What is audio distortion, and why does it matter?
Audio distortion refers to the alteration or deformation of an audio signal’s original waveform. While any kind of audio processing can alter an audio signal, the term “audio distortion” is typically used to describe either intentional or undesired sonic destruction.
Perfect sound curve vs Distorted sound curve
There are two types of audio distortion: linear and non-linear. Linear distortion is a change in the amplitude of a signal, while non-linear distortion is a change in the frequency content of a signal. While both are forms of audio distortion, when most people talk about audible distortion, they're talking about non-linear distortion.
Linear distortion doesn't add anything to a signal. Instead, it directly changes it. This can be in very noticeable ways like changing the pitch or volume of a sound, or in more subtle ways like altering the phase of a signal.
Non-linear distortion adds additional frequencies to the signal. These can sound like a grainy texture on top of the sound, almost like you're listening to an old vinyl record. It can add a buzzing, hissing, or crackling sound to recordings.
That said, non-linear distortion isn't always unpleasant. Most pop music production uses various forms of non-linear distortion throughout the mixing process, even in what sound like clean, pristine recordings.
In this article, we're focusing on the unpleasant side of audio distortion with the three most common forms you're likely to encounter. These are total harmonic distortion, clipping, and speaker distortion.
Total Harmonic Distortion (THD)
Harmonic distortion is sounds that have been added to the original frequencies present in a signal. Total harmonic distortion is a measurement of how much of an audio signal consists of those newly added harmonic frequencies compared to the original.
Harmonic frequencies aren't always unwanted. If music was all fundamental frequencies, it would be pretty boring. The harmonics in sound are what give instruments their unique timbres. The low E string on a guitar is roughly 83 Hz, but harmonics at different frequencies are what give it the sound of a guitar string.
This isn't all harmonics are useful for. As an example, audio engineers will often enhance the harmonic frequencies around bass instruments to ensure you can hear them on smaller speakers that physically can't reproduce the fundamental frequency.
When components in your stereo system add harmonics to a signal, they're not usually pleasant. This is why manufacturers aim to keep the total harmonic distortion of their components as low as possible. The THD of a class D amplifier, for example, is regularly far below one percent.
There's a reason for this one percent figure: that's the level where people will start to notice it. While only some people can hear distortion well below one percent, most will notice when it goes above that figure.
Harmonic distortion is present in any product that includes an amplification stage, from your phone to your home theater system. Fortunately, you won't often hear it, and you don't need to worry about it.
Clipping is what happens when a signal pushes beyond a specific threshold. This threshold is often the power rating of an amplifier, but clipping can happen to an audio signal at many different stages for varying reasons.
No matter how or why the signal is exceeding the threshold, this effectively chops off the peaks of the signal. Where in a signal chain the clipping is happening will determine how abruptly this happens, meaning clipping can range from barely noticeable distortion to a loud, ragged sound that has you diving for the power button on your stereo.
Digital clipping is typically the most unpleasant variety. This can happen in software, a component's analog to digital (ADC) or digital to analog (DAC) converters, or in a standalone product like an external DAC. No matter what, it's not something you want to hear.
Of course, how unpleasant clipping sounds is often a result of how you use it, and in the analog world it can sound great. Guitar distortion and overdrive are both forms of clipping, letting a guitar sing in a way that it couldn't otherwise.
That said, when clipping happens in your stereo system, due to a component mismatch or simply turning the volume up too loud, it's never a good thing.
Speaker distortion is exactly what it sounds like, and it sounds like your speaker is trying to rip itself apart.
The good news is, most of the time, what you're hearing as a speaker distorting may actually just be clipping. This is probably your amplifier struggling to drive the speaker and distorting as it is exceeding its maximum power rating to try to bring sound to the speaker. Turn down the volume and this will stop.
Other times, what you're hearing is non-linear movement in the speaker driver, which is meant to move linearly. This can be a result of using too powerful an amplifier or an ohm mismatch between speaker and amplifier.
In a speaker, as the electrical signal hits the driver's voice coil, the driver should move in turn with the waveform. If the audio signal is too powerful, the motion can start to bottom out at the extremes, meaning it can't push more air. This is where you start to hear actual speaker distortion.
If this continues, this can physically damage the speaker, so if you think you hear speakers distorting, be sure to turn the volume down as quickly as possible. Even if it turns out to be clipping, this usually means something isn't right in your stereo or home theater system.
What creates distortion in the different components that make up a loudspeaker is a very in-depth and very technical topic, but these are a few things that can give you an idea of what is distortion.