What is RT60?
RT60 stands for Reverberation Time 60. It is defined as the time it takes for a sound pressure level to decay by 60 decibels (dB) after the sound source has abruptly stopped. A 60 dB drop effectively means the sound has faded to one millionth of its original intensity, disappearing into the background noise floor.
Different rooms require different RT60 targets. A cinema or recording studio requires a 'dead' room with an RT60 of 0.2 to 0.4 seconds. A symphonic concert hall requires a 'live' room with an RT60 of 1.8 to 2.2 seconds to allow the music to blend.
Sabine vs. Eyring Formulas
Wallace Clement Sabine developed the foundational formula for reverberation in the late 1800s. The Sabine equation works exceptionally well for 'live' rooms with low absorption. However, in heavily treated, acoustically 'dead' rooms, the Sabine formula becomes mathematically inaccurate.
For highly absorbent rooms (like broadcast studios), acousticians use the Eyring formula, which modifies the math to account for sound dying out before it can even make a complete transit across the room.
How to Calculate Sabine RT60
To calculate RT60, you need the physical volume of the room and the total sound absorption provided by all the surfaces (walls, floor, ceiling, people). Absorption is measured in Sabins. One Sabin equals one square foot of 100% sound-absorbing material (like an open window).
The Formula (Imperial): RT60 = 0.049 x (Volume in cubic feet) / (Total Absorption in Sabins).
Worked Example: A small classroom is 20ft long, 15ft wide, and 10ft high. Volume = 3,000 cu ft. The walls are hard drywall (low absorption coefficient of 0.05), but the ceiling is acoustic tile (coefficient of 0.70). You calculate the surface area of each material and multiply by its coefficient to get Sabins. Let's say the total sum of all Sabins in the room is 350. Calculation: RT60 = 0.049 x 3000 / 350. RT60 = 147 / 350 = 0.42 seconds. This is an excellent reverberation time for a classroom.
Octave Bands and Frequency Dependence
Reverberation is not a single number. Low-frequency bass waves bounce around a room much differently than high-frequency treble waves. Thick concrete walls reflect bass, while thin carpets easily absorb treble.
Because of this, acousticians must calculate the RT60 across multiple frequencies, typically standard octave bands (125Hz, 250Hz, 500Hz, 1kHz, 2kHz, 4kHz). Materials have distinct absorption coefficients for each frequency, meaning a room could have a tight 0.5s RT60 at 2kHz but a muddy 2.0s RT60 at 125Hz.
Frequently asked questions
What is the difference between a Sabine and a metric Sabin?
An imperial Sabin is based on one square foot of absorption. A metric Sabin is based on one square meter of absorption. If you use metric dimensions, the Sabine constant in the formula changes from 0.049 to 0.161.
Do people in the room affect RT60?
Significantly. The human body and clothing are highly absorptive. A theater that sounds incredibly echoey when empty will often sound perfectly balanced when all the seats are filled with patrons.
How do I lower the RT60 in a room?
To decrease reverberation, you must increase the Total Absorption in the denominator of the formula. You achieve this by adding soft, porous materials like fiberglass acoustic panels, heavy drapery, or carpet.
Why is the Eyring formula better for dead rooms?
The Sabine equation theoretically allows an RT60 greater than zero even if the room is lined with 100% absorptive material, which is physically impossible. The Eyring equation uses a natural logarithm that correctly predicts an RT60 of zero in an entirely anechoic space.