For guitarists, stomp-box effects are the sum of both aural and tactile experiences. You turn a knob with one hand while plucking a string with the other and perk your ears to the wonders produced. However, I feel safely perched on a limb when saying that many aspiring guitarists have no idea of what is actually at play when experimenting with these tools.
Part of this may be that so many effects manufacturers print abstract terminology on their products, but there’s a good chance that your guitar tutor never told you a thing about this stuff either. So, in this tutorial I will briefly explain the basis for many modulation effects in hopes of making your effects experience partly cognitive as well.
The word modulation implies change. When using a wah pedal, your foot governs how much change will happen to a filter depending on the latitude of your heel versus your toes. So then, what’s controlling change in an auto-wah? It’s pretty much the same effect after all, sans the foot pedal to lean on. In this case we are controlling change with a voltage known as a low frequency oscillation or LFO for short. An oscillation implies a change over time just like modulation does, so it makes sense that they should relate to each other.
Let’s take a quick look at how that works. If something oscillates (or changes) more than about 30 times a second – and with enough force – we perceive the event as sound and if it’s continuous we will hear a clear pitch. In fact those of us who haven’t spent too much time with our ears pressed to speaker cones can hear up to 20,000 oscillations per second (Hz). Just try counting the number of times your guitar strings oscillate once you get them moving!
So, let’s bring this back to the LFO. Well the L stands for low and low usually implies less than 30 Hz (changes per second). This means that it’s a change that we do not hear as a sound… but we can hear it as a change in another sound. If your foot is moving back and forth two times every second on that wah pedal it’s not really producing a sound itself, but it is changing the sound your guitar ultimately produces by altering the state of the wah’s filter.
Moving away from the wah example, let’s look at the tremolo effect. This is a constant change in volume. One way of getting this effect would be to have a friend furiously turning the volume of your amp up and down… but a smarter way of doing it would be using an LFO to change the volume of the signal at some point before it even gets to the amp – and that’s precisely what a tremolo pedal does. There are a few ways the LFO might adjust a parameter such as volume over time. We describe this motion in terms of shape. Try to imagine someone changing the volume of your signal in the motions illustrated in the image.
There are several types of effects that make use of LFOs. For instance choruses, flangers and vibratos which produce very short delays constantly speeding up and slowing down, based on what an LFO is doing. The auto-wah may use an LFO to continually adjust a filter’s position (not to mention a phaser) and even the whammy pedal (or harmonizer), which literally uses an LFO to dopplerize a sound.
Other than shape, there are two more parameters to the LFO. These are rate or speed (changes per second) and intensity (which designates the amount of change). Manufacturers who use abstract terms actually make experimentation a lot of fun as you try to decode the labeling on your new stomp-box, but beware, some may have one knob controlling two or three different parts of the effect just to keep you guessing.
So go get your favourite phaser or flanger out and see if you can tell the shape of the LFO driving the effect. Perhaps you can even change the shape. If you’d like more insight into what some of your effects are doing try researching envelope followers for another dimension of modulation.
Marcel Bellvé is a lecturer at SAE Auckland and writes production music for film and television. You can contact Marcel at firstname.lastname@example.org