This page provides an overview of the most common audio effects used in sound production, with links to more detailed tutorials.
Equalization means boosting or reducing (attenuating) the levels of various frequencies in a signal. At it's most basic, equalization can mean turning the bass/treble controls up or down. Advanced equalizers have fine controls for specific frequencies.
Common uses for equalization include correct signals which sound unnatural and reducing feedback.
More info: Audio Equalization
Compression & Limiting
Compression means reducing the dynamic range of a signal. All signal values above a certain adjustable threshold are reduced in gain relative to lower-level signals. This creates a more even signal level, reducing the level of the loudest parts.
More info: Audio Compression
Limiting is an extreme form of compression. Rather than smoothly reducing the gain of successively higher levels, all signal above the threshold is limited to the same gain. This creates a very hard cut-off point, over which there is no increase in level.
More info: Audio Limiting
Expansion & Noise Gating
Expansion means increasing the dynamic range of a signal. High level signals maintain the same (or nearly the same) levels, low level signals are reduced (attenuated). This creates a greater range between quiet and loud. Expansion is the opposite of compression.
Noise gating is an extreme form of expansion — signals below a certain point are either heavily attenuated or eliminated completely. This leaves only higher level signals and removes background noise when the signal is not present.
More info: Audio Expansion
Delay / Echo
Delay is a simple concept — the original audio signal is followed closely by a delayed repeat, just like an echo. The delay time can be as short as a few milliseconds or as long as several seconds. A delay effect can include a single echo or multiple echoes, usually reducing quickly in relative level.
Delay also forms the basis of other effects such as reverb, chorus, phasing and flanging.
Reverb is short for reverberation, the effect of many sound reflections occurring in a very short space of time. The familiar sound of clapping in an empty hall is a good example of reverb.
Reverb effects are used to restore the natural ambience to a sound, or to give it more fullness and body.
More info: Audio Reverb
The chorus effect is designed to make a signal sound like it was produced by multiple similar sources. For example, if you add the chorus effect to a solo singer's voice, the results sounds like.... a chorus.
Chorus works by adding multiple short delays to the signal, but rather than repeating the same delay, each delay is "variable length" (the speed and length of the delay changes). This adds the randomness required for the chorus sound. Varying the delay time also varies the pitch slightly, further adding to the "multiple sources" illusion.
More info: Chorus Effect
Phasing & Flanging
Phasing, AKA phase shifting, is a sweeping, whooshing effect often used in music. The effect is created by mixing the original signal with another version of itself which has been phase-shifted. This results in various out-of-phase interactions over time which gives the sweeping effect.
Phasing is created by adding evenly-spaced notches in the frequency response and moving them up and down the frequency spectrum.
Flanging is a specific type of phasing which uses notches that are "harmonically related", i.e. related to musical notes.