The tools on Reverb-Calculator.com calculate RT60 — reverberation time — using established acoustic physics formulas. No microphone, no audio capture, no signal processing. You enter room dimensions and surface material properties; the tools return RT60 in seconds. This page explains the physics behind the calculation, the formulas used, how absorption coefficients work, what the results mean, and where the formulas’ assumptions break down.
Written and maintained by Noah Bennett, founder of Reverb-Calculator.com.
What Is RT60?
RT60 is the time it takes for sound pressure level to decay by 60 decibels in an enclosed space after the sound source stops. It is the single most important measurement in architectural acoustics — the quantity that determines whether a room sounds live or dead, whether speech is intelligible or muddy, whether music sounds rich or congested.
The 60 dB decay reference was established by Wallace Clement Sabine during his pioneering research at Harvard University beginning in 1898. Sabine was attempting to solve the intelligibility problems in a Harvard lecture theatre and in doing so developed the first quantitative theory of room acoustics. His reverberation equation remains the foundational formula of the field more than 125 years later.
The Sabine Formula
The Formula
RT60 = 0.161 × V / A (metric units: metres and seconds)
RT60 = 0.049 × V / A (imperial units: feet and seconds)
Where:
- RT60 = reverberation time in seconds
- V = room volume in cubic metres (or cubic feet for imperial)
- A = total sound absorption in metric Sabins (or imperial Sabins for imperial)
- 0.161 = the metric Sabine constant (derived from the speed of sound in air at approximately 20°C)
- 0.049 = the imperial Sabine constant (same physics, different unit system)
The most common calculation error when using RT60 calculators is mixing unit systems — entering a volume in cubic metres and an absorption value calculated from areas in square feet, or using the wrong constant for the chosen unit system. The tools on this site specify units explicitly and apply the correct constant automatically.
The Sabin: Unit of Absorption
The Sabin is the unit of sound absorption used in the Sabine formula. One metric Sabin represents the equivalent absorptive area of one square metre of a perfectly absorptive surface (absorption coefficient = 1.0). Total absorption A is calculated by summing the contribution of each room surface:
A = Σ (Sᵢ × αᵢ)
Where Sᵢ is the area of surface i in square metres and αᵢ is the absorption coefficient of that surface at the relevant frequency.
For example: a 20 m² wall with absorption coefficient 0.05 contributes 1.0 Sabin. A 15 m² ceiling with acoustic tile at absorption coefficient 0.70 contributes 10.5 Sabins. The total A is the sum across all surfaces.
Worked Example
A rectangular room: 8m long × 6m wide × 3m high
Volume: 8 × 6 × 3 = 144 m³
Surface areas:
- Floor (8 × 6): 48 m²
- Ceiling (8 × 6): 48 m²
- Two long walls (8 × 3 × 2): 48 m²
- Two short walls (6 × 3 × 2): 36 m²
- Total surface area: 180 m²
Absorption coefficients (example — hard-surfaced unfurnished room):
- All surfaces average absorption coefficient: 0.05
Total absorption: A = 180 × 0.05 = 9.0 Sabins
RT60: 0.161 × 144 / 9.0 = 2.57 seconds
This is far too long for a recording studio (target: 0.2–0.6s) and even long for a concert hall (target: 1.5–2.5s). Adding absorptive treatment — acoustic panels, carpeting, furnishings — increases A and reduces RT60.
What Makes RT60 Longer or Shorter
RT60 increases with:
- Larger room volume (more space for sound to bounce around)
- Hard, reflective surfaces (low absorption coefficients — concrete, glass, bare drywall)
- Fewer furnishings and occupants
RT60 decreases with:
- Higher total absorption (more or more absorptive surfaces)
- Soft, porous materials (carpets, curtains, acoustic foam, fabric panels)
- Smaller room volume
- More occupants and furnishings (bodies and clothing are significantly absorptive)
The Eyring Formula
The Sabine formula assumes that absorption is uniformly distributed across all surfaces and that the sound field is diffuse (sound energy arriving at all points from all directions with equal probability). These assumptions hold well for lightly treated rooms with average absorption coefficient below approximately 0.15–0.20.
For rooms with higher average absorption — such as recording studios, heavily treated home theatres, or vocal booths — the Eyring formula provides more accurate results:
RT60 = 0.161 × V / (−S × ln(1 − ᾱ))
Where:
- S = total surface area in square metres
- ᾱ = mean absorption coefficient (weighted average across all surfaces)
- ln = natural logarithm
Why Eyring Is More Accurate for High Absorption
The Sabine formula treats absorption linearly — doubling A halves RT60. But at high absorption values, the relationship between absorption and reverberation time is non-linear. The Eyring formula accounts for this non-linearity through the natural logarithm term.
