What Is Reverberation Time? RT60 Explained

Reverberation time is the time required for sound in an enclosure to decay by 60 decibels after the source stops. It is expressed in seconds and commonly written as T60 or RT60. A longer value means sound persists for longer; a shorter value means the room absorbs or loses sound energy more quickly.

RT60 turns complaints such as “this room is echoey,” “speech is blurred,” or “the restaurant becomes unbearably loud” into a measurable room-acoustic problem. However, there is no universal ideal value. The right reverberation time depends on room volume, frequency, occupancy, and whether the space is intended for speech, amplified sound, cinema, rehearsal, or music performance.

Key takeaways

• RT60 represents a 60 dB decay, equivalent to the time-mean-square sound pressure falling to one-millionth of its initial value.
• Reverberation time is frequency-dependent and should be reviewed as a curve or band table, not only as one average.
• EDT, T20, and T30 use different parts of the decay curve; T20 and T30 are extrapolated to represent a 60 dB decay.
• In SI units, the Sabine formula is T60 = 0.161V/A, where V is room volume in m³ and A is equivalent sound absorption area in m² sabins.
• Sabine is an early-design model, not a guarantee. Its assumptions weaken when absorption is high, unevenly distributed, or the room is non-diffuse.
• Speech-focused rooms generally need shorter decay than symphonic music spaces, but every project target must come from the applicable brief or standard.
• Acoustic panels reduce reverberation only through their tested absorption, installed area, mounting, position, and frequency response.
• Reverberation control does not replace sound insulation, HVAC-noise control, or sound-system design.

Table of Contents

1. What is reverberation time?
2. How Sabine established RT60
3. Reverberation vs echo
4. What does RT60 mean?
5. Why reverberation time changes with frequency
6. EDT vs T20 vs T30 vs T60
7. How is reverberation time measured?
8. The Sabine reverberation time formula
9. Worked RT60 and panel-area example
10. When the Sabine formula is not enough
11. What is a good reverberation time?
12. How acoustic materials change RT60
13. How to reduce excessive reverberation
14. Project measurement and procurement checklist
15. Common reverberation-time mistakes
16. Frequently asked questions

What Is Reverberation Time?

The Acoustical Society of America’s terminology database defines reverberation time for a stated frequency or band as the time required for the level of time-mean-square sound pressure in an enclosure to decrease by 60 dB after the source stops. Its unit is the second, and its symbol is T60.

In everyday terms, reverberation time measures how long a room holds sound. A voice reaches a listener directly, but it also strikes the ceiling, floor, walls, glazing, furniture, people, and other surfaces. Part of the energy is absorbed, part is scattered, and part returns as reflections. After the source stops, those reflections continue briefly and form a decay tail.

Reflected sound can support music but interfere with speech when one syllable masks the next. Reverberation is therefore not automatically “bad”; the design question is whether decay is appropriate, balanced by frequency, and sufficiently consistent for the intended activity.

How Sabine Established RT60

Modern architectural acoustics is closely associated with Wallace Clement Sabine. In 1895, Harvard President Charles William Eliot asked the young physics instructor to improve the poor acoustics of a lecture room in the newly built Fogg Art Museum. Harvard’s own Department of Physics history describes this assignment as the beginning of a new science.

Sabine investigated how room volume and sound-absorbing materials influenced decay. His work established the practical relationship now expressed by the Sabine reverberation equation. The history matters because the formula was not invented as an abstract mathematical exercise; it emerged from a real client problem—speech was difficult to understand in a reflective lecture hall.

Reverberation vs Echo

Reverberation and echo both involve reflected sound, but they are not interchangeable.

Acoustic effect	What the listener perceives	Typical cause	Typical investigation
Reverberation	A dense sound tail or general persistence after the source stops	Many closely spaced reflections throughout the room	Decay time by frequency and position
Echo	A distinct repetition separated from the original sound	A strong, delayed reflection from a distant or focused surface	Reflection path, delay, direction, and surface geometry
Flutter echo	A rapid ringing or “zing” between surfaces	Repeated reflections between large parallel hard surfaces	Parallel geometry and lateral treatment
Background noise	Continuous sound even when the test source is off	HVAC, equipment, traffic, services, occupants, or masking	Noise spectrum, source-path-receiver analysis
Sound transmission	Sound arriving from another room or outdoors	Weak boundary, opening, penetration, or flanking path	STC/Rw or field isolation analysis—not RT60 alone

A room can have excessive reverberation without a clearly separate echo. It can also have a reasonable mid-frequency RT while one wall produces a strong discrete reflection. Listening, geometry review, impulse-response analysis, and decay measurements should be interpreted together.

