RT60 Reverberation Time (60 dB decay)

How RT60 is measured

The standard method derives RT60 from the Schroeder reverse-time integral applied to a captured room impulse response. The impulse can be a balloon pop, starter pistol, or — more commonly today — an exponential sine sweep deconvolved against the original signal to recover the impulse. Measuring a true 60 dB decay requires an unrealistically loud source above the room's noise floor, so most modern tools measure T20 (the time for the level to drop from −5 dB to −25 dB, then multiplied by 3) or T30 (−5 dB to −35 dB, multiplied by 2) and extrapolate. Audyssey MultEQ XT32, Dirac Live, YPAO RSC, and Anthem ARC Genesis all derive RT60 estimates this way during automated room calibration.

Why a single number understates the problem

RT60 is frequency-dependent. A treated home theater can measure a clean 0.4 s at 500 Hz and a problematic 0.9 s at 100 Hz — and the second number is what makes bass sound muddy and dialogue intelligibility drop in scenes with a bass-heavy score. When evaluating a room, look at the RT60 curve across the spectrum, not the single 500 Hz value most marketing copy and product reviews quote. The frequency dependence comes from absorption efficiency: porous absorbers (wool, foam, fabric panels) work above ~250 Hz; below that, absorption requires depth proportional to wavelength, which is why bass-band reduction needs traps with at least 4 inches of depth, ideally placed at corners where particle velocity is highest.

Targets for home theater

The reference recommendation in ITU-R BS.1116-3 for critical listening rooms is RT60 between 0.2 and 0.4 s averaged across 200 Hz to 4 kHz, with deviation across that band of less than 0.05 s. Most home theaters live above this — 0.4 to 0.6 s is typical for a well-treated dedicated room; 0.7 to 1.0 s is typical for a converted living room with carpet and soft furnishings; bare-walled rooms commonly hit 1.2 to 1.8 s. The Acoustical Society of America's room-acoustics guide notes that intelligibility drops sharply above 0.8 s in the 250 Hz to 2 kHz band, the speech-dominant range.

Worked example

A 4,000 ft³ basement theater with drywall, drop ceiling, and tile floor measures roughly 1.0 s at 500 Hz and 1.4 s at 125 Hz before treatment. Adding 8 fabric-wrapped 2-inch panels (~32 ft² total) brings 500 Hz to ~0.5 s but barely moves 125 Hz. Adding 4 corner-placed 4-inch superchunk bass traps brings 125 Hz to ~0.8 s. A second pass of bass traps (8 total) is typically required to bring the low band to 0.5 s. Total absorption needed scales with the Sabine equation, A = 0.161 V / RT60 (metric), where V is room volume and A is total absorption in metric Sabines.

Common confusions

RT60 is not the same as early decay time (EDT), which integrates only the first 10 dB of decay and correlates better with perceived reverberance. EDT is what you actually hear; RT60 is what most measurement tools report. They diverge when a room has strong early reflections that decay quickly: the EDT will be short while RT60 still measures long because of slowly-decaying late energy. Cinema-grade rooms target both. RT60 also is not "reverb" as the term is used in audio plugins; plugin reverbs simulate impulse responses and may report a "decay time" parameter that approximates RT60, but the simulation is the inverse of measurement.

What automated room correction does and doesn't

Audyssey, Dirac, YPAO, and ARC Genesis can identify RT60 problems and apply EQ to compensate for the resulting frequency-response colorations, but EQ cannot remove reverberation from the room. Only treatment can. The audible effect of correction is typically smoother frequency response and tighter bass; the reverberation tail remains. This is why measurement-driven room correction is necessary but not sufficient for serious home theater.