At low absorption (ᾱ < 0.15), the two formulas give nearly identical results. As ᾱ increases, the difference grows. At ᾱ = 0.50, Sabine gives a result approximately 30% longer than Eyring. At very high absorption (ᾱ → 1.0), the natural logarithm term approaches infinity, correctly reflecting that a fully absorptive room has essentially zero reverberation.
Which formula to use:
- ᾱ < 0.15: Either formula; Sabine is simpler
- 0.15 < ᾱ < 0.30: Sabine gives reasonable results; Eyring preferred
- ᾱ > 0.30: Use Eyring for meaningful accuracy
Absorption Coefficients
An absorption coefficient (α) is a dimensionless value between 0 and 1 expressing the fraction of incident sound energy a material absorbs at a given frequency. A coefficient of 0 means all sound is reflected; a coefficient of 1 means all sound is absorbed.
Key properties of absorption coefficients:
Frequency dependence. Absorption coefficients vary with frequency. Most common materials absorb high frequencies (above 1 kHz) more effectively than low frequencies. Thin acoustic foam, for example, may have α = 0.05 at 125 Hz but α = 0.90 at 4 kHz. This means RT60 varies with frequency — a room may be well-treated at high frequencies but still have excessive low-frequency reverberation.
Installation dependence. The same material installed differently can have different absorption coefficients. An acoustic panel mounted directly on a wall behaves differently from the same panel mounted with an air gap behind it (air gap improves low-frequency absorption).
Typical coefficient ranges:
- Bare concrete, glass, ceramic tile: 0.01–0.05 across most frequencies
- Painted drywall: 0.03–0.07
- Hardwood floor: 0.03–0.10
- Carpet on concrete: 0.02–0.35 (increases significantly with frequency)
- Acoustic ceiling tile: 0.25–0.90 (highly frequency dependent)
- Dedicated broadband acoustic panels: 0.50–0.99 (varies by product and frequency)
The NRC (Noise Reduction Coefficient) is a single-number average of a material’s absorption coefficients at 250, 500, 1000, and 2000 Hz. It is useful as a quick comparison between products but does not capture low-frequency performance or frequency-dependent variation. Always use full octave-band data for accurate RT60 calculations when available.
RT60 Targets by Room Type
Different rooms require different RT60 values depending on their primary use. These are the established target ranges from acoustic engineering practice and international standards:
| Room type | RT60 target range | Primary requirement |
|---|---|---|
| Vocal booth | 0.2–0.4 s | Very dry, minimal reflections |
| Recording studio | 0.3–0.6 s | Clean capture, controlled room sound |
| Mixing/control room | 0.2–0.4 s | Accurate monitoring, low coloration |
| Home theatre | 0.3–0.5 s | Speech and music clarity, controlled decay |
| Conference room | 0.4–0.6 s | Speech intelligibility, low distraction |
| Classroom | 0.6–0.9 s | Speech intelligibility at moderate volume |
| Lecture theatre | 0.8–1.2 s | Speech projection over larger distances |
| Multipurpose hall | 1.0–1.5 s | Flexible use — speech and unamplified music |
| Chamber music hall | 1.3–1.8 s | Small ensemble acoustic richness |
| Concert hall (orchestral) | 1.5–2.5 s | Full orchestral acoustic depth |
| Cathedral / large church | 2.5–8.0 s | Liturgical atmosphere, organ resonance |
What the Formulas Cannot Determine
Flutter echo and standing waves. The Sabine and Eyring formulas calculate average reverberation in a diffuse field. They cannot detect flutter echo (rapid repetitive reflections between parallel walls) or room modes (standing wave resonances at specific low frequencies determined by room dimensions). These acoustic problems require geometric analysis beyond RT60 calculation.
Frequency-specific RT60. The single RT60 value produced by these formulas uses a single average absorption coefficient. For frequency-specific RT60 — the variation in reverberation time across octave bands — each frequency band must be calculated separately using frequency-specific absorption coefficients for each surface.
Irregular room geometries. The Sabine and Eyring formulas assume rectangular or near-rectangular rooms with reasonably uniform surface treatment. Rooms with domed ceilings, angled walls, curved surfaces, or highly localised absorption areas may show significant deviation from calculated RT60.
Perceptual acoustic quality. RT60 is the most important single metric — but not the only one. Speech intelligibility also depends on clarity (C50), strength (G), and early decay time (EDT). Music quality depends on C80, lateral energy fraction (LF), and interaural cross-correlation (IACC). These metrics require impulse response measurement (ISO 3382) rather than formula calculation.
Related Pages
- Reverb Calculator FAQ — common questions about RT60 and room acoustics
- About the Author — Noah Bennett’s background and expertise
- Editorial Guidelines — research standards for all content
- Data Security — how the tools handle input data
- Contact — report a technical issue or formula error
This How It Works page is written and maintained by Noah Bennett, founder of Reverb-Calculator.com. Last updated: June 2026.