BPM → delay/reverb (ms)

What it's for

To set delay and reverb «in time» with the song: at a given BPM, each note value (1/4, 1/8, triplets…) maps to a precise time in milliseconds.

How to read it

1/4 is the quarter note (the beat), 1/8 the eighth, «.» means dotted (duration × 1.5), «T» a triplet. The Hz are for synced filters or LFOs.

Where the sum comes from

It all starts from one division: sixty thousand milliseconds in a minute, divided by the beats per minute, gives the length of a quarter note. At 120 BPM that is 500 milliseconds, at 90 it is 667. From there every other figure is a half or a double: the eighth is 250 at 120 BPM, the sixteenth 125, the half note 1,000.

The two variants that appear in every plugin are the dot and the triplet. The dot lengthens the figure by half, that is multiplies it by 1.5; the triplet shortens it to two thirds. They sound very different: a dotted-eighth delay gives the wide pulse typical of certain effected guitars, the triplet gives a nervous run.

Why a delay in time is heard less

An echo that lands exactly on a subdivision merges with the rhythm and disappears as an effect: it is perceived as depth, not as repetition. An echo out of time, instead, sounds like a mistake and clogs everything. That is why syncing is nearly always the starting point, even when you later decide to break away on purpose.

The deliberate exception is the slightly detuned delay, out by a few milliseconds: it shifts the echo just enough not to be perfectly aligned, and gives the movement analogue delays had as a defect. Many plugins have a control for it, usually called «offset» or «humanize».

Reverb: the tail is what counts

On reverb the number to sync is not the pre-delay but the tail, that is how long it takes to die away. A tail longer than the bar overlaps the next chord and muddies the harmony; a tail set to a rhythmic figure ends in the right silence before the next hit, and the piece breathes.

Pre-delay, on the other hand, is the silence between the direct sound and the start of the reverb, and it is what keeps a voice separate from the room: a few tens of milliseconds are enough to keep the word intelligible while still leaving space around it. It too can be set in time, usually to a sixteenth or a thirty-second.

Hertz, and what they are for

The hertz column is the inverse of time: one cycle every 500 milliseconds makes 2 Hz. It is used for LFOs, the slow oscillations that drive a tremolo, a filter or an auto-pan, and which synthesisers set in hertz rather than in rhythmic figures.

A less obvious use is the release time of a compressor: bringing it close to the length of an eighth or a sixteenth makes the compressor breathe with the track instead of against it. It is one of the simplest ways to make a mix feel tighter without touching a single fader.

Nearby tools

The tempo of a track is found with BPM tap counter and held with Metronome. For note frequencies there is Note to frequency, and for the audio side of video Video loudness.