What Causes Distortion? An Audio Engineer Explains Speaker Distortion

What causes distortion? This is an actually very, very complex question if we want to dive deep into the details of distortion. Because in an audio or video system, every component can lend a certain amount and a certain type of distortion. But today, I want to just talk a little bit about focusing on distortion and what causes it in loudspeakers.

Now, loudspeakers are inherently what's known as a non-linear device to a certain extent. Which means that they're not perfect, they have a certain level of distortion. And frankly, it would be impossible to have a loudspeaker with zero distortion. And frankly, very difficult even for a loudspeaker to have distortions as low as the levels we see in most electronics like power amplifiers or preamplifiers, for instance.

So anything you can do to reduce the distortion in the loudspeaker is going to make a big difference to the whole system. But what causes that distortion? So the first thing is that the drive units themselves, the woofer, tweeter, mid-range, whatever, all of those parts, and the components that make up a drive unit, all have certain levels of their own distortion.

For instance, you probably are aware that a drive unit, the conventional moving coil driver, like this woofer, is an electrical motor, so there's a magnetic system. You put an alternating current through the voice coil. And because it's suspended that voice coil in a magnetic circuit, it causes movement.

So if you remember some of your high school, you know, physics or science classes, you probably remember, you know, coiling wire around a nail, and putting some battery to that wire and then being able to pick up metal filings, because you created an electromagnet.

So this is an electrical motor or electro-magnetic motor system. Now, the magnetic circuit, which is these metal parts and the black disc here, which is the actual ceramic magnet, it inherently is not perfectly linear, particularly when you get to high currents or high power levels, or the system heats up.

So that's one area where there can be some distortions. The voice coil moving in the magnetic field itself, as it moves more and more, it actually creates its own current because it's moving, which then interferes with the magnetic field. So the two actually upset each other to a certain extent, and can cause some distortion.

Now, obviously, the radiating and the moving surfaces, the cone, the dust cap, the rubber surround, the spider suspension, all of those things can create and do create levels of distortion. Those distortions can be the fact that if I put a perfect sine wave in, the cone is not going to move perfectly in and out in a perfectly what's called pistonic fashion, so it's moving perfectly.

Also, at different frequencies there'll be different resonances built into the cone, the dust cap, the voice coil, the voice coil former, the surround. Yeah, you're probably going, "Wow, that's a lot of things causing distortion." And I mean, this is one of the reasons why loudspeakers tend to have a higher level or magnitude of distortion than electronics do.

Now, the same thing is going to apply for, you know, a tweeter, and it doesn't matter what the materials are, it doesn't matter if it's a ribbon or a planar tweeter, or a full electrostatic loudspeaker. All of them will have their own areas and components that will generate distortion. But here in our conventional moving coil loudspeaker with a woofer and a tweeter, once we mount those parts into a cabinet, now the cabinet actually has some levels of distortion.

Again, where distortion is things that are created that are not part of the original signal that went into the speaker to begin with. So one of those can be vibrations in the cabinet, resonances in the cabinet, and we can do lots of things to reduce those levels of distortion. But even if you made a speaker out of concrete, it's going to have its own resonance signature, and that's a form of distortion.

Now, one last thing that's in most conventional loudspeakers, not all of them, is the crossover unit, the filter network that divides up the frequencies that the different drive unit should handle. So for instance, mid and bass frequencies only go to the woofer in this system, and high frequencies only go to the tweeter.

The components that we use to make up a filter system can be resistors, capacitors, inductors, what are known as passive electrical components. All of those components themselves have levels of distortion. Now, it's much lower than the levels of distortion in a woofer or a drive unit for instance, but it is still a component.

So I hope that gives you just a real quick overview of where does distortion come from? What creates distortion in the different components that make up a loudspeaker? Like I said, it's a very, very in-depth, very technical topic, but hopefully those are a few things that can give you an idea of what causes distortion.

Thanks a lot for sending your questions in.