The RT60 formula is the mathematical foundation of room acoustics. It lets you predict how long sound will reverberate in a space based on the room’s size and the materials inside it. Understanding the formula—and its assumptions—is key to applying it correctly and knowing when it might give you misleading results.
There are two main formulas in use: the Sabine formula, which works well for most rooms, and the Eyring formula, which is more accurate in highly treated spaces. Both are grounded in physics, but they make different assumptions about how sound behaves.
The Sabine Formula Explained
The Sabine formula is the standard equation for estimating RT60:
RT60 = 0.161 × V / A
This equation is elegantly simple, but each component carries real physical meaning:
- RT60 is the reverberation time in seconds
- V is the room volume in cubic meters
- A is the total sound absorption in sabins
- 0.161 is a constant that converts the relationship into practical units
The logic is intuitive: a larger room (bigger V) with more absorption (larger A) will have a longer or shorter reverb time depending on which factor dominates. Doubling the room volume doubles RT60 if absorption stays the same. Doubling the absorption halves RT60.
The formula was developed by Wallace Clement Sabine, an American physicist who worked at Harvard in the 1890s. Sabine studied the acoustics of the Boston Symphony Hall and realized that a simple mathematical relationship existed between room volume, surface absorption, and how long sound decays. His work became the foundation of modern architectural acoustics.
Breaking Down Each Component
Room Volume (V)
V is straightforward: the cubic volume of your room, measured in cubic meters. Multiply length × width × height. If your room is 4 m long, 5 m wide, and 3 m tall, V = 60 m³.
Larger rooms naturally have longer RT60 because sound has to travel farther and bounce more times before energy is dissipated. A concert hall (thousands of cubic meters) will have much longer RT60 than a small bedroom (20–50 m³).
Total Absorption (A)
A is the sum of all sound absorption in your room, measured in sabins. One sabin is the absorption of one square meter of a perfectly absorptive surface (theoretical—no real material is 100% absorptive).
To find A, calculate the absorption of each surface:
Absorption = Surface Area × Absorption Coefficient
Then sum all surfaces. Example:
- Floor (carpet, 20 m²): 20 × 0.4 = 8 sabins
- Walls (drywall, 60 m²): 60 × 0.08 = 4.8 sabins
- Ceiling (drywall, 20 m²): 20 × 0.08 = 1.6 sabins
Total A = 8 + 4.8 + 1.6 = 14.4 sabins
Absorption depends on both the material and the frequency. Soft, porous materials like foam and carpet absorb more sound than hard materials like concrete and glass. Low frequencies are harder to absorb than mid and high frequencies, so absorption coefficients are always frequency-dependent.
The Constant 0.161
The constant 0.161 isn’t arbitrary—it comes from the logarithmic physics of sound decay. Here’s where it comes from:
Sound intensity decays logarithmically in a room. When we say “60 dB decay,” we’re referring to a specific logarithmic threshold (60 dB = a factor of 10^6 reduction in intensity). The speed of sound in air is approximately 343 m/s at room temperature. When you work through the mathematics of how many reflections occur before sound drops 60 dB in a room, you get:
0.161 = ln(10^6) / (3 × 343 m/s × ln(10))
If you’re working in feet instead of meters, the constant changes to approximately 0.049. So if V is in cubic feet and A is in sabins, use RT60 = 0.049 × V / A.
The constant embeds the speed of sound and the logarithmic decay threshold. Change the speed of sound (as happens at higher altitudes or different temperatures) and the constant would technically change slightly—but for standard conditions, 0.161 (metric) is universal.
Sabine vs. Eyring: Which Formula to Use
The Sabine formula works well for most rooms, but it has a limitation: it assumes that absorption is low to moderate (typically under 20% of the total surface area). In heavily treated studios or rooms with very absorptive materials covering most surfaces, the Sabine formula becomes less accurate.
