Reverberation time

How do you use the tool?

Enter the room dimensions (L × W × H): the tool derives volume and surfaces. Choose the materials of floor, ceiling and walls, add the openings (windows, doors) and treatments (carpets, panels, audience), and set temperature and humidity. T60 is computed in real time with the three formulas over the six octave bands 125–4000 Hz.

1. Enter the room dimensions (L × W × H). The tool automatically computes volume, floor and ceiling surface and total wall surface.
2. Choose the structural materials: floor, ceiling and walls. To assign different materials to the four walls, enable “Split walls by material”.
3. Add the openings (windows, doors): they occupy a portion of the wall and apply their own α, subtracting area from the structural wall.
4. Add treatments and furnishings: carpets, panels, curtains, audience, seats. They are additive — “coverings” (carpets, panels) replace the α of the covered portion; audience and seats add absorption without occupying surface.
5. Set temperature and humidity if relevant (needed for large rooms or analysis above 1 kHz).
6. T60 is computed in real time with the three formulas for 6 octave bands.

For non-rectangular or complex rooms, switch to “Manual inventory” mode at the top.

When the tool helps

• Estimating the T60 of a classroom, meeting room, canteen or gym before an acoustic correction intervention.
• Quickly comparing the effect of panels, curtains or suspended ceilings on an existing room.
• Understanding, for teaching purposes, how volume, structural materials and furnishings contribute to the result.

When it is not the right tool

• Regulatory compliance checks (e.g. the Italian UNI 11532-2:2020 for school building acoustics): a model adequate to the methodology is needed, and a competent acoustic technician responsible for it.
• Concert halls, auditoria, recording studios: the design requires 3D models and in-situ measurements.
• Very small rooms (below ~30 m³) or very elongated ones: the sound field is not diffuse and T60 poorly describes perception.

Which formulas compute T60?

The tool applies the three classic formulations. Sabine: T60 = 0.161·V/A, simple and robust for weakly absorptive rooms. Eyring – Norris: uses −S·ln(1−ᾱ), preferable at medium-high ᾱ. Millington – Sette: treats each surface separately. The constant 0.161 equals 24·ln(10)/c with c ≈ 343 m/s; air absorption 4mV is optional.

Sabine (1898):

$$T_{60}=\frac{0,161V}{A}A=\sum _i{\alpha }_i{S}_i+4mV$$

Eyring – Norris (1930):

$$T_{60}=\frac{0,161V}{-S_{\text{tot}}\ln (1-\bar{\alpha })+4mV}\bar{\alpha }=\frac{A_{\text{surf}}}{S_{\text{tot}}}$$

Millington – Sette (1947):

$$T_{60}=\frac{0,161V}{-{\sum }_i{S}_i\ln (1-{\alpha }_i)+4mV}$$

The constant 0.161 = 24·ln(10)/c with c ≈ 343 m/s. For openings (α = 1) the Millington ln(1−α) term is singular: the tool treats it as finite total absorption and flags it with an asterisk.

Which formula to use

• Sabine — simple and robust, valid for weakly absorptive rooms (ᾱ ≲ 0.2): most ordinary rooms. It is the summary value in the Result box.
• Eyring — preferable when ᾱ is medium-high (above ~0.3): it corrects the T60 overestimation Sabine produces in heavily treated rooms.
• Millington — treats each surface separately; useful when the materials differ widely, but unstable in the presence of openings (α = 1).

The three formulas converge when absorption is low and uniform, and diverge when it is high or concentrated on a few surfaces: the spread of the three curves in the chart is itself informative.

Terms

• T60 — time for the sound level to decay by 60 dB after the source stops (measured as T20/T30 and extrapolated, ISO 3382-2).
• αᵢ — absorption coefficient of the i-th material, band by band, between 0 and 1 (ISO 354).
• ᾱ — mean absorption coefficient, A_surf / S_tot.
• 4mV — air absorption; m depends on frequency, temperature and humidity.
• Diffuse sound field — the ideal condition in which acoustic energy is uniformly distributed in space and across all directions of propagation. The three T60 formulas assume it holds.
• Room modes — at low frequencies the room resonates only at discrete frequencies tied to its dimensions: the field is not diffuse and the very notion of T60 loses much of its meaning.
• Schroeder frequency — the transition frequency (f ≈ 2000·√(T60/V)) above which room modes overlap densely and the field can be called diffuse; below it, individual modes dominate.
• Surface covering treatment — carpets, wall or ceiling panels, curtains. They cover a portion of the structural surface and replace its α with their own. Example: an 8 m² carpet on a 60 m² floor → 52 m² keep the floor α, 8 m² apply the carpet α.
• Volumetric absorber treatment — seated audience, scattered seats, objects in the volume. They add absorption without occupying structural surface: their equivalent α·S area sums to the total.

What are the tool’s limitations?

The three formulas assume a diffuse sound field: in very elongated rooms, below ~30 m³ or at low frequencies (under 125 Hz, dominated by room modes) the assumption breaks down. The rectangular mode presumes a cuboid geometry. It is a demonstrative tool: compliance checks (e.g. UNI 11532-2:2020) need a model adequate to the methodology and a competent acoustic technician responsible for it.

The reliability of the estimate depends on the room geometry and the chosen mode:

Room	Mode	Reliability
Classroom, office, meeting room — regular shape	Rectangular room	Good
Canteen, gym, large regular room	Rectangular room	Good
L-shaped plan, sloped ceiling, tiered seating	Manual inventory	Fair
Concert hall, auditorium, recording studio	Out of scope	Insufficient — needs 3D modelling
Room below ~30 m³ or very elongated	Out of scope	Insufficient — non-diffuse field

• Diffuse sound field — the three formulas assume uniformly distributed energy. In very elongated or non-uniform rooms the assumption breaks down.
• Sabine overestimates T60 for very absorptive rooms (ᾱ > ~0.3): since −ln(1−ᾱ) > ᾱ, Eyring’s denominator is larger and its T60 smaller. Eyring corrects it and is preferable at medium-high ᾱ; Millington treats each surface separately but is unstable with openings (α = 1).
• Low frequencies (< 125 Hz) — statistical theory loses validity: room modes dominate.
• “Rectangular room” mode — assumes a cuboid geometry. For L-shaped plans, sloped ceilings, stages or tiered seating the approximation introduces errors not quantifiable a priori: use “Manual inventory”.
• Position of openings and treatments — the tool considers only the aggregate area, not the position on the walls. For low-frequency modal analysis position matters, but it is outside the scope of a diffuse-field calculation.
• Tabulated α coefficients — measured in a reverberation room; in situ, mounting, edges and backing cavities can give a different effective α.
• Demonstrative tool, not for expert use. For certification and compliance checks (e.g. the Italian UNI 11532-2:2020, referenced by the CAM Edilizia of DM 24/11/2025) a model adequate to the methodology is needed, and a competent acoustic technician on the ENTECA register responsible for it: “certified software” is not a category the regulation provides for.

References

• Sabine W.C. (1900) — Reverberation. The American Architect.
• Eyring C.F. (1930) — Reverberation Time in “Dead” Rooms. J. Acoust. Soc. Am.
• Millington G. (1932) — A Modified Formula for Reverberation. J. Acoust. Soc. Am.
• Fitzroy D. (1959) — Reverberation Formula Which Seems to be More Accurate with Nonuniform Distribution of Absorption. J. Acoust. Soc. Am. 31, 893.
• Arau-Puchades H. (1988) — An improved reverberation formula. Acustica 65, 163–180.
• Schroeder M.R. (1962) — Frequency-Correlation Functions of Frequency Responses in Rooms. J. Acoust. Soc. Am. 34, 1819.
• ISO 3382-2:2008 — Measurement of reverberation time in ordinary rooms.
• ISO 354:2003 — Measurement of sound absorption in a reverberation room.
• ISO 9613-1:1993 — Absorption of sound by the atmosphere.

For the full Italian regulatory framework — UNI 11532-1:2018 and 11532-2:2020, CAM Edilizia DM 24/11/2025 and the DM 18/12/1975 (technical standards for school buildings, formally superseded by Law 23/1996 yet still applied transitionally) — and for measurement techniques, see the Reverberation time wiki.