Distortion: refers to any signal that appears in a reproduced sound that was not in the original recording or program source.
Audio reproduction is affected by distortion, and every component in the chain adds some level of distortion. The higher the quality of the device, the less distortion or deviation from the original. An old adage among audio enthusiasts is that a system is only as good as its weakest link, so if you spend tons of money on your amplifiers and speakers but you are streaming a lo-res file for example, the sound you get out of your system will only be as good as your streamed lo-res files.
Since the level of distortion determines how good our listening experience can be, let’s look at the three types of distortion that affect audio.
A harmonic is a signal that is created from a fundamental signal. Amplifier circuits tend to be the biggest culprit in the creation of harmonics in your audio chain. An amplifier creates even order harmonics of a fundamental signal. For example, at 100 Hz an amplifier circuit creates a harmonic 200, 400, 800, 1600, 3200, 6400, 12800 Hz. Each harmonic gets lower in amplitude, so the harmonic at 200Hz will cause you far more grief than the harmonic at 800Hz, and by 12.8kHz the harmonic energy is so low you won’t hear it. Each instrument, especially acoustic instruments, also create their own harmonics, so oftentimes the artificial harmonic from the amplifier circuit is not noticeable.
Harmonic distortion can also be filtered out, as in the case of a Class D amplifier that is being used to drive a subwoofer or other LF frequency driver. In that case, since the designer only cares about fundamental frequencies below 200Hz (for example), everything above 200Hz is filtered out (lo-pass filter) so we never hear the distortion.
Clipping can also produce a tremendous amount of harmonic distortion. In the extreme, distortion caused by clipping can completely destroy the listening experience and even damage equipment (especially speakers). Clipping is caused by overloaded components, or weak components that are being asked to do more than they were designed to do. A low power amplifier driving a high-quality pair of speakers will often clip. The danger here is the energy not being used by the speaker is transferred into heat energy. Build up enough heat energy in a speaker coil and the windings will open and you’ve got yourself a dead speaker.
Harmonic distortion (listed as Total Harmonic Distortion (THD)) in component specifications is expressed as a percentage of the original signal and is typically produced in electronic components, but can also be produced by loudspeakers.
IM is produced when two or more tones or frequencies are produced at the same time, which is almost always the case in a musical passage. A non-linear characteristic of some component in the chain or circuit creates sum and difference frequencies which are harmonically and musically unrelated to the original tones. Because they are unrelated to the original, unlike harmonic distortion, IM is far more noticeable and objectionable.
Intermodulation Distortion (IM) is also expressed as a percentage of the original signal and is typically produced in electronic components such as amplifier circuits.
TD occurs when a component in the chain cannot respond quickly enough to a rapid change in the signal. This is a particular problem in loudspeakers where insufficient damping or other design flaws create a mechanical resonance. Even though TD is most noticeable at the loudspeaker, the loudspeaker actually depends on the amplifier to provide damping. The damping factor is the ratio of speaker impedance to the internal resistance (not impedance) in the amplifier output. A damping factor of at least 20 is your best bet.
Transient Intermodulation Distortion is produced in an under-designed amplifier when a time delay in the circuit’s feedback loop causes a brief overload in some other circuit element. An example of this would be when a percussive element in a passage, such as a cymbal, becomes distorted while the strings or other instruments sound normal.
When you’re scoping out new equipment for your audio system, these are three culprits that are often overlooked but can give you a bad listening experience. This is the reason buying quality components makes a huge difference in how your music sounds. You’ll spend a little more money for the engineering and design of quality components that don’t introduce noticeable amounts of distortion, but your ears and quality of life or worth it.