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What exactly is RT60, and why is it the first thing this simulator calculates?
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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, a long RT60 makes speech muddy, while a very short one can make music feel lifeless. Try moving the sliders for wall or ceiling materials above—you'll see the RT60 change instantly.
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Wait, really? So the materials on the surfaces are that important? What's this "absorption coefficient" I'm setting for them?
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Exactly! Every material, from concrete to acoustic foam, absorbs a fraction of the sound energy that hits it. That fraction is the absorption coefficient, α. A value of 0 means it reflects all sound (like a hard floor), and 1 means it absorbs all sound (theoretically perfect absorber). In this simulator, when you change the "Floor" or "Walls" dropdown, you're changing their α value at 500 Hz, a key mid-frequency.
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I see the simulator gives two answers: Sabine and Eyring. Which one should I trust?
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Great observation. Sabine's formula is simpler and works well for rooms with relatively low absorption, like a typical classroom or office. Eyring's formula is more accurate for very "dead" rooms with high absorption, like a recording studio. If you set all your surfaces to highly absorptive materials in the simulator, you'll see the two results diverge—that's when you'd rely on Eyring.
The total absorption $A$ is the sum of contributions from all surfaces: floor, ceiling, walls, and even objects like seats.
$$A = \sum_i \alpha_i S_i = \alpha_{floor}S_{floor}+ \alpha_{ceiling}S_{ceiling}+ \alpha_{walls}S_{walls}+ \text{(seats)}$$
The Eyring formula refines this for highly absorptive rooms by using the mean absorption coefficient $\bar{\alpha}= A / S_{total}$ and a logarithmic term, where $S$ is the total surface area of the room.
$$RT_{60}= \dfrac{-0.161 \cdot V}{S \ln(1-\bar{\alpha})}$$
Common Misconceptions and Points to Note
When you start using this tool, there are a few points you should be mindful of. First, the tendency to think "selecting the material is all that's needed." In an actual room, furniture and people become significant sound absorbers. For example, the reverberation time is completely different between an empty conference room and one packed with chairs and people. Think of the simulation as a baseline for an "empty room state." A good tip is to design for a time about 0.5 seconds shorter than the target to allow a margin for real-world use.
Next, "judging based on a single numerical value while ignoring frequency characteristics." When you select "concrete" in the tool, its absorption coefficient is low, but in reality, it's often higher at low frequencies (like 125Hz) than at mid-frequencies (500Hz or 1kHz). Conversely, carpet absorbs high frequencies well but absorbs very little low frequency. So if you feel the reverberation is "generally long," it might actually be specific low frequencies that are booming. Always check the frequency-specific results and look at the overall balance.
Finally, the danger of "placing too much absolute trust in the calculation results." Both the Sabine and Eyring formulas assume an ideal state called a "diffuse sound field," where "sound spreads uniformly throughout the room." However, in reality, especially in small rooms or long, narrow corridors, standing waves (room modes) occur, creating unevenness in the sound. Even if the calculated RT60 is 2 seconds, depending on where you sit, you might experience "excessive reverberation" or "difficulty hearing." Simulation is the first step. What's important is to develop the habit of taking a step back after calculating and asking, "Does the diffuse field assumption truly hold for this room's shape?"
Related Engineering Fields
Calculating reverberation time is actually just the gateway to acoustics, and the fields connected from here are incredibly broad. First, "Noise Control / Environmental Acoustics." Designing soundproof rooms to reduce factory machinery noise or road traffic noise uses the exact opposite thinking of this tool. That is, by increasing the equivalent absorption area $A$ to bring reverberation as close to zero as possible, you prevent sound from building up and becoming louder.
Next, it's also deeply related to "Electroacoustics / Loudspeaker System Design." How the direct sound from a speaker mixes with the reverberant sound reflected from walls determines speech clarity. Especially in conference systems or public address systems, if the reverberation time is too long, speech becomes difficult to understand. Estimating RT60 with this tool is sometimes used as foundational data to optimize parameters for audio processing like "delay time" and "ducking."
Delving deeper into the physics, it connects directly to the world of "Numerical Acoustic Simulation (CAE)." This tool's calculation is an aggregate approach called "statistical acoustics," but to understand sound propagation in more detail, simulations using "geometric acoustics" (ray tracing) or "wave acoustics" (Finite Element Method FEM or Boundary Element Method BEM) become necessary. For example, designing the shape of a concert hall's ceiling reflector to be subtly curved to deliver reflected sound to specific seats cannot be done without such advanced CAE.
For Further Learning
Once you're comfortable with this tool and think "I want to know more!", try moving to the next step. First, "deepen your understanding of the mathematical background a little." Look into how the Sabine formula is derived. In that process, the concept of "exponential decay of acoustic energy density" appears. This is described by the same mathematical model (the differential equation $\frac{dE}{dt} = -k E$) as radioactive decay or RC circuit discharge. Noticing these "instances of the same equation appearing in different fields" can dramatically broaden your perspective on engineering.
The next recommended practice is to "try simple measurements in spaces around you." There are free smartphone apps that can measure RT60, so you can compare simulation results with actual measurements. Measuring your living room and your bathroom will let you experience firsthand that the bathroom reverberation is longer, just as calculated. This "calculation → measurement → analysis" loop is the most effective learning method.
To advance your learning further, look into the "Three Elements of Room Acoustics." Reverberation time (RT60) is one of them; the other two are "Clarity (C50, C80)" and "Spatial Impression (LF, IACC)." These are metrics for quantitatively evaluating whether sound is "heard clearly" or "feels richly enveloping." In modern acoustic design, these are evaluated comprehensively alongside RT60. Once you've solidified the basics with this tool, the natural next step is to challenge yourself with more advanced simulation tools or theories that can calculate and evaluate these parameters.