RT60 Calculator — Sabine & Eyring

RT60 is the time it takes for sound in a room to decay by 60 decibels after the source stops — a drop to one millionth of the original sound intensity. It is the single most-cited number in room acoustics because it captures, in one figure, how long the room keeps talking after you stop. It is frequency dependent: nearly every real room has a longer RT60 at 125 Hz than at 2 kHz, so a single quoted value is conventionally the mid-band average around 500 Hz to 1 kHz.

Below an average absorption coefficient of about 0.2 — which covers most furnished rooms — they agree closely and either is fine. Above roughly 0.3, use Eyring: Sabine systematically overestimates because it assumes sound loses only a fraction of its energy per reflection, which stops being true once surfaces are genuinely absorptive. The clearest way to see the difference is to set ᾱ to 0.99 in the calculator above. Eyring collapses towards zero, which is what an anechoic chamber actually does. Sabine still reports a substantial reverberation time for a room that has essentially none.

Because both formulas assume a diffuse field — sound energy uniformly distributed and arriving from all directions equally. Real rooms violate that in three common ways: absorption concentrated on one surface (a carpeted floor with bare walls), a room shape far from cubic, and non-uniform coupling to adjacent spaces. Rectangular rooms with absorption spread over several surfaces typically land within 10 to 20 percent. A room with all its absorption on one plane can be off by a factor of two, and no statistical formula will fix that — measure it.

The Schroeder frequency divides your room into two acoustic regimes, and it is derived from RT60 and volume, so this calculator already has everything it needs. Below it, room modes are sparse and separately audible as discrete booms and nulls — that is where you need modal treatment such as bass traps and placement changes. Above it, modes overlap densely enough to behave statistically, and that is the region where RT60 is a meaningful number at all and where broadband absorption works. Applying an RT60 target below the Schroeder frequency is applying a statistic to a region that is not statistical.

Critical distance is where the direct sound from a source and the reverberant field of the room are equally loud. Closer than that, you mostly hear the source; further away, you mostly hear the room. It is why near-field monitoring works and why a microphone placed across a live room sounds washed out. It depends on the directivity factor Q of the source as well as the room, which is why the calculator takes Q as an input — a horn-loaded source with Q of 5 pushes its critical distance more than twice as far as an omnidirectional one in the same room.

The inputs are metric — metres and cubic metres — because the 0.161 constant in both formulas is metric. To convert, multiply feet by 0.3048. A 16 by 13 by 10 foot room is 4.88 by 3.96 by 3.05 metres. If you prefer to work in imperial throughout, the equivalent Sabine constant is 0.049 with volume in cubic feet and absorption in square-foot sabins.