Any control knob, on any effect, can be modulated. Audio signal processors are often modulated by low frequency oscillators (LFO)s. These LFOs generally range from .1 Hz to 10 Hz (speed, cycles per second). The first string, open, on a bass guitar, in standard tuning, is around 41 Hz (E).
Triangle Wave Modulation
Figure 105 : The triangle wave. Imagine the effect control is a volume knob on a stereo. At point A the volume knob is on ten, at point B the knob is on zero.
Most LFOs in audio signal processors, are triangle waves LFOs, because they create a constant, periodic, sweeping sound that covers the whole spectrum, and doesn’t speed up or slow down at different parts of the sound file.
Square Wave Modulation
Square wave modulation causes the effect parameter to cycle between maximum, and minimum, nowhere in between. Imagine this LFO is used to modulate a tremolo (volume) effect. The volume is either wide open (point A) or completely off (point B).
Sine Wave Modulation
Figure 107 : The sine wave.
Sine waves, at higher frequencies, sound like flutes. A guitar string, if it didn’t have any harmonics, would sound close to a sine wave. This type of modulation is often chosen, because it speeds up and slows down at different parts of the sound file. The MXR flanger is modulated with a sine wave.