Room Acoustics — RT60 Reverberation Time Calculator
Set room dimensions and surface materials to compute RT60 across 6 octave bands (125–4000 Hz) using both Sabine and Eyring formulas. Check whether your design meets concert hall, classroom, or studio targets.
Sabine RT60 (top), room eigen-frequencies (middle), and the Schroeder transition frequency (bottom).
What is Reverberation Time (RT60)?
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What exactly is RT60, and why is it such a big deal in room acoustics?
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Basically, RT60 is the time it takes for a sound to decay by 60 decibels after the source stops. It's the single most important number for describing how "live" or "dead" a room sounds. In practice, if it's too long, speech becomes muddy; if it's too short, music sounds flat. Try moving the room dimensions (L, W, H) in the simulator above—you'll see the RT60 change instantly.
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Wait, really? So the size of the room directly changes the reverb time? What about the stuff on the walls?
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Exactly! The volume (V) is a key factor, but the materials are crucial. A large, empty gymnasium has a very long RT60 because sound bounces off hard walls. A same-sized room filled with plush seats and acoustic panels has a much shorter RT60. In the simulator, when you change the material for each surface, you're changing its absorption coefficient ($\alpha$), which directly feeds into the calculation.
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I see two formulas, Sabine and Eyring. Which one is "correct," and why does this tool show both?
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Great question. Sabine's formula is the classic, simpler model and works well for rooms with relatively low absorption. Eyring's is a more refined model that's better for very "dead" rooms, like recording studios. A common case is a vocal booth: Sabine might predict an unrealistically short RT60, while Eyring gives a more accurate result. The simulator lets you compare both side-by-side across different frequencies—try setting all materials to "Heavy Curtain" and see the difference!
Physical Model & Key Equations
The foundational Sabine equation assumes a diffuse sound field and calculates RT60 based on the room's total volume and the area-weighted average absorption.
$$T_{60}= \frac{0.161\,V}{\sum S_i \alpha_i}$$
Where $V$ is the room volume (m³), $S_i$ is the area of surface *i* (m²), and $\alpha_i$ is the absorption coefficient of that surface (a number between 0 and 1). The constant 0.161 comes from the speed of sound in air at 20°C.
The Eyring equation is a statistical revision that accounts for the fact that in very absorptive rooms, the assumption of many weak reflections breaks down. It uses the mean absorption coefficient $\bar{\alpha}$.
Here, $S$ is the total surface area of the room, and $\bar{\alpha}$ is the average absorption coefficient ($\sum S_i \alpha_i / S$). The term $-\ln(1-\bar{\alpha})$ corrects for high absorption. When $\bar{\alpha}$ is small, Eyring and Sabine give nearly identical results.
Frequently Asked Questions
Sabine's formula assumes a diffuse sound field where reverberation decays uniformly. It is effective in rooms with evenly distributed sound-absorbing materials, such as typical conference rooms and concert halls. However, in rooms with extremely high absorption coefficients (0.5 or above) or complex shapes, errors become significant. In such cases, consider using Eyring's formula or other alternatives.
When natural modes are densely clustered at low frequencies (especially below 100 Hz), issues such as 'booming' or 'muddiness' occur, where sound resonates strongly at specific frequencies. If the calculation results show wide mode spacing (e.g., gaps of 20 Hz or more), sound pressure unevenness due to standing waves is likely in that frequency band. Use this as a reference for designing with additional sound-absorbing materials or diffusers to suppress the effects of modes.
Enlarging the room increases the volume V, which lengthens RT60 (assuming the same sound-absorbing material area). Additionally, natural mode frequencies are inversely proportional to dimensions, so expanding the room increases the number of low-frequency modes and raises mode density. Conversely, shrinking the room shortens RT60, reduces the number of modes, and shifts them toward higher frequencies. Use real-time calculations to explore optimal dimensions while observing these changes.
The absorption coefficient varies with frequency, but this tool simplifies it by using a single value (0 to 1). Since many materials have low absorption at low frequencies (e.g., 125 Hz) and high absorption at high frequencies (e.g., 4 kHz), it is recommended to input a value for a representative frequency (e.g., 500 Hz) based on the actual application, or to test under multiple conditions. Additionally, an absorption coefficient of 1 means perfect absorption, which is nearly impossible to achieve with real materials.
Real-World Applications
Concert Halls & Opera Houses: These spaces aim for a relatively long, warm reverberation (typically 1.8–2.2 seconds at mid-frequencies) to blend musical notes and create a rich, enveloping sound. Engineers use these calculations to balance room volume, shape, and strategic placement of reflective and absorptive materials.
Classrooms & Lecture Theaters: Speech intelligibility is paramount. A target RT60 of 0.6–0.8 seconds is common. Too much reverb makes the teacher's words overlap, hurting comprehension. Acoustic ceiling tiles and carpet are standard solutions modeled by these equations.
Recording Studios & Vocal Booths: These are "dead" rooms requiring very short RT60 (0.3–0.5 seconds) to capture the dry, direct sound of an instrument or voice without room coloration. Here, the Eyring formula is often more accurate due to the extremely high absorption from bass traps and broadband panels.
Home Theaters & Listening Rooms: The goal is a balanced RT60 (often around 0.4–0.6 seconds) that provides clarity for movie dialogue and a tight, accurate bass response for music. Simulations help decide where to place absorptive panels and diffusers to control specific frequency bands, which you can explore in the tool's octave band view.
Common Misconceptions and Points to Note
There are a few key points you should be aware of when starting to use this simulator. First, "Sabine's formula is not a universal solution." The equation underlying this tool assumes an ideal state where absorption is uniform and sound energy is perfectly diffuse (evenly spread) within the room. Real rooms have furniture, large openings, and uneven distribution of absorptive materials. For instance, if one entire wall of a meeting room is covered with acoustic panels and the opposite wall is glass, the calculated value and the actual listening experience will likely differ. Use it strictly as a "first approximation."
Next, the point that "It's not safe to assume everything is fine above the Schroeder frequency." It's true that above the Schroeder frequency, modes become dense and the sound field tends to become smoother. However, poor reflection patterns in the mid-to-high frequencies can still cause issues like muddiness in specific seats or an overly dry, harsh impression. RT60 is merely an average value of energy decay rate and does not tell you about the "quality" of sound.
Finally, don't forget that "absorption coefficients vary significantly with frequency." While the tool uses a single value per material, actual carpet, for example, absorbs high frequencies well but barely absorbs low frequencies. The cause of low-frequency boominess is often insufficient low-frequency absorption. If you aim for an RT60 target of 0.5 seconds across all frequency bands, you'll likely need to consider separate low-frequency absorbers (like membrane/panel resonators or Helmholtz resonators).