc).

*Clipping and Distortion*
Warning: Much of the following will directly oppose everything you've
ever been told about power, distortion, and clipping. The fact remains
that power is almost completely misunderstood, even by many experts in
the industry.

Let us start the next section by breaking down distortion generated by
the amplifier and clipping, which are really the same thing. As we said
early, distortion occurs when the output signal is different than the
input signal. I will give a very simple example here and introduce you
to a concept that may be new to you.

When we play a signal on your speaker, the resulting sound consists of a
number of frequencies with various amplitudes. If we play a 100 Hz sine
wave through a speaker, then the fundamental frequency is 100 Hz. Now,
we may also see response at other frequencies that are typically a given
order higher than the fundamental. Assuming a fundamental frequency of
100 Hz, we can say that the second harmonic is 200 Hz, the third
harmonic is 300 Hz, the 4th is 400 Hz, and so on. The presence of any
of these additional harmonics is considered distortion as they were not
present in the original signal. Typically, the amplitude of these
harmonics decrease as the harmonic itself increases, ie. second harmonic
distortion is often higher than third, third is higher than fourth, etc.

Clipping is itself a form of distortion. In the above example, we
assumed a 100 Hz signal. However, if we were to increase the gain until
we have fully clipped the signal, the result would be the fundamental
frequency (100 Hz) and its higher order harmonics. If you've ever heard
a speaker reproducing a clipped signal, you probably already know that
is sounds like distortion, but you've probably also been told something
else: that distortion or clipping an amplifier will immediately damage
your speaker. Hold on to your hats: it doesn't! Perhaps a little
further explanation is required?

Here is a picture demonstrating what a sine wave looks like, as well as
a square wave (fully clipped signal), a triangle wave, and a sawtooth wave.


- Image: Wikipedia

That square wave sure looks ugly! More importantly, you should notice
one very important thing, though, when comparing a sine wave and a
square wave: the area under the curve from any point A to any point B
will always be greater with a square wave than with a sine wave. In
other words, a square wave carries more power over a given time. I like
to call it "increased average power over time" because it sounds cooler
that way. Hopefully the visual makes this next sentence understandable
as well: if the amplitude of the signal increases (the power increases)
within a given period of time, then the average power over time has
increased as well.

*How a Speaker is Damaged*
Let's take a moment now to examine why and how speakers are damaged by
power. Essentially, there are two types of failure:

1) Mechanical - This means that the speaker has physically been driven
beyond its limits; usually stuff starts banging together. For example,
a speaker can be mechanically damaged if the former is smashed into the
backplate a time or two. This occurs from applying too much
instantaneous power (read: power at any given moment) in a certain
enclosure. Since excursion (which is how much the coil moves) increases
as frequency decreases, we know that a speaker is more likely to have
over-extended itself at low frequencies. This is increasingly likely if
the enclosure is larger than typical for that subwoofer, or if you are
playing frequencies that fall below the "tuning frequency" of your
vented enclosure.
2) Thermal - Simply put, this happens when more power is dumped into the
voice coil than the coil can handle.

But stop! Thermal damage doesn't just come from too much power. Have
you ever wondered why a speaker can take ungodly amounts of power for a
brief moment without the coil exploding? Well, that's because it is
average power over time that matters. There are many subwoofers that
can take 10 kW or more for a second or two, but not many that can take
it for a full minute. And here we come full circle: whether it is a
sine wave, square wave or any wave in between, the speaker will not be
damaged thermally if the input average power over time is less than the
driver can handle. That's right: a low amplitude clipped signal will
not damage your speaker at all! You can test this for yourself: get a
speaker, hook it up, play a clipped signal, and keep the volume low.
You will not see any damage at all.

*When is it too much? When is it not enough?*
So the natural question arises: what is too much average power over
time? To be honest, it is tough for any layperson to make a good
estimate. Manufacturers typically publish power handling specifications
like "100 Watts RMS," which is misleading because there really is no
such thing as "Watts RMS" (a story for another day, or you can read here).
However, we know that we can apply more or less power if we adjust the
time over which this power is applied accordingly, which is why I
support buying just enough.

Dynamite in a monkey's hand is quite dangerous and too much power in a
novice's hands is equally so (minus the casualties). One has to wonder,
though: what is really gained by exceeding the manufacturer's own
recommendations? Is the associated thermal risk of increasing power
really worth it? The answer is simply no, unless you are chasing tenths
in SPL competitions. When more power is applied to the voice coil, the
voice coil becomes hot (duh!). At a certain point, this heat begins to
increase the resistance of the voice coil to the current flowing through
it. So we add more power, and more power, and more power... and before
you know it, the resistance of the voice coil has increased such that
adding more power will not gain any more output. Better yet is the
nature of the human ear: an increase in sound pressure of 1-3 dB is just
barely audible for most humans. To achieve such an increase, we are
looking at doubling the power we are already applying and, thus, putting
the coil at greater risk of thermal failure.

In the event you can't achieve the output you're looking for within the
manufacturer's recommended power handling specifications, your best
solution is usually to look for a different speaker. Of course,
achieving high-output is an equally complicated concept, but more power
than a speaker is rated for is rarely the solution (and will most
certainly void your warranty).

*Conclusion*
In short, you should now understand that only power can damage a
speaker. There are no mystery clipping gnomes that will kill your
speaker at the first sign of distortion. Furthermore, you should be
aware that power is just a way to get a speaker moving: there is no
underpowering or overpowering, there are just various levels of output
and a damaged speaker. Next week we will look even more closely at some
of these concepts, but please ask questions or correct me if something
is unclear. It is time to stop perpetuating these myths that never seem
to die and a good education is the right battle-axe to wield in this fight.