🔊Acoustics Calculator - RT60 Reverb Time, Room Modes, dB Math

How to use Acoustics Calculator (RT60, Room Modes, Decibels)

1. 1Pick a room preset, or enter the dimensions and choose a main and patch material for each surface
2. 2Set the number of people and the room use, then read the RT60 bar chart against the recommended range
3. 3Open Room modes to see the low-frequency standing waves and the Bonello criterion
4. 4Use Sound levels for dB addition, background subtraction, distance loss, dB(A) and noise dose
5. 5Use Speed & wavelength for the speed of sound vs temperature and humidity, other media and a wavelength table

FAQ

What is the difference between the Sabine and Eyring formulas?

Sabine's RT60 = 0.161V/A assumes a diffuse field with little absorption and works well in live rooms with a mean absorption coefficient below about 0.2. With heavy absorption it overestimates the reverb; even a fully absorbing room would still have a non-zero Sabine RT. Eyring replaces A with −S·ln(1 − ᾱ), which is more realistic for studios and home theaters. Look at both: they agree in live rooms, and when they differ a lot, trust Eyring.

What reverberation time should a room have?

It depends on the use. Speech needs short reverb: classrooms under about 0.6 s per ANSI S12.60 (0.7 s above 283 m³), meeting rooms 0.6–1.0 s, control rooms 0.2–0.4 s, home theaters 0.25–0.5 s. Music wants more: chamber halls 1.2–1.7 s, symphony halls 1.8–2.2 s, churches with organ 2–3 s. The usual single figure is the mid-frequency RT, the average of the 500 Hz and 1 kHz bands.

What are room modes and why is bass uneven in small rooms?

Parallel surfaces trap standing waves at f = (c/2)·√((nₓ/L)² + (n_y/W)² + (n_z/H)²). Modes along one axis are axial and carry the most energy; tangential and oblique modes involve two and three axes. In a small room the low-frequency modes are sparse, so some notes boom and others vanish. The Bonello criterion checks that the number of modes per 1/3-octave band never decreases with frequency, a quick test of the room's proportions. Above the Schroeder frequency the modes overlap and statistical acoustics applies.

Why do two 80 dB sources add up to 83 dB, not 160 dB?

Decibels are logarithmic, so you must add energies: L = 10·log(10^(L₁/10) + 10^(L₂/10)). Doubling identical sources adds 10·log 2 ≈ 3 dB, and ten of them add 10 dB. Removing background noise is also an energy subtraction, and when the total is less than 3 dB above the background the result is unreliable.

How do noise dose and TWA work?

Dose is the day's exposure as a percentage of the allowed amount: D = Σ Cᵢ/Tᵢ, where Cᵢ is time spent at a level and Tᵢ is the time allowed at that level. NIOSH sets 100% at 85 dBA for 8 hours with a 3 dB exchange rate; OSHA uses 90 dBA for 8 hours with a 5 dB exchange rate. TWA expresses the day as an equivalent steady 8-hour level. Above 100% you need noise control or hearing protection. This is an estimate only; occupational assessments need proper measurements.