lesson
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How did classic records turn a single, dry electric guitar into the massive swirl of Nirvana's Come As You Are or the jet-engine sweep of Van Halen's Unchained?
They used modulation effects β audio processors that constantly alter a sound's volume, timing, or phase using an internal control signal called an LFO (Low-Frequency Oscillator).
Tremolo: Amplitude Modulation
Tremolo is the simplest modulation effect: it periodically raises and lowers the overall amplitude (volume) of a signal without altering its pitch or timing.
An LFO (a sub-audible wave cycling typically between 0.5Β Hz and 15Β Hz) turns the gain up and down automatically to create a rhythmic, pulsing sensation.
πInteractive diagram
Tremolo only modulates volume. But what happens if we modulate the timing of an audio signal instead?
Flanging: The Comb Filter Jet Sweep
In 1966 at Abbey Road Studios, recording engineer Ken Townsend invented automatic double tracking by manually pressing on the tape reel flange to slow down one tape machine relative to another.
A flanger splits the input into two paths: a dry signal and a duplicate delayed by a tiny, LFO-modulated time delay of 0.1Β toΒ 10Β ms.
When you recombine the dry and delayed copies, frequencies whose delay creates a half-cycle (180β) phase offset cancel out completely, producing a series of evenly spaced cancellation notches called a comb filter.
πInteractive diagram
Adding feedback (regeneration) feeds the delayed output back into the input, sharpening the peaks between the notches to generate that signature metallic, jet-engine whoosh.