Tremolo is the pulsing effect that powered the signature guitar sound of Bo Diddley in the '50s, the rhythm guitar in Pink Floyd's "Money," and more recently, the intro to Green Day's "Boulevard of Broken Dreams" and the rhythm behind Audioslave's "Gasoline." Sure, you can get tremolo by turning your volume control up and down, with perfect rhythm, 4-7 times a second — but it's a lot easier to do it electronically.
The upper waveform shows a power chord, while the lower waveform shows the same chord processed through tremolo — you can see how the level pulsates.
Confused about the difference between vibrato and tremolo? Here's the scoop: tremolo is a rhythmic level change, while vibrato is a rhythmic pitch change. Incidentally, the first tremolo (by DeArmond) was mechanical, and sloshed liquid around between two wires to create the pulsing effect — we've come a long way since then!
Rate adjusts the pulse's rhythm. Although some tremolo rates go into the audio range, classic tremolos seldom go above 10Hz or so.
Depth determines whether the pulsing is really deep (like the intro to "Boulevard of Broken Dreams") or more subtle. With old tremolos based on optical technology, turning up the rate automatically decreased the depth. Modern tremolos don't have those kinds of limitations, and can have heavy depths at fast rates.
Waveform describes the pulse's "shape." The most common options are triangle and sine waves, which produce the pulsing effects shown in the picture. Square waves are less common, but can chop the signal dramatically between extreme high and low levels.
Three chords with tremolo: triangle wave (left), sine wave (center), and on the right, a signal being hacked into pieces by a square wave.
Tap tempo lets you sync the rate to a song's rhythm by tapping along with the tempo; your taps set the rate.
Tap tempo is your friend. Synchronizing tremolo to the rhythm can give a cohesive sound that's tighter than James Brown's horn section...
...except for when tap tempo isn't your friend. Tap tempo wasn't around for the classic tremolo sounds of the '50s and '60s, yet sometimes the lack of sync produced a chaotic vibe that fit perfectly with the revolutionary anarchy of early rock 'n' roll. Some of Bo Diddley's recordings capture that spirit perfectly.
Most modern tremolo users prefer a jagged, pulsing kind of distortion. But back in the '50s, clean tremolo effects were used on slow songs for a more romantic feel. Try tremolo on just one pickup (neck or bridge) with no distortion.