Methodology

The tempo method

Tempo-synced reverb and delay rest on one fact: a minute contains 60,000 milliseconds. If a song plays at BPM beats per minute, and each beat is one quarter note, each quarter note lasts 60,000 divided by the tempo. That single division is the engine of the whole site.

Every other note value is a fixed fraction or multiple of the quarter note — a half note is 2 quarters, a whole note is 4, an eighth is half a quarter, down to 1/512, which is 1/128 of a quarter. The ladder the site uses, shown at the worked-example tempo of 128 BPM:

Two standard variations complete the grid: a dotted note adds half its length again (× 1.5), and a triplet note fits three into the time of two (× 2/3). The site also shows each value as a frequency, because LFOs and some modulation sections are set in hertz:

Worked example: 128 BPM

Display rounding. The math runs at full precision internally; only the display is rounded, to two decimal places with trailing zeros trimmed — hence 984.38 ms, 62.5 ms, 15.63 ms and 3.91 ms. Rounding never feeds back into later steps: decays are computed from unrounded values. The accepted tempo range is 40 to 208 BPM; the same grid is printed in full on the BPM to milliseconds chart.

How the reverb presets were derived

The presets are built backwards from a musical decision: the total reverb time — pre-delay plus decay — should land on a note value, so the tail finishes on the grid instead of smearing across the next beat. Each preset starts from a total in quarter notes, takes a small note value as its pre-delay, and the decay is whatever remains. That relationship is an invariant on every page of this site:

The pattern is deliberate. Bigger spaces get a longer total and a longer pre-delay: a hall's reflections genuinely arrive later, and the longer gap keeps the dry signal clear before the tail blooms. Smaller spaces pair a short total with a pre-delay felt as thickness rather than heard as a gap. The type pages — plate, hall, room, spring and chamber — reuse the framework with their own totals, from a 16-quarter Cathedral down to a 0.25-quarter Vocal Booth; the vocal presets assign longer pre-delays because keeping a lyric intelligible matters more than simulating a space. The pre-delay calculator isolates that one variable.

An honest statement about what these presets are. They are musical starting points derived from the tempo framework above — not acoustic measurements of real halls, plates or rooms; no physical space was measured to produce them. Real spaces do not quantize their decay to a song's tempo, and that is precisely the point: tempo sync is a mixing technique, and these are the values an experienced engineer would start from before adjusting by ear. Plugins differ in how their decay control maps to a true tail length, so expect to trim by ear even with exact numbers.

The snare rule derivation

The snare calculator applies one rule: a snare's reverb tail should die away just before the next snare hit, so the groove stays punchy and the tail never piles up under the following hit. The total reverb time is therefore set equal to the interval between snare hits, and that interval is fixed by the drum pattern, measured in quarter notes:

The pre-delay defaults to a 1/64 note — short enough to keep the hit itself dry and distinct — and can be set from a 1/32 to a 1/512 note. Decay follows the same invariant: total minus pre-delay. At 120 BPM with a standard backbeat, that gives a 1000 ms total, a 31.25 ms pre-delay and a 968.75 ms decay. The rest of the kit is derived as fractions of the snare interval: kick 0.25× so its low end stays tight, toms 0.5×, overheads and room mics 1.5× for space — capped at 2 bars (8 quarters) so a slow half-time pattern cannot produce an absurd tail. Double-time is the honest edge case: its interval equals the every-beat pattern, as the table shows; only the density of hits changes, not the spacing the tail must fit into.

The RT60 method

The RT60 calculator does a different job from everything above: it estimates how long sound takes to decay by 60 dB in a physical room. That is acoustics, not tempo, so it uses the two classical formulas — Sabine first, the industry default for a first estimate:

Here V is the room volume and A is the total absorption in sabins, built surface by surface: each surface contributes its area multiplied by its material's absorption coefficient, and people add 0.45 m² sabins each, directly to A. The Eyring formula refines Sabine by accounting for how absorption compounds over repeated reflections, using the average coefficient ᾱ = A ÷ S, where S is total surface area:

The coefficients are standard published values at 500 Hz, from the Engineering ToolBox — Sound Absorption Coefficients table, with the concepts referenced to Wikipedia — Reverberation: smooth concrete 0.03, painted brick 0.05, gypsum drywall 0.10, wood floor 0.10, heavy carpet on pad 0.30, heavy curtains 0.50, glass 0.05, acoustic ceiling tile 0.70. The single-frequency choice is a stated simplification: real rooms ring longer at low frequencies and die faster at high ones, and a six-band model would need data most users do not have. 500 Hz sits mid-range for speech and music, so it is the honest single number. Read results against the usual bands: under 0.3 s very dry, 0.3–0.6 s a controlled studio, 0.6–1.0 s a live room, 1.0–1.8 s hall-like, beyond 1.8 s speech muddies.

Verification

Every value on this site is computed from the formulas above, in your browser, at the moment you ask. There is no server lookup and no stored table that could drift out of date on one page while staying correct on another: the combined calculator on the home page, the individual tools and the printed chart all run the same division, the same multipliers and the same decay invariant. Change the tempo anywhere and the figures are re-derived from scratch, not scaled from a cached row.

Outputs are checked against worked examples computed by hand from this page — the 128 BPM figures, the 120 BPM preset table and the golden room are the reference cases, chosen because their arithmetic can be verified with a pencil. If you find a value that does not match these formulas, that is a bug by definition: report it through the Report an Error page with the tempo or room you entered and the value you expected.

Sources

The tempo method itself is arithmetic — 60,000 ÷ BPM needs no citation — but the practices and acoustic material around it rest on four published sources, each used for what it actually supports:

• Wikipedia — Reverberation — what reverberation time means, and the Sabine and Eyring formulas and their assumptions.
• Engineering ToolBox — Sound Absorption Coefficients — the 500 Hz absorption coefficients used by the RT60 calculator.
• iZotope — Learn — mixing guides supporting the preset practice: pre-delay for clarity, decay chosen against the track.
• Sound on Sound — mixing technique supporting tempo sync itself: tying echo and tail times to the song's grid.

Where a source and this framework disagree — real rooms, again, do not decay in quarter notes — the site says which side it is on and why.

Frequently asked questions