What Does RT60 Mean?

The “60” in RT60 describes a 60 dB reduction, not a target sound level and not the moment at which sound becomes absolutely zero. If the decay starts from a reference level of 95 dB, a full 60 dB decay would reach 35 dB.

Because decibels are logarithmic, a 60 dB reduction in time-mean-square sound pressure corresponds to the sound-energy-related quantity falling to one-millionth of its initial value. Sound pressure amplitude falls to one-thousandth. This is why the definition can be consistent even though the room’s background noise often prevents the complete decay from being directly observed.

RT60 also does not tell you how loud the original source was. A quiet source and a loud source can produce the same decay rate in a linear, time-invariant room if there is enough dynamic range to measure them reliably.

RT is a property of the room condition

The result belongs to a stated condition:

• Empty or occupied;
• Curtains open or closed;
• Retractable seating deployed or stored;
• Doors open or closed;
• Stage shell installed or removed;
• HVAC operating or off;
• Furniture present or absent.

Always state the measurement condition alongside the number because seating, occupants, curtains, and movable elements can materially change the decay.

Why Reverberation Time Changes With Frequency

Materials do not absorb all frequencies equally. A thin porous panel may perform strongly at mid and high frequencies but provide less absorption at 125 Hz. Upholstered seating, curtains, perforated systems, membrane absorbers, cavities, and the room’s air contribute differently across the spectrum.

For that reason, a professional report may show RT at octave-band center frequencies such as 125, 250, 500, 1,000, 2,000, and 4,000 Hz. Performance-space assessments may use additional bands and parameters depending on the standard and purpose.

The user-supplied training diagram illustrates a common downward-sloping RT curve: longer decay at low frequencies and shorter decay through the mid/high range. That shape is not automatically “good.” A balanced target curve depends on room use, and excessive low-frequency persistence can remain troublesome even when a mid-frequency average looks acceptable.

Why one average can mislead

Two rooms can share a mid-frequency RT of 0.8 seconds while behaving very differently at 125 or 4,000 Hz. Examine the curve rather than only one average.

EDT vs T20 vs T30 vs T60

A real room rarely offers a clean, measurable 60 dB decay above its background-noise floor. Acoustic standards therefore allow shorter portions of the decay curve to be evaluated and extrapolated.

Parameter	Decay range commonly evaluated	Extrapolation	What it can indicate
EDT	0 to −10 dB	Multiplied to represent 60 dB	Early decay, often related to the listener’s initial impression
T20	−5 to −25 dB	20 dB range multiplied by 3	Reverberation estimate where available dynamic range is limited
T30	−5 to −35 dB	30 dB range multiplied by 2	Reverberation estimate using a longer measured decay segment
T60	Conceptual full 60 dB decay	No extrapolation if directly measurable	Standard definition of reverberation time

These are not four unrelated room properties. They are ways of describing decay using different sections of the response. Differences among them can still reveal non-linear or non-uniform decay, coupled volumes, noise-floor interference, or other room behavior requiring interpretation.

The top-ranking US search result for the query emphasizes that T30 needs enough separation between the maximum response and background noise. A practical measurement plan should preserve a suitable margin below the evaluated decay range rather than allowing the curve to disappear into the noise floor.

How Is Reverberation Time Measured?

ISO 3382-2:2008, confirmed current by ISO in 2022, specifies methods for reverberation-time measurement in ordinary rooms. ISO 3382-1:2009 covers performance spaces and includes other room-acoustic parameters derived from impulse responses.

Two common measurement approaches are:

1. Interrupted-noise method: A suitable broadband or band-limited noise field is established and then stopped. The decay is recorded and evaluated.
2. Integrated impulse-response method: The room impulse response is captured using an appropriate excitation and processed to obtain the decay curve.

The test requires more than a clap and a phone timer. Reliable results depend on suitable source directivity and level, calibrated measurement equipment, source and microphone positions, frequency-band filters, background noise, averaging, room condition, and data quality.

Why multiple positions matter

RT can vary spatially, especially in large, irregular, coupled, or non-diffuse rooms. A single microphone point may sit near a strong mode, local reflection, balcony edge, stage opening, or absorptive surface. The number and distribution of positions should match the standard, room type, and purpose of the assessment.

For performance spaces, reverberation time also sits alongside other parameters. Clarity, definition, strength, early lateral energy, stage support, background noise, and spatial variation may matter. A hall is not fully described by RT60 alone.

The Sabine Reverberation Time Formula

For a preliminary estimate in SI units, the Sabine equation is:

T60 = 0.161V / A

where:

• T60 = reverberation time in seconds;
• V = room volume in cubic metres, m³;
• A = total equivalent sound absorption area in square metres, m² sabins.

