What is reverb pre-delay?

Pre-delay is the silent gap between the direct sound and the start of the reverb tail. In a real room it is the time for the first reflection to travel from the source to a wall and back to the listener. Adding pre-delay keeps the dry signal clear and pushes the reverb behind it.

How is pre-delay calculated from room size?

Sound travels at about 343 metres per second in air at 20 Celsius. The pre-delay is the extra distance a reflected ray travels compared with the direct sound, divided by the speed of sound, times 1000 for milliseconds. A reflection path 5 metres longer than the direct path gives about 14.6 ms of pre-delay.

Why does pre-delay make vocals clearer?

Without pre-delay the reverb starts the instant the voice does and smears the words. A 20 to 40 ms pre-delay separates the dry vocal from the wash of reflections, so the listener hears the words first and the space afterwards. It is one of the most effective tricks for a present, intelligible lead vocal.

Does temperature change the result?

Yes, slightly. The speed of sound rises about 0.6 metres per second for each degree Celsius, so a warm room shortens pre-delay a touch. The tool lets you set temperature, but for mixing the difference between 15 and 25 Celsius is only a fraction of a millisecond and rarely matters.

How does pre-delay relate to reverb decay time?

Pre-delay sets where the reverb starts; decay (RT60) sets how long it lasts. They are independent. A large hall reverb often pairs a longer decay with a moderate pre-delay, while a tight room uses short values for both. Matching pre-delay to a plausible room size keeps the two consistent.

Reverb Room Size to Pre-Delay Calculator

Pre-delay is the small gap before a reverb tail begins, and in the real world it is set by physics: the time for the first reflection to bounce off a wall and reach your ears. This tool uses the actual speed of sound to turn a room size and listener position into a natural pre-delay value for your reverb plugin.

How it works

Sound travels at roughly 343 m/s in air at 20 Celsius. When a source plays, the direct sound reaches the listener first; the earliest reflection has to travel out to a surface and back, covering a longer path. The pre-delay is the difference in arrival times:

pre-delay (ms) = (reflected path - direct path) / speed of sound x 1000

The tool models a simple rectangular room. The direct path is the source-to-listener distance, and the first reflection path is approximated from the room geometry and the distance to the nearest wall.

Temperature

The speed of sound depends on air temperature:

speed (m/s) = 331.3 + 0.606 x temperature_C

You can set the temperature, though for mixing the effect is tiny — a few tenths of a millisecond across normal room conditions.

Using the result

Type the calculated pre-delay straight into your reverb’s pre-delay control. As a guide:

Small rooms / booths: 5 to 15 ms — tight, intimate.
Studios / live rooms: 15 to 30 ms — clear vocals, present space.
Halls / churches: 30 to 80 ms — big, cinematic, with obvious separation.

Worked example

In a 10 m room with the listener 3 m from the source and the nearest wall 4 m away, the first reflection travels roughly 8 m versus a 3 m direct path — about 5 m extra. At 343 m/s that is 5 / 343 x 1000 = 14.6 ms of pre-delay, a natural value for a medium studio space.

All maths runs locally in your browser.