The Sabine formula is an equation that predicts how long sound reverberates in a room. It’s named after Wallace Clement Sabine, an American physicist who developed it in the late 1890s while studying the acoustics of concert halls. The formula became the foundation of room acoustics and is still used today in studio design, architectural acoustics, and audio engineering.
The formula is:
RT60 = 0.161 × V / (S × α)
Where:
- RT60 = reverberation time in seconds (time for sound to decay 60 dB)
- V = room volume in cubic meters (length × width × height)
- S = total surface area in square meters (all walls, floor, ceiling)
- α = average absorption coefficient (0–1 scale)
- 0.161 = a constant for metric units
RT60 is the time it takes for a sound to decay to one-thousandth of its original level (a 60 dB drop). In other words, if you clap loudly, RT60 is how long until the echo fades to barely audible.
Components of the Formula
Understanding each part helps you predict and design reverb in rooms.
Room volume (V) is straightforward: multiply the room’s length, width, and height. A 5 m × 4 m × 3 m room has a volume of 60 cubic meters. Larger rooms have longer reverb times because sound has more distance to travel before hitting a wall.
Surface area (S) is the total area of all surfaces the sound bounces off. For the same 5 × 4 × 3 room:
- Floor: 5 × 4 = 20 m²
- Ceiling: 5 × 4 = 20 m²
- Two long walls: 5 × 3 × 2 = 30 m²
- Two short walls: 4 × 3 × 2 = 24 m²
- Total S = 94 m²
Absorption coefficient (α) is the trickiest part. It’s a number from 0 to 1 that describes how much sound a material absorbs. Reflective materials like concrete or drywall have low α (around 0.05–0.1). Absorptive materials like foam, fiberglass, and heavy curtains have high α (0.4–0.9). A room’s average α is the weighted average of all surface materials.
How to Measure Room Volume and Surface Area
Measure the room’s dimensions with a tape measure or laser. Length × width × height gives volume in cubic meters (or feet cubed—convert later if needed).
Calculate surface area by measuring each wall, the floor, and the ceiling. For irregular rooms (sloped ceilings, alcoves), break it into simpler shapes and add them together.
For studio design, you don’t need perfect precision—estimates within 10% are fine. The Sabine formula gives approximate results; real-world measurements will vary.
Finding Absorption Coefficients
Absorption coefficients are published values for materials. A standard reference is the ISO 11654 standard, which lists absorption coefficients for common materials at different frequencies.
Common values at mid-frequency (500 Hz):
- Hard concrete or tile: α ≈ 0.01–0.03 (very reflective)
- Drywall or plaster: α ≈ 0.05–0.1
- Wood paneling: α ≈ 0.1–0.2
- Carpet on hard floor: α ≈ 0.3–0.5
- Acoustic foam (2 inches): α ≈ 0.5–0.7
- Fiberglass (4 inches): α ≈ 0.7–0.9
- Curtains or heavy fabric: α ≈ 0.4–0.8
To find the average α for your room, list each surface, its area, its material, and its α value. Multiply each area by its α, sum the products, and divide by total surface area.
Example: A 60 m³ room with 94 m² of surface:
- 40 m² drywall walls (α = 0.08): 40 × 0.08 = 3.2
- 20 m² floor with carpet (α = 0.4): 20 × 0.4 = 8
- 20 m² ceiling with acoustic tile (α = 0.6): 20 × 0.6 = 12
- 14 m² window/door (α = 0.05): 14 × 0.05 = 0.7
- Sum = 24.9 / 94 = average α ≈ 0.26
Calculating RT60 Step-by-Step
Using the example above: RT60 = 0.161 × 60 / (94 × 0.26) = 0.161 × 60 / 24.44 = 9.66 / 24.44 ≈ 0.40 seconds.
This room has an RT60 of about 0.4 seconds—quite dry, suitable for a podcast studio or intimate recording space. A control room target is typically 1.2–1.6 seconds. A concert hall might be 1.8–2.5 seconds depending on the venue’s purpose.
Use the Sabine formula to predict how adding absorption (more foam, carpet, curtains) changes RT60. If you add 30 m² of acoustic foam (α = 0.7) by replacing some drywall:
- New α calculation: (3.2 + 8 + 12 + 0.7 – (30 × 0.08) + (30 × 0.7)) / 94 = (23.1 – 2.4 + 21) / 94 = 41.7 / 94 ≈ 0.44
- New RT60: 0.161 × 60 / (94 × 0.44) ≈ 0.21 seconds.
The foam dramatically shortened reverb time. Use our reverb time calculator to experiment with different scenarios.
Using the Result in Studio Design
If you calculate that your room’s natural RT60 is 2 seconds but you want it to be 1.2 seconds (for a mixing environment), you need to add absorption. The formula helps you predict how much.
For room treatment planning, calculate your target RT60, then work backward: determine what average α you need, then select materials and amounts to hit that target. The Sabine formula is your planning tool.
For live spaces (concert halls, churches), you might accept a longer RT60 (1.5–2+ seconds) because the space is meant to be reverberant. For dead spaces (studios, podcasting), you might target 0.8–1.2 seconds for tight, controlled recording.
Read more about room acoustics and treatment in this guide.
Limitations of the Sabine Formula
The Sabine formula is an estimate, not gospel. It works well for mid-frequency sound (500–2000 Hz) but becomes less accurate at very low frequencies (bass) and very high frequencies (treble).
It assumes sound bounces uniformly and evenly—ignoring diffusion, diffraction, and the room’s geometry. A real room has parallel walls, diffusers, and corners that create modes and reflections the formula can’t fully capture.
It treats the room as a uniform acoustic space, ignoring seats, furniture, and people (which all absorb sound differently).
For precision, measure RT60 in your actual room using a sound level meter and a test signal. But for planning and estimation, the Sabine formula is invaluable.
Frequently Asked Questions
Is the Eyring formula better than Sabine?
The Eyring formula is an alternative that works better in highly absorptive rooms where the Sabine formula can overestimate RT60. Eyring is more accurate when absorption coefficients are high (>0.2 average). For typical rooms, Sabine is fine. For very absorbent rooms (heavily treated studios), consider Eyring.
What’s the ideal RT60 for a mixing studio?
Typically 1.2–1.6 seconds at mid-frequency (500 Hz). This is dry enough for mixing (clear bass and midrange definition) but not so dead that it sounds artificial. Some engineers prefer 1.0–1.2 sec (very tight), others 1.4–1.6 sec (slightly more spacious). Measure your room and adjust treatment accordingly.
Can I calculate RT60 for different frequencies separately?
Yes, and you should. Absorption coefficients vary by frequency. Low frequencies are harder to absorb, so α at 125 Hz is lower than α at 500 Hz. The Sabine formula can be applied per frequency band, but you need absorption data for each frequency. Our calculator does this for you. Try the RT60 tool here.
Does furniture change RT60?
Yes. Furniture, people, and soft furnishings all absorb sound and lower RT60. The formula doesn’t account for these. If you calculate RT60 in an empty room, it will be longer than when the room is furnished and occupied. For a control room, measure RT60 with typical furniture and people present for accurate results.