Equivalent sound absorption area is calculated by summing the contribution from surfaces and relevant objects:

A = Σ(Sᵢ × αᵢ)

where Sᵢ is the area of surface i in m² and αᵢ is its absorption coefficient at the frequency being evaluated.

If a room has a 100 m² ceiling with α = 0.70 at 1,000 Hz, that ceiling contributes approximately 70 m² sabins at 1,000 Hz under the model assumptions. It does not contribute the same value automatically at 125 Hz, because absorption is frequency-dependent.

What the equation tells us

Design change	All else equal	Expected effect on T60
Increase room volume	More volume per unit absorption	Longer RT
Add effective absorption	Larger A	Shorter RT
Remove absorptive seating or curtains	Smaller A	Longer RT
Add a high-absorption finish over too little area	Small increase in A	Limited change
Use a product in a different mounting from its test	Unknown change in band coefficients	Prediction becomes unreliable

The constant 0.161 applies to a common SI form under ordinary atmospheric assumptions. Other unit systems use a different constant, and large rooms may require an air-absorption term.

Worked RT60 and Panel-Area Example

Consider a meeting room with:

• Volume V = 300 m³;
• Existing equivalent absorption A₁ = 40 m² sabins at 1,000 Hz;
• Target reverberation time T₂ = 0.70 s at that frequency.

Step 1: Estimate the existing reverberation time

T₁ = 0.161 × 300 / 40

T₁ = 1.21 s

Step 2: Calculate the total absorption required for the target

Rearrange the Sabine formula:

A₂ = 0.161V / T₂

A₂ = 0.161 × 300 / 0.70 = 69.0 m² sabins

The room therefore needs an estimated net increase of:

ΔA = 69.0 − 40.0 = 29.0 m² sabins

Step 3: Account for the surface being replaced

Assume the proposed panel has α = 0.80 at 1,000 Hz and replaces a hard finish with α = 0.10. The net absorption gain is:

0.80 − 0.10 = 0.70 m² sabins per m² of treatment

Required treatment area:

29.0 / 0.70 = 41.4 m²

This example is intentionally simplified. A real design repeats the calculation by relevant frequency band and accounts for actual tested mounting, doors, glazing, floor and ceiling finishes, furniture, occupants, air absorption where relevant, geometry, and the suitability of the Sabine model.

When the Sabine Formula Is Not Enough

Sabine assumes a sufficiently diffuse sound field and treats absorption through an equivalent-area model. The user-supplied training material gives average absorption coefficient below about 0.20 as a practical limitation for straightforward use. This should be treated as a rule of thumb—not a universal pass/fail boundary—because room geometry and absorption distribution also matter.

Accuracy can reduce when:

• Average absorption is high;
• Absorption is concentrated on one surface;
• The room is long, flat, irregular, or strongly coupled to another volume;
• Large reflectors, concave surfaces, balconies, or openings create non-diffuse behavior;
• Low-frequency modes dominate;
• The decay curve has more than one slope;
• Air absorption matters in a large volume;
• Occupancy and variable elements change substantially.

The Eyring alternative

For a diffuse room with more substantial average absorption, the Eyring equation is often written in SI form as:

T60 = 0.161V / [−S ln(1 − ᾱ)]

where S is total surface area and ᾱ is the average absorption coefficient. At low values of ᾱ, the Eyring and Sabine estimates approach each other. Neither formula captures every geometrical reflection, room mode, focusing effect, or coupled-volume decay.

For critical halls, studios, worship spaces, large atria, complex multipurpose rooms, or projects with contractual acceptance criteria, combine calculation with appropriate simulation and field measurement.

What Is a Good Reverberation Time?

There is no universally good RT60. A suitable value supports the intended activity without excessive masking, loss of clarity, lack of musical support, or an unnaturally dead acoustic character.

Leeyin’s Chinese teaching charts demonstrate a useful and general approach to advancing teaching:

Functional emphasis	Indicative teaching range	Design tendency
Cinema	About 0.5–0.8 s	Shorter decay supports amplified clarity
Reports, meetings, and speech	About 0.8–1.3 s	Prioritize intelligibility
Drama and traditional theatre	About 1.0–1.5 s	Balance speech clarity and theatrical support
Dance, opera, festival, and folk performance	About 1.3–1.7 s	More persistence may support performance
Early classical and symphonic music	About 1.7–2.2 s	Longer decay may support blend and envelopment

These ranges are educational, not universal specification values. Volume, occupancy, amplification, musical style, stage design, seating, frequency, and local standards can shift the target. A small rehearsal room and a large symphony hall should not receive the same criterion merely because both contain music.

