This is a short list of descriptions of some audio effects that I put together for my work with audio processing on Maple so that I could easily look and come up with something in code. I left out some, like distortion and tremolo, that are either very obvious to me or too general (or have too many forms) to classify with a short description.

Noise Gate
Require signal level to reach a threshold before passing it. It’s good to have a lower threshold to turn it off than used to turn it on to eliminate “chatter” or switching on and off during sound that’s around the threshold. Sometimes a hold-time before the signal can be switched off is desirable.

Octave-up
Apply full-wave rectification and mix with original signal or FFT and shift all the bins to the bin with twice the frequency.

Octave-down
Convert signal to square wave, count the period, stretch by a factor of 2, and mix. Add another square wave stretched by a factor of four to create two octaves down. For a more natural sound, use the stretched square waves to modulate the polarity of the original signal. Doing this with the one octave down square wave produces frequencies at one half and three halves ratios of the “fundamental frequency” of the input signal. Octave-down effects are commonly used for kick drums to produce a “heavier” sound.

Phaser
The electronic phasing effect is created by splitting an audio signal into two paths. One path treats the signal with an all-pass filter, which preserves the amplitude of the original signal and alters the phase. The amount of change in phase depends on the frequency. When signals from the two paths are mixed, the frequencies that are out of phase will cancel each other out, creating a phaser’s characteristic notches. Changing the mix ratio changes the depth of the notches; the deepest notches occur when the mix ratio is 50%. The phasors (not phasers) in the output of the discrete Fourier transform could also be rotated before the inverse transform is applied to achieve phasing.

Chorus
The chorus effect is produced by taking an audio signal and mixing it with one or more delayed, pitch-modulated copies of itself. The pitch of the added voices is typically modulated by an LFO, which makes the overall effect similar to that of a flanger, except with longer delays and without feedback.

Flanging
Flanging is an audio effect that occurs when two identical signals are mixed together, but with one signal time-delayed by a small and gradually changing amount, usually smaller than 20 milliseconds. This produces a swept comb filter effect: peaks and notches are produced in the resultant frequency spectrum, related to each other in a linear harmonic series. Varying the time delay causes these to sweep up and down the frequency spectrum. Part of the output signal is usually fed back to the input, producing a resonance effect which further enhances the intensity of the peaks and troughs. The phase of the fed-back signal is sometimes inverted, producing another variation on the flanging sound.

Reverberation
Feedback delay networks can be used to simulate or implement reverb. A convolution reverb can also be used, which can be understood by the fact the convolving the response of an environment to infinitely short impulse with a signal transforms the signal to how it would be in that environment. Gated reverb is an effect commonly applied to drums to make them sound more “punchy”. It is strong reverb that is often gated by the level of the original signal.

Ring Modulation
Ring modulation is achieved by multiplying two signals. Often one of the signals is something simple like a sine-wave or another simple waveform. The product is the sum and difference of and between the frequencies present in each waveform. Can be used to make speech sound robotic.

Vocoder
A vocoder is an analysis/synthesis system, mostly used for speech. In the encoder, the input is passed through a multi-band filter, each band is passed through an envelope follower, and the control signals from the envelope followers are communicated to the decoder. The decoder applies these (amplitude) control signals to corresponding filters in the (re)synthesizer.

Comb Filter
A comb filter adds a delayed version of a signal to itself, causing constructive and destructive interference. The frequency response of a comb filter consists of a series of regularly-spaced spikes, giving the appearance of a comb.