A sine wave is the purest tone possible, just a single frequency. No musical instrument produces a really pure sine wave, although tuning forks get close. The sine wave is smoothly rounded everywhere, with no sudden changes in direction.
If we take that same sine wave and electronically clip off the tops and bottoms at a set clipping level, a crazy thing happens. Just snipping off those tips has a big effect on the sound. A pure sine wave is somewhat generic sounding. Distorting the sine wave by clipping introduces a whole bunch of new sounds called “partials” or “harmonics”. The sound is much more interesting to listen to, as long as we don’t get too carried away with it.
In music, there are fundamentals and partials. A fundamental is the basic tone of any given note – for instance, the frequency A-440 is the standard for musical tuning, which is a sine wave that has a frequency of 440 Hz (Hz is short for Hertz, and stands for one cycle per second). We recognize that a tone of A-880 is one octave higher than A-440. Musicians refer to a musical note that has both A-440 and A-880 mixed as having a fundamental and the first partial. A frequency of three times the basic note frequency is also referred to as the third partial of the basic note. Four times the fundamental is the fourth partial and so on. Since harmonic is another name for a partial, when you hear the term “total harmonic distortion” it means whatever is left after taking out the fundamental sine wave in some waveform.
So there are really two kinds of distortion: simple harmonic distortion, which generates distortion related to the fundamental frequency and adds character to instrument sound, and intermodulation distortion (IMD), which produces non-musically related frequencies at multiples of the sum and differences of two frequencies, considered to
sound harsh to the ear.
Ultimately, the name of the game is to produce harmonic distortion, which makes musically good sounding notes, and to minimize IMD, which makes unmusical sounds and
harsh buzzes. Unfortunately, no matter how hard we try, we can never get all harmonic distortion and no IMD.
One way to keep mostly harmonic distortion is to not have sharp corners on the waves. Tubes help with this process. Tubes generally produce very softly rounded, squashed tops when done on square waves. A pure sine wave is almost devoid of information, having no wiggles or corners in that smoothly rounded top. However, if we took a square wave and massively amplified it, or a triangle wave, or a wave with spikes all over its top and did the same amplify/clip operation on it, we’d get exactly the same result as with the sine wave with massive clipping.
There are other distortion tricks besides clipping a signal. To get an octave sound, a process called full-wave rectification is used. What happens here is the negative half-wave of an alternating current is converted into a positive half-wave. This trick has been used to one degree or another in a number of pedals. Having the signal completely fullwave rectified eliminates the fundamental frequency. But if we half-wave rectify a sine wave, the result still has a lot of the fundamental in it, in addition to a noticeable octave sound. We can also partial full-wave rectify a signal, which makes for an even more noticeable octave sound.
Here’s the big trick: filtering of the signal both before and after the distortion usually has at least as much, if not more, to do with how the distortion sounds than the actual method of distortion. A combo amp with a speaker or two and an open back implements a multi-pole low-pass filter all by itself. This is at least one of the reasons that mic''ing
an amp is preferable to running a distortion signal directly into a PA mixer. The speaker cabinet’s low-pass action smooths off those offending treble shrieks. An electronic multipole low-pass filter is the essence of all the cabinet simulators you hear these days.
Filtering before the distortion is also interesting. Hendrix used a wah pedal before his Fuzz Face. The band-pass effect makes the signal bigger at the frequencies boosted by the wah, so those frequencies get distorted most. You get an interesting change in distortion depending on what notes you hit. This cuts a lot of the harsh sounding IMD, too.
If you are studying science in high school or physics in college and your teacher doesn’t like your long hair and your iPod, bring in a Tube Screamer and present this little distortion lesson to your class. Everyone will love you for it.
Rick Wheeler currently works as Larry Carlton’s guitar tech and front of house engineer. He is also an accomplished jazz guitarist, vocalist, and educator. You can contact Rick at firstname.lastname@example.org