Whilst looking at the massive range of effects equipment and accessories that can be used to help affect or adjust the sound of an electric guitar, you may have come across a suite of tools called modulation related effects. Modulation on its own makes little sense, and unless you are an experienced player of electric guitars, or know fair bit about electric guitar music, you may well wonder what modulation effects are, and how they work.
One of the earliest examples of a modulation effect was the rotary speaker. Today, the effect produced by a rotary speaker can be re-created electronically, but often these new electronic sounds can be labelled in such a way that, unless one knows where the original sound came from, it is hard to envisage exactly what the purpose is of the effect. You may seen digital systems offering a rotary sound, or rotary modulation, but what is this exactly? The original rotary speaker was literally a speaker which rotates round. By physically rotating the speaker around, a doppler effect can be produced (as though you were walking around the speaker yourself, with the sound becoming more distant, then gaining in volume, whilst at the same time shifting from left to right.)
Physically rotating the speaker in this way wasn't always possible - with cables and leads sometimes becoming rather complex, and the sheer weight of the speaker preventing the system from working safely or easily. In these cases the same sort of effect was achieved by placing a rotating baffle in front of the speaker. This directs the sound from left to right, and then covers it up completely before opening up again from the left. This creates the same doppler effect with the sound fading and shifting at the same time.
You may have already heard of these rotation effects, or used it yourself within a digital system. Very likely you may well have not realised exactly what the sound was trying to achieve - but by visualising this rotating speaker, it is possible to understand what the sound is achieving, and be more critical when listening to this effect and deciding on the parameters to use.
By speeding up this rotation effect, a number of other effects can be achieved - many of them named perhaps rather oddly. Speeded up rotation modulation can be translated in to any of a number of other sound qualities, such as flanging, phasing, chorus, vibrato and in some cases tremolo. There are various combinations of using this effect - such as the Leslie Speaker which has a bass speaker unit which includes a rotating baffle unit inside, with a horn speaker above it which actually rotates fully itself. These speakers produce such a unique combination of effects that the actual sound achieved is virtually impossible to accurately reproduce electronically.
Fender also produced a speaker unit which incorporated a rotating baffle inside the speaker casing itself, although they did not include a rotating horn. Modern digital ways of recreating this sound effect base the manipulation of the sound on three factors - the speed of the sound produced, the volume, and the modulation. It might seem odd to consider the speed of the sound is important - after all, you don't imagine that the notes can come much quicker than you play them on the electric guitar.
However, the doppler effect is based entirely on speed of sound. As an object, like an ambulance siren, that is coming towards you is pushing the sound waves in front of it faster, whereas those behind the vehicle as it passes you and travels further away are dragged out, creating slower sound waves. The speed of the sound wave affects its pitch - which is the pitch change we hear in the doppler effect. The rotating speaker or rotating baffle re-create this pushing or pulling of the sound waves as the sound is either pushes towards you as it rotates in your direction, or pulled away as the baffle cover the sound and projects it further away.