Acoustics

Reverberation time RT60

RT60 changes with frequency, volume, and use. One number for a hall type is a discussion target only. Sabine fails when absorption is high, and ISO 3382-1 is the measurement reference, not this page.

RT60 is the time for energy to fall 60 dB after the source stops, and it is not one number for the hall. A serious measurement reads it in bands, and T20 or T30 are extrapolated because noise hides the full fall. ISO 3382-1 describes that for performance spaces.

Definition

The definition ties the time to a fall, not to the operating level. A speech room and an unamplified music room of the same volume are not discussed with the same tail: speech wants the syllable to die, and music may want the energy to remain. This page sets no universal target in seconds for any hall type. A mid-band figure opened in a meeting is a discussion target until the brief and the measurement fix it.

The metric Sabine equation RT = 0.161 V / A ties the time to volume V and to total absorption A. The picture is sound: a larger volume lengthens the time if absorption per square metre stays put. The formula assumes a diffuse field and modest, distributed absorption. When panels cover a large area, or absorber gathers in the ceiling, it drifts from measurement. Eyring handles a higher mean and is still not the room.

Common mistakes

One second for the whole project — A lecture, a sports hall, and music do not share a target. One figure deletes use and volume.

Sabine after dense coverage — The formula is used where its assumption is weak, then the team is surprised by measurement. The formula’s limit is written with the result.

Reading only the mid band — The bass stays long and speech sounds coloured despite a neat mid-band figure.

Mixing RT60 with STI — A shorter time does not guarantee understanding if noise is high. The index is in IEC 60268-16.

When needed

• Before volume is frozen — Added height lengthens RT60 if absorption does not rise. The architectural decision comes before the panels.
• When the programme differs — A lecture by day and music at night in the same hall. One figure does not serve both settings, and variable acoustics may be the discussion.
• After a finish change — A mineral tile, a curtain, or a carpet changes A in a band, not in every band. Recalculate, then measure.
• Before judging the sound system — STI is affected by the tail and by noise. Raising the gain does not shorten RT60. IEC 60268-16 is a separate question.

Where used

• Performance spaces — Theatres and music rooms, where ISO 3382-1 is the reference for measuring parameters, including decay.
• Speech rooms — Lecture and conference. Discussion tends toward a shorter mid-band tail so syllables separate. The value lives in the brief, not here.
• Multipurpose halls — A compromise, or curtains and movable absorption. Each setting is measured. No setting is perfect for both programmes.
• Small rooms — Modes show, and Sabine is weaker. Measurement and a geometric model matter more than one second.

Parts

• Volume — V in the formula. More seats in the same volume tend toward a shorter time because the audience is absorption, if the surfaces stay.
• Absorption A — Areas times frequency coefficients from ISO 354 reports. Without those data A is unknown. This page does not fill the table.
• Distribution — Scattered absorption is closer to the Sabine assumption. Concentrated absorption leaves live surfaces and late paths.
• Audience and seats — Occupancy is part of the criterion. Empty and full are two figures. The report states which was measured.
• Noise that hides the fall — T20 and T30 exist because a full 60 dB may not be visible above HVAC. The method is named.

Engineering

• Frequency dependence — Bass longer than the mid band is common in rooms untreated at low frequency. An average of 500–1000 Hz hides it.
• Sabine at high absorption — When mean α is large or absorber covers most of the surface, the field is not diffuse and the result drifts from 0.161 V / A.
• Volume and use together — A larger hall tends toward a longer time for the same finish. Speech and music do not share one target because of that. There is no universal table here.
• Clarity is not the time — C50 and STI describe speech in other ways. A short RT60 with high noise can still fail understanding.
• Prediction then measurement — The calculator on this site is instructional. ISO 3382-1 is the measurement in a performance space. This page does not perform the standard.

Materials

• Live surfaces — Solid plaster, dense wood, and glass reflect. Keeping them is a decision about the tail, not a forgotten treatment.
• The absorber and its build-up — Thickness, cavity, and face change the band. Thin PET felt is not bass treatment. The coefficient comes from the report.
• Variable elements — Heavy curtains and rotating panels. Measure them open and closed. The mechanism’s name is not two figures.

Related systems

• Treatment — Panels are one way to change A. Coverage on the drawing is not the result.
• Isolation — More isolating walls can reflect more inside. Isolating the neighbour does not shorten the room’s tail.
• The sound system — A Line Array or a Point Source lights surfaces. Poor aim asks absorption to hide a mistake.
• HVAC — The noise floor decides whether T30 is valid. Lowering noise is part of being able to measure, not of elegance.

Questions

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