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
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.
To understand the idea behind power, let’s define a few things first, shall we?
What is 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.
What is 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.
What is 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.
What is 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 Vs. DistortionWarning: 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.
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:
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.
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). 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).
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.
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.