Choruses create a “doubled” sound by adding a delayed copy of the sound. The delay time is very short (usually less than 40ms) so it blends with the original sound (and doesn’t sound like two different sounds). Flangers are a special case of chorus effects where the delay time is usually less than 15ms. To add movement and interest, the delay time is slowly changed. This also changes the pitch of the delayed sound, which helps make the sound richer. More sophisticated chorus effects add multiple delayed copies (also called “voices”) for an ever richer and smoother sound. The downside is that the sound becomes more diffuse and washed out. It can also blur the sense of pitch in the sound.
Use chorus to widen the stereo image when you want the sound bigger and more diffuse. Don’t use chorus when you want to keep the sound focussed and direct.
Phasers usually operate by a complex method of using allpass filters to cause phase shifting. You don’t need to understand exactly how it works – just the sound. Phasers add movement by modifying the frequency spectrum, and changing it over time. You might think of it as a complex EQ that keeps shifting and changing.
Use a phaser to widen the stereo image when you want to sound to stay focussed and direct. Don’t use a phaser if you don’t want to change the tone of the sound.
This is a broad term that has been used to refer to several different techniques. Some tools widen a mono sound by delaying one side (left or right). I recommend against this technique because it causes the audio to sound like it’s originating from one side (even though there’s equal energy on both sides) and the tone of the sound will drastically change if your mix is collapsed to mono (or if you do any further stereo width adjustment further down the track, such as that in mastering). Some other tools adjust the stereo image by using mid/side encoding to separate the “centre” audio from the “side” audio (which represents the stereo width). They then enhance the side audio which widens the stereo image. Basic tools do this by simply raising the volume of the side audio. More sophisticated tools use EQ to let you control the stereo width at different frequency areas. The advantage of this approach is that it can sound very natural if the original audio is already stereo.
Use these tools when you have a sound that’s already stereo (such as a stereo recording of an instrument or a complete mixdown) that you want to widen with a natural sound. Don’t use these tools if your original audio is mono (including mono which has been processed by stereo effects).