What Is RT60?
RT60 (Reverberation Time 60) is the time it takes for sound in a room to decay by 60 dB after the source stops. It is the most widely used metric for characterizing the acoustic behaviour of an enclosed space.
A room with a long RT60 sounds reverberant and "live"; a room with a short RT60 sounds dry and controlled. Neither extreme is inherently bad — the target depends on the room's purpose.
The Sabine Formula
Wallace Clement Sabine developed the first reverberation equation in the late 1800s:
- V — room volume in m³
- A — total absorption in sabins (m²), the sum of each surface area multiplied by its absorption coefficient
The Sabine formula assumes a diffuse sound field and works best in rooms with low average absorption (typically α < 0.2). It tends to overestimate RT60 in heavily treated rooms because it does not account for the fact that a sound ray loses a significant fraction of energy on every reflection.
The Eyring Formula
Carl Eyring refined the model for rooms with higher absorption:
- S — total surface area in m²
- αavg — area-weighted average absorption coefficient
When αavg is small, the Eyring formula gives results nearly identical to Sabine. As absorption increases, the Eyring result becomes shorter (more accurate). For studio control rooms where αavg can reach 0.3 – 0.5, the Eyring formula is the better choice.
Target RT60 Values
There is no single "correct" RT60 — it depends on the function of the space:
- Mixing / mastering control room — 0.2 – 0.4 s. Tight, controlled decay for accurate monitoring. The lower end suits smaller rooms.
- Recording live room — 0.4 – 0.8 s. Enough ambience to keep recordings sounding natural without excessive reverb.
- Vocal booth — 0.1 – 0.3 s. Very dry to capture clean, isolated vocal takes.
- Podcast / broadcast studio — 0.2 – 0.4 s. Speech intelligibility demands short decay.
- Home theater — 0.3 – 0.5 s. A balance between dialogue clarity and cinematic immersion.
- Concert hall — 1.5 – 2.5 s. Long reverberation enriches orchestral music.
Frequency-dependent RT60 is equally important. Most studios aim for a relatively flat decay curve, meaning that RT60 at 125 Hz should be no more than 1.3 times the midband value (the so-called "bass ratio").
How to Measure RT60
The standard approach uses an omnidirectional measurement microphone, an audio interface, and software such as REW (Room EQ Wizard):
- Step 1: Place the microphone at the primary listening position. The mic should be at ear height when seated.
- Step 2: Generate a measurement signal. REW uses a logarithmic sine sweep that covers the full audible range.
- Step 3: From the captured impulse response, the software calculates the Schroeder decay curve — a backwards integration of the squared impulse response.
- Step 4: RT60 is derived from the slope of the decay curve. Because measuring a full 60 dB drop often requires impractically high levels, most tools extrapolate from T20 (20 dB decay) or T30 (30 dB decay).
Take measurements at multiple positions (at least three) and average the results for a reliable picture of the room. Use the RT60 calculator to estimate your room's reverberation time based on surface materials, and the Absorbers tool to evaluate treatment options.
Common Mistakes
- Treating only mid/high frequencies — foam panels do almost nothing below 500 Hz. RT60 might look great at 2 kHz but remain excessive at 100 Hz.
- Over-treating — a room that is too dead feels uncomfortable and can cause ear fatigue. Bass absorption is essential, but keep some reflective surfaces (especially in a live room).
- Ignoring frequency dependence — a single broadband RT60 number hides problems. Always examine octave-band or third-octave-band data.