Knowledge
What is RT60?
What RT60 is, why it changes with frequency, how ISO 3382 measures it, and when the Sabine formula misleads.
RT60 is the time sound level takes to fall sixty decibels after the source stops. A figure without a frequency, a room volume, and a use does not describe a hall.
Definition
By the standard definition, reverberation is sound continuing in a room because of reflections. RT60 quantifies how fast that energy disappears. A serious measurement follows a stated method. For performance spaces the usual reference is ISO 3382-1, not a phone in the photographer’s pocket.
T20 and T30 are taken from part of the decay curve and scaled to a 60 dB fall, because background noise often hides the full drop. The number can therefore change with the method even inside the same standard.
Why it matters
Speech needs the previous syllable to die before the next one stacks on it. Music may need some tail so the room does not sound dead. One handsome number does not serve both in the same volume.
A long RT60 plus HVAC noise crushes the speech transmission index. Wall treatment that ignores volume can kill the high octaves and leave the low ones blooming.
Principle
The metric Sabine equation is RT = 0.161 V / A. V is volume in cubic metres. A is the sum of areas times their absorption coefficients (sabins). More energy fits in a larger volume; it disappears faster as absorption rises.
The formula assumes a diffuse field and modest, distributed absorption. When panels cover a large area, or absorption sits only in the ceiling, the formula drifts away from measurement. Eyring handles a higher mean absorption. It is still not a full geometric model.
Measurement
Source and method — An interrupted source or an impulse response, with enough source and receiver positions that one seat is not treated as the room.
Noise floor — If the curve does not fall cleanly above the noise floor, the figure is not valid however neat the plot looks.
Frequency — RT is normally reported in octave or third-octave bands. A casual average from 250 to 4000 Hz can hide a peak at 125 Hz.
Units
Seconds — RT60 is written in seconds. A bare “1.2” in a specification table is a copy error.
Decibels in the definition — The sixty decibels are the size of the fall in the definition. They are not the sound level required in the room.
Design
Volume first — Before panels, ask whether the volume suits the seat count and the use. More absorption does not fully repair a wrong volume.
Distribution — Absorption only on the rear wall leaves early side reflections untreated and can dull the back rows.
Low frequencies — A thin panel absorbs the mid and high bands. Bass needs thickness, a spacing off the hard surface, or a calculated resonant absorber.
Audience — An audience absorbs. A room tuned empty and handed over full will measure differently. The criterion has to name the condition.
Use
Lecture and spoken theatre — Discussion tends toward a shorter mid-band decay so syllables stay distinct. The value is fixed in the brief, not in a public pamphlet.
Unamplified music — Larger rooms may seek a longer tail, balanced across frequency. That is a different design target, not an error in a speech table.
Multipurpose — An honest answer may be variable acoustics: curtains or changeable panels, accepting that each setting is a compromise.
Limits
Not isolation — RT60 describes sound inside the room. It does not say whether the hall is heard in the corridor or the next cinema.
Not STI — A short decay with high noise can still fail speech. Intelligibility is asked of a speech index, not of reverberation time alone.
The calculator is preliminary — The site calculator applies Sabine and Eyring to numbers you type. Replace textbook coefficients with laboratory data, then with measurement.
Common mistakes
One number for the job — Writing RT60 = 1.0 s on every item from the lecture hall to the sports hall deletes the meaning of a criterion.
Panels on the drawing with no build-up — A wood look can be reflective. Absorption comes from what is behind the face: cavity, density, open fabric.
Measuring the director’s seat — One position near the source, or inside a balcony shadow, does not represent the room.
Example
A room of 900 m³ with 180 sabins of absorption gives, by Sabine, 0.161 × 900 / 180 ≈ 0.81 s. If half of that absorption is an audience that is absent on the night of the test, the measured figure will be longer. The example explains the formula. It is not a target for your project.
Questions
Is a shorter RT60 always better?
No. Too short kills musical support and can make a sound system harsh. Short or long is judged against the use.
Do curtains absorb every frequency?
A heavy, folded curtain affects the mid and high bands more than the lows, unless it is deep and spaced off the wall.
Why do the contractor’s and consultant’s measurements differ?
Source position, point count, frequency range, chairs or workers present, and instrument setup. Without a method report the numbers are not comparable.
Does concrete cause a long RT60?
Concrete is reflective, but the length comes from volume and the lack of total absorption. A small concrete surface in a furnished room does not decide the result.
Where do I calculate a first figure?
In the reverberation calculator on this site, then discard the result when a confirmed volume and laboratory coefficients arrive.