Reverb Pre-Delay and Decay: Timing Reverb to Tempo

What reverb pre-delay does

Pre-delay is the time between the dry sound and the start of the reverb, set in milliseconds. It separates the direct sound from its reflections, so a vocal or snare keeps its attack clear before the room fills in behind it.

Short pre-delays, under about 20 ms, blend the source into the space. Longer pre-delays, from about 30 to 120 ms, push the reverb behind the source and keep words intelligible. In a real room the gap depends on distance to the walls; in a plug-in it is a free parameter, which is why it can be set from the tempo.

What decay time measures

Decay time is how long the reverb tail takes to fade, and most plug-ins define it in the style of RT60: the time for the level to fall by 60 dB. A 2-second decay means the tail is 60 dB quieter 2 seconds after the sound stops.

RT60 is the acoustics measure of reverberation time for a real room. Plug-ins use the same idea, though the exact definition varies by manufacturer, and the audible tail in a dense mix ends well before the full 60 dB drop. Treat a tempo-synced decay as a starting point and trim by ear.

The tempo-synced method

Pick the pre-delay as a short note value, pick the note or bar where the tail should end, and subtract. The total of pre-delay plus decay then equals a musical length, so the reverb clears on the grid.

pre-delay = 1/64, 1/32 or 1/16 note · decay = target note length − pre-delay

The note lengths come from the quarter note, 60,000 ÷ BPM: a 1/16 note is a quarter of it, a 1/32 note an eighth of it and a 1/64 note a sixteenth of it. At 120 BPM the quarter note is 500 ms, the 1/32-note pre-delay is 62.5 ms, and a tail that ends on the next beat needs 500 − 62.5 = 437.5 ms of decay. The reverb timing calculator runs this subtraction for any tempo.

Pre-delay and decay at 90, 120 and 140 BPM

Faster tempos need shorter pre-delays and shorter decays to stay on the grid. The tables use the same formulas as the calculator, rounded to two decimals.

Choose the target by the part: a quarter-note tail keeps a busy snare tight, a half-note or one-bar tail suits vocals, and two bars suit pads and slow ballads.

When not to sync reverb to tempo

Skip tempo sync when the reverb is meant to place a sound in a realistic room rather than to add a rhythmic tail. Real rooms have fixed decay times that do not change with the song.

• Acoustic ensembles recorded in one space sound natural with one consistent room reverb at any tempo.
• Free-time music with rubato or no click has no grid to land on.
• Very short ambiences, under about 300 ms, are heard as tone and space, not as rhythm.
• Long ambient tails, several bars long, blur across many beats; exact sync makes no audible difference.

Using timed reverb with a timed delay

Set the delay and the reverb from the same BPM so echoes and tail clear together. A common setup is a dotted-eighth delay feeding a reverb with a 1/32-note pre-delay and a half-note tail.

At 120 BPM that means a 375 ms delay, a 62.5 ms pre-delay and a 937.5 ms decay. The echo time calculator lists every echo setting, and how to set delay time to tempo explains the dotted and triplet ratios. To measure the tempo first, use the tempo detection tool.

Questions