Example of standard-linked classroom targets

The UK Department for Education’s current Building Bulletin 93 provides maximum mid-frequency reverberation-time criteria tied to room type and whether the project is new build or refurbishment. Selected new-build examples include:

School room type	BB93 new-build maximum mid-frequency RT
Nursery and primary classroom/general teaching room	≤ 0.6 s
Secondary classroom/general teaching room	≤ 0.8 s
Open-plan teaching area	≤ 0.5 s, subject to the standard’s additional provisions
Small lecture room, fewer than 50 people	≤ 0.8 s
Large lecture room, more than 50 people	≤ 1.0 s
Performance/recital room	1.0–1.5 s

These values demonstrate why “best RT60” cannot be answered without naming the room and governing document. They are not automatically applicable outside the BB93 project scope.

Performance spaces need more than RT

ISO 3382-1 specifies measurement of reverberation time and other parameters in performance spaces. A concert hall with a target RT may still fail if clarity, strength, spatial impression, stage support, uniformity, or background noise is unsuitable.

ISO 23591:2021 further differentiates music rehearsal criteria for amplified music, quiet acoustic music, and loud acoustic music, and includes room-volume and dimensional considerations. This is another reason not to use one generic “music room” RT.

Open-plan offices also need more than RT

ISO 3382-3:2022 evaluates open-plan offices using spatial speech-decay and intelligibility-related parameters. ISO explains that reverberation time alone is not sufficient for these spaces; workstation screens, layout, ceiling absorption, background sound, and speech propagation all affect distraction and privacy.

How Acoustic Materials Change RT60

Acoustic absorbers reduce reverberation by increasing the room’s equivalent sound absorption area. Candidate systems include acoustic ceilings, wall panels, suspended clouds, baffles, spatial absorbers, perforated systems with backing, upholstered elements, and other tested assemblies.

Their contribution depends on five linked variables:

1. Frequency response: Use the coefficient at the frequency being calculated.
2. Installed area: A high-performing product over a tiny area may not materially change the room.
3. Mounting: Air cavity, backing, suspension, and edge exposure can change absorption.
4. Net gain: Subtract the absorption of the finish being covered or removed.
5. Distribution: Placement affects reflections, flutter, and spatial consistency even when total sabins are unchanged.

NRC and αw are not room RT values

NRC and αw summarize product or assembly absorption. RT60 describes the decay in the completed room. A panel with NRC 0.90 does not mean the room will have RT 0.90 seconds, nor does it guarantee a fixed reduction in seconds.

Review Understanding Noise Reduction Coefficient (NRC) Rating and What Does αw (Alpha-w) Mean? before transferring laboratory values into room calculations.

ISO 354:2003 and ASTM C423-23e1 cover reverberation-room absorption measurement under their respective systems. Always match the proposed product thickness, backing, cavity, mounting, and layout to the applicable report.

How to Reduce Excessive Reverberation

Step 1: Define the acoustic problem

Record the room use, volume, geometry, finishes, occupancy, source types, and complaint. Determine whether the issue is reverberation, a discrete echo, background noise, sound-system coverage, or sound entering from another space.

Step 2: Measure the existing decay

Measure RT by frequency and across representative positions. Document room condition and background noise. In an early concept stage where measurement is impossible, build a frequency-specific absorption schedule and state the uncertainty.

Step 3: Set a defensible target

Use the applicable code, standard, client brief, venue requirements, or acoustic consultant’s criterion. Define frequency averaging, occupancy state, tolerance, and verification method—not only one number.

Step 4: Calculate additional absorption

Estimate the required total A from the target and volume, subtract existing equivalent absorption, and convert the net requirement into treatment area using band-specific tested data. Include the finish displaced by the new treatment.

Step 5: Select the complete assembly

Check acoustic performance together with fire classification, emissions, durability, cleanability, humidity resistance, impact risk, suspension safety, access, appearance, and maintenance. The visually preferred surface is useful only if the installed build-up delivers the design contribution.

Step 6: Coordinate placement

Ceiling absorption can provide broad coverage in offices, classrooms, restaurants, and halls. Wall treatment may control strong lateral reflections and flutter. Suspended elements can add effective exposed area where ceiling services or heritage finishes limit conventional coverage.

Step 7: Verify the completed room

Repeat measurements using the agreed method and room state. Compare band results and spatial variation with the acceptance criteria. If the result misses the target, diagnose whether the cause is product substitution, insufficient area, mounting change, construction gaps, furