When a speaker doesn’t sound quite right, you may guess that it is experiencing distortion–and you’d probably be correct. However, there are a lot of different kinds of distortion (some you can’t even hear) and reasons why distortion happens.
To understand this better, let’s start with a simple definition of what distortion is: Distortion in a sound system is an unintended change of the signal. This could be in the electrical signal feeding into a speaker or the shape of the soundwave coming out of it.
Distortion fits into two broad categories: linear and nonlinear.
Linear distortion refers to changes in the amplitude (volume) of the signal. This has to do with the frequency response and how each frequency is transmitted by the speaker. Typically, this type of distortion is not as big of a deal and can usually be corrected with digital signal processing (DSP).
One kind of linear distortion is bandwidth distortion. This refers to a system not reproducing sound correctly outside of a specific frequency range (really low or really high frequencies). In many loudspeaker applications, this doesn’t matter as the frequencies are too high or low for human ears to hear. However, in some more extreme engineering applications, this becomes important and needs to be given attention. On the low end, infrasonic frequencies may not be audible but may be felt as vibrations.
Dynamic distortion is another type of linear distortion. It has to do with limiting the extremes of a signal. It’s often intentional in musical recordings as the medium (radio/video broadcast, Bluetooth, etc) is bandwidth limited. Think about how live music often sounds more exciting. This is because the greater acoustical output from things like drum solos contrast with softer portions of a song to make the entire presentation audibly intriguing. Pop and rock music is frequently compressed on recordings so, it is in your best interest to make sure your system doesn’t compress it further by having inadequate power or thermal issues with the speaker driver.
Nonlinear distortion is when extra frequencies are produced that aren’t intended–or are not in the original signal being fed into the speaker. Nonlinear distortion can generally be divided into either harmonic or intermodulation distortion.
Harmonic distortion is when a fundamental signal at one frequency is reproduced by a speaker in multiples, or harmonics, and at higher frequencies than what was intended. For example, imagine a 300 Hz tone is the fundamental signal going in, but the output includes that tone plus overtones at 600 Hz and 900 Hz caused by the mechanics and the electronics the tone is being played through. We call these harmonic overtones.
Harmonic distortion is measured in dB (just like normal amplitude). Total harmonic distortion (THD) is a percentage of how dramatic the distortion is compared to the original signal (with each order of harmonics contributing to the percentage). Too much harmonic distortion is a very bad thing in a loudspeaker intended to reproduce a signal faithfully and without “color” added.
The biggest contributors to harmonic distortion are the mechanical components of a speaker. These are the things like the magnet, cone, surround, voice coil, etc. A well-designed speaker made with quality parts should have a THD below 2% in its intended frequency range. You can use advanced testing tools like Klippel QC and SoundCheck by Listen, Inc. to get really detailed insights into THD and what is causing it in a speaker.
Now let’s talk about intermodulation distortion. This is when there are two (or more) signals mixing. This can affect the amplitudes of the original frequencies and create new frequencies above and below in frequency, and that were not in the original signal. These are sometimes called sidebands.
One instance where intermodulation distortion can be a problem is in a poorly designed speaker that includes both a tweeter and woofer. The signal being fed into this speaker will be split between the two drivers based on the frequencies. They better operate nicely together or there will be unwanted distortion.
Interested in seeing how sensitive you are to distortion? Try Klippel’s fun distortion listening test here. Maybe you’re cut out to be a quality control technician at a speaker factory?
Distorting Distortion’s Reputation
Not all distortion is bad. Guitars rock, in part, because they have a unique “fuzzy” sound which is a form of distortion, and guitar speakers are built specially to handle this. It’s impossible to completely eliminate all distortion from a speaker or a sound system. However, with the right materials, design, and testing you can essentially eliminate audible distortion to the point where it won’t be a problem.