Room Acoustics Calculator

How Room Acoustics Are Calculated

Room acoustics determine how sound behaves in a space. The key measurement is Reverberation Time (RT60), the time it takes for sound to decay by 60 dB after the source stops.

Sabine Formula: RT60 = 0.161 × V / A

Where:

• RT60 = reverberation time in seconds
• V = room volume in cubic meters (length × width × height)
• A = total acoustic absorption in Sabins (m²)
• 0.161 = Sabine’s constant (metric version)

Absorption Coefficient Examples:

• Bare concrete: α = 0.02 (very reflective)
• Carpet: α = 0.35–0.55
• Acoustic foam panel: α = 0.70–0.95
• Upholstered audience: α = 0.80
• Absorption (Sabins) = Surface Area × α

Worked Example: A 5m × 4m × 2.5m home studio (V = 50 m³). The four walls come to 2 × (5 × 2.5) + 2 × (4 × 2.5) = 45 m², and the ceiling adds another 20 m²:

• Bare walls + ceiling = 65 m² × 0.03 = 1.95 Sabins
• Carpet floor = 20 m² × 0.45 = 9.0 Sabins
• Total A = 10.95 Sabins
• RT60 = 0.161 × 50 / 10.95 = 0.74 seconds

Target RT60 Values:

• Home studio / podcast booth: 0.2–0.4 s
• Live music room / rehearsal: 0.4–0.6 s
• Concert hall (classical): 1.5–2.2 s
• Cathedral: 4–10 s

Excessive reverb muddies recordings. Add bass traps in corners and broadband absorbers on first reflection points to reduce RT60.

Room Modes

RT60 is only half the story, and in a small room it is the less important half. Sound bouncing between two parallel walls reinforces itself at frequencies where the distance is a whole number of half-wavelengths. Those are the room’s axial modes, and they are what makes one bass note in a song boom while the note below it disappears.

f = n × c / (2 × d)

where c is the speed of sound (344 m/s, 1130 ft/s), d is the distance between the two surfaces, and n = 1, 2, 3… for each successive mode. A 20-foot length puts its first mode at 28 Hz, the second at 57, the third at 85. Every pair of parallel surfaces has its own series.

What makes a room’s modes good or bad is spacing, not count. Trouble comes when modes from different dimensions land on the same frequency:

• Two dimensions nearly equal. A 9 ft × 9 ft cross-section gives the width and the height identical mode series, so every one of them doubles up. Square rooms and cubes are the worst offenders.
• A whole-number ratio. If the length is exactly twice the height, the length’s second mode sits on the height’s first, and so on all the way up.

Published ratios exist to avoid both. Sepmeyer’s 1 : 1.28 : 1.54 and 1 : 1.60 : 2.33, and Bolt’s 1 : 1.14 : 1.39, spread the modes about as evenly as three numbers can. If you are building a room from scratch, start there. If you are stuck with the room you have, corner bass traps are the fix, and they need real depth to work at these frequencies. Start at 100 mm of rigid mineral wool, spanning the corner rather than sitting flat against one wall.

Above the Schroeder frequency the modes crowd together closely enough that the room stops behaving modally and RT60 becomes the meaningful measurement instead. That crossover is usually somewhere between 100 and 300 Hz in a domestic room, and the calculator works it out for yours.