RT60 Reverberation Guide | TheaterOwl
Calculate and tune RT60 reverberation time using the Sabine formula and acoustic panel placement.
RT60 measures how long it takes a sound to decay by 60 decibels after it stops. In home theaters, an RT60 in the 0.3 to 0.5 second range keeps dialogue intelligible and prevents bass blur. Hardwood floors and bare walls push RT60 above 1 second, smearing both dialogue and music. Carpet, drapes, and acoustic panels bring it back into the cinematic range. This guide covers the Sabine formula, target values for different room types, and how to choose treatment that fits the math.
Sabine Formula Basics
RT60 = 0.161 * V / A, where V is room volume in cubic meters and A is total absorption in sabins. A 4 x 5 x 2.7 meter room has 54 m^3 volume. With bare walls (about 5 sabins total) the RT60 hits 1.7 seconds — far too live for a home theater. Adding 20 sabins of treatment drops it to 0.43 seconds, ideal. The formula assumes diffuse sound fields and works best for mid frequencies (250 Hz to 4 kHz); bass and very high frequencies need separate analysis because Sabine breaks down outside that band.
Reading and Using Absorption Coefficients
Every material has an absorption coefficient at each frequency band, ranging from 0 (fully reflective) to 1 (fully absorbent). A 2-inch 24 x 48 inch Rockwool panel has alpha of about 0.95 at 1 kHz, contributing 8 square feet * 0.95 = 7.6 sabins. Drywall has alpha of 0.04 at the same frequency. Manufacturers publish coefficient tables, but real-world performance varies with mounting (panels spaced off the wall absorb more bass), so plan with 10 to 20 percent margin for error.
Where to Place Panels First
Treat first reflection points before broadband absorption. From your seating position, mirror the side walls — if you see a speaker driver in the mirror, that wall location is a first reflection point. Treating those points first removes the most coloration per panel installed. After first reflections, treat the ceiling above the front speakers (an 'acoustic cloud' panel), then rear-wall slap echo. Bass traps in corners come fifth in priority but deliver the largest improvement in bass clarity.
Avoiding Over-Damping
Going below an RT60 of 0.25 seconds makes the room sound dead and fatiguing — a common mistake in DIY treatment is over-paneling the walls. Keep a balance of absorption and diffusion. Diffusion preserves a sense of space without muddying dialogue, and rear-wall diffusion behind seating works particularly well in rooms with at least 8 feet of distance behind the seats. Quadratic Residue Diffusers (QRD) and skyline diffusers scatter sound across a wide angle without absorbing energy, so they reduce flutter without killing room ambience.
Frequency-Specific Targets
A single RT60 number is misleading because real rooms have different decay times at different frequencies. Aim for a flat RT60 curve from 250 Hz to 4 kHz at the home theater target of 0.3 to 0.5 seconds. Below 250 Hz, modal behavior dominates and RT60 typically rises to 0.6 to 1.0 second — that is normal but can be improved with bass traps. Above 4 kHz, high-frequency absorption from carpets and soft furnishings naturally keeps decay short; you rarely need dedicated treatment in that band.
Measuring RT60 With REW
Room EQ Wizard (REW) with a UMIK-1 USB measurement microphone is the standard tool. Run an MLS or sine sweep from your AVR, position the mic at the listening position, and REW will produce an RT60 vs frequency graph with 1/3 octave resolution. Compare your measured curve to the target 0.3 to 0.5 second band. The waterfall plot reveals individual modal ringing — long decays at specific frequencies indicate where bass traps and modal corrections are needed. A measurement session takes about 30 minutes and replaces weeks of trial-and-error panel placement.
FAQ
What target RT60 should I aim for?
Home theater: 0.3 to 0.5 seconds across 250 Hz to 4 kHz. Music listening room: 0.4 to 0.6 seconds. Recording studio control room: 0.2 to 0.4 seconds. Living room with mixed use: 0.5 to 0.7 seconds is acceptable. Below 0.25 seconds the room sounds unnaturally dead; above 0.8 seconds dialogue becomes hard to follow.
Can I measure RT60 with a phone app?
Yes for relative comparisons. Apps like Decibel X or AudioTool give a usable single-number RT60 if you have a transient sound source like a balloon pop. For precision work, use REW with a calibrated USB measurement mic — phone microphones are noise-floor limited and inaccurate at low frequencies.
Do bass traps count toward RT60?
Yes, but they primarily affect the low-frequency RT60 below 250 Hz. Treat low frequencies and midrange independently to avoid uneven decay times. A typical 4 to 6 inch thick corner bass trap delivers strong absorption from 80 to 500 Hz; thicker traps extend lower.
How thick should acoustic panels be?
2-inch panels absorb effectively above 500 Hz. 4-inch panels extend down to 250 Hz. For broadband absorption including the lower midrange, mount 2-inch panels with a 2 to 4 inch air gap behind them — the gap acts as an absorber for the lower band that the panel alone cannot catch.
Will adding furniture reduce RT60?
Yes, especially upholstered furniture, area rugs, and floor-to-ceiling drapes. Bookshelves act as diffusers. Hard furniture (glass tables, wood) has minimal effect. A heavily furnished living room typically has RT60 of 0.5 to 0.8 seconds versus 1.5 to 2.0 seconds for an empty room of the same size.
Why does my room sound boomy even with treatment?
Standing waves at room modes (typically 30 to 80 Hz for typical rooms) are not solved by mid-frequency panels. Add corner bass traps, consider double bass arrays (front and rear subs canceling modes), or use parametric EQ to notch the offending frequencies in the AVR's DSP.