The Eyring formula was developed to handle high-absorption rooms:
RT60 = 0.161 × V / (-S × ln(1 – α))
Where:
- S is the total surface area of the room (m²)
- α is the average absorption coefficient across all surfaces
- ln is the natural logarithm
- The negative sign and logarithm account for non-linear absorption behavior
The Eyring formula doesn’t assume a linear relationship between absorption and decay time. In heavily treated rooms, doubling absorption doesn’t exactly halve RT60—the effect is non-linear, and Eyring captures this.
When to Use Sabine
Use Sabine for most practical scenarios: typical home studios, untreated rooms, rooms with moderate treatment. It’s simple, fast, and accurate enough for these contexts.
When to Use Eyring
Use Eyring when:
- Your room is heavily treated (foam covering 50%+ of surfaces, or high-absorptivity materials everywhere)
- Absorption coefficients are above 30% for most materials
- You’re designing a vocal booth or anechoic chamber where absorption is maximized
For a standard home studio or mixing room, Sabine will serve you fine. For detailed comparison and when to use each formula, see the Eyring formula reference.
Frequency-Dependent RT60 Calculations
One important reality: the RT60 formula assumes a single absorption coefficient (A), but in real rooms, absorption varies by frequency. High frequencies are absorbed more easily than low frequencies, so RT60 is longer in the bass than in the treble.
Professional acoustic design calculates RT60 at six standard frequencies:
- 125 Hz (low bass)
- 250 Hz (bass)
- 500 Hz (lower midrange)
- 1 kHz (midrange reference)
- 2 kHz (upper midrange)
- 4 kHz (high treble)
For each frequency, you use the absorption coefficient at that frequency and calculate a separate RT60. This gives you a frequency response of reverberation time.
Example: A room might have RT60 = 1.2 seconds at 125 Hz (boomy lows), but only 0.6 seconds at 4 kHz (controlled highs). This tells you that the room needs more low-frequency absorption to balance the decay. Explore absorption coefficients across frequencies to understand this variation.
Limitations and Assumptions
The Sabine and Eyring formulas are estimates, not absolute predictions. They assume:
- Diffuse sound field: Sound is bouncing uniformly in all directions, not creating standing waves or focusing points. Real rooms have room modes and reflections that can violate this.
- Even distribution of absorption: The formula assumes absorption is distributed evenly throughout the room. In reality, all the absorption might be on one wall or concentrated in corners, which changes the actual decay pattern.
- No large obstacles or furniture: The formulas don’t account for the effect of individual pieces of furniture, people, or equipment that might block or absorb sound unevenly.
- Rectangular room geometry: Non-rectangular rooms, angled ceilings, or irregular shapes aren’t directly handled by the formula. You’d need to break them into simpler sections.
Because of these assumptions, expect measured RT60 (using actual sound decay in the room) to differ from calculated RT60 by about 10–15%. This is normal and acceptable for most purposes.
Frequently Asked Questions
What’s the difference between the constant 0.161 and 0.049?
0.161 is the constant for metric units (meters, cubic meters). 0.049 is the equivalent constant for imperial units (feet, cubic feet). If you’re calculating RT60 using room volume in cubic feet and absorption in sabins, use 0.049 instead of 0.161.
Why is the formula called “Sabine” if it’s just V divided by A?
The simplicity is actually the genius. Before Sabine derived this relationship mathematically, there was no way to predict how a room would sound acoustically. Sabine’s breakthrough was proving that a simple, linear relationship existed between room size, absorption, and decay time. The formula bears his name because he discovered the underlying principle, even though the math itself is straightforward.
Can I use the Sabine formula for very small rooms like a vocal booth?
Yes, the formula works at any scale. A 1 m³ room (very small) and a 1000 m³ hall both follow the Sabine relationship. The only caveat is that very small rooms may have strong room modes that skew the actual decay, making measured RT60 differ from calculated.
What happens if my calculated A (total absorption) is higher than Sabine recommends?
If A becomes very high relative to room volume, Sabine’s prediction becomes less reliable and Eyring becomes more accurate. Highly absorptive rooms don’t follow a simple linear decay model. If you’ve heavily treated a space, measure actual RT60 rather than relying solely on calculation.