Tremolo
From Audacity Manual
Tremolo modulates the volume of the selection at the depth and speed selected in the dialog. This is the same as the tremolo effect familiar to guitar and keyboard players.
Waveform type
Determines the "shape" of each tremolo (how it progresses between the lowest and highest volume as determined by the wetness level). Choice of sine, triangle, sawtooth, inverse sawtooth, square.
Starting phase (degrees)
Sets where to start the tremolo in the waveform cycle. The default (zero) starts the tremolo at the start of the cycle (as the waveform starts to rise from the lowest point).
Wetness level (percent)
Sets the depth of tremolo. 0% is no tremolo, 100% sweeps between zero and maximum volume based on the original amplitude level.
Frequency (Hz)
Controls the speed of the oscillation; use higher frequencies for faster oscillation.