Matching Reverb Pre-Delay and LFOs to BPM: Calculating ms and Hz

What changes when reverb and LFOs follow the tempo

When a reverb tail spills into the next beat, the attack of the kick and snare blurs and the whole mix sounds muddy. Set the length so the tail dies away just before the next phrase, and the rhythm stays clear while the sense of space remains. Effects that cyclically move the volume or stereo position, like tremolo and auto-pan, also feel good as part of the rhythm when the speed of the movement lines up with the beat.

That said, tempo-matched values are only a starting point. Using the calculated value as a reference and adjusting by ear around it matters more than hitting the millisecond exactly. Knowing the reference gets you to the sound you want faster than turning knobs at random.

Pre-delay is “the gap between the dry sound and the reverb”

Pre-delay is the time between the dry sound and the start of the reverb. A longer pre-delay keeps the attack of the dry sound from being buried in the reverb, so vocal consonants and the crack of the snare come forward. A shorter one blends the dry sound and reverb straight away, making the instrument sound pushed further back.

• Short pre-delay (0–10 ms): sounds distant, like it’s playing close to a wall
• Medium pre-delay (10–40 ms): a natural room or hall sound
• Long pre-delay (40–100 ms or more): the dry sound stays clearly up front and the reverb blooms afterward. A standard for vocals

To match the tempo, set the pre-delay to a short note value such as a 64th or 128th note. At BPM 120 a 64th note is 31.25 ms and a 128th about 15.6 ms. Aligning it with a regular note length makes the onset of the reverb ride on the finest subdivisions of the rhythm.

Guide values for reverb pre-delay and decay by BPM

A common approach to tempo-matched reverb is to make “pre-delay + decay (the time for the tail to die away) = exactly one note length”. For each type of space, the pre-delay and total length are set like this:

Calculated this way, the pre-delay and decay for common BPMs come out as follows (pre-delay rounded to one decimal place, decay to a whole number).

The decay here is a rough guide to how long until the tail is essentially inaudible. Plug-ins define decay differently (for example, the time for the sound to drop by 60 dB), so the same number can sound a little different. After entering it, play the song and check that the kick and snare on the next beat aren’t muddied; if they are, shorten the decay. Enter a BPM into the BPM Tap Tempo & Delay Time Calculator and it shows pre-delay, decay and total for all four types of space at once.

The formula for matching LFO speed (Hz) to BPM

Tremolo, auto-pan, the movement of chorus and flanger, filter wah, and synth vibrato are all driven cyclically by an LFO (low-frequency oscillator). LFO speed is usually set in Hz (hertz), meaning “cycles per second”, so you need to convert note lengths to Hz.

The formula is “Hz = 1000 ÷ note length (ms)”. Since note length is “60000 ÷ BPM × multiplier”, this simplifies to “Hz = BPM ÷ 60 ÷ multiplier”. At BPM 120, a quarter note (500 ms) is 1000 ÷ 500 = 2 Hz, an eighth note (250 ms) 4 Hz, and a sixteenth note (125 ms) 8 Hz.

Guide values by effect when matching the tempo

If a DAW plug-in has a “Sync” or “Tempo Sync” button, turn it on and pick a note value, and it follows the tempo. On gear that can’t sync, such as external pedals or analog synths, set it to the calculated Hz. If the tempo is even slightly off, the position of the movement drifts as the song goes on, so measuring the song’s BPM accurately comes first.

Applying it to compressor release and sidechaining

Note-length calculations also help when setting a compressor’s release time. Release is the time it takes for the gain to recover after being reduced. If it’s too long, the level hasn’t fully recovered when the next kick arrives, and everything sounds permanently squashed. If it’s too short, the recovery stands out as “pumping”.

• Drum bus compression: a good starting point is a release of an eighth or sixteenth note, so it fully recovers before the next hit (250 ms or 125 ms at BPM 120)
• Sidechaining bass or synths to the kick: with four-on-the-floor, a release slightly shorter than a quarter note makes the level swell between kicks, giving that classic dance-music pulse
• Vocal compression: usually thought of in terms of word length, so there’s no need to fuss over the tempo

How a compressor behaves also depends on input level, attack and ratio, so the calculated value is just a starting point. Adjusting while watching that the gain-reduction meter returns close to 0 just before the next kick makes it easier to get the result you’re after. How to match delay times is covered in how to calculate delay time.

FAQ

How many ms of reverb pre-delay should I use?

Around 10–40 ms for a natural sound, or 40–100 ms if you want vocal words to stay clear. To match the tempo, one approach is a 64th note for a hall (31.25 ms at BPM 120) and a 128th note for a room (about 15.6 ms).

How do I calculate LFO Hz from BPM?

“Hz = 1000 ÷ note length (ms)”. At BPM 120 a quarter note is 500 ms, so 2 Hz; an eighth note is 4 Hz and a sixteenth 8 Hz. For one cycle per bar, it’s BPM ÷ 240 Hz.

What happens if I match reverb decay to the tempo?

The tail dies away just before the next beat or phrase, so the attack of the kick and snare stays clean and the space opens up while the rhythm stays clear. It’s common to make pre-delay plus decay add up to a note length such as 2 bars or 1 bar.

How do I match tremolo speed to a song?

Measure the song’s BPM, convert an eighth or sixteenth note to Hz, and set that as the rate. At BPM 120 that’s 4 Hz for eighths, 8 Hz for sixteenths and 6 Hz for eighth triplets. On gear with sync, just pick the note value.

Should compressor release be matched to the tempo?

For drums and sidechaining, aiming for a length slightly shorter than an eighth or quarter note, so the level fully recovers before the next hit, keeps things stable. But input level and attack also affect how it behaves, so make the final adjustment with the meter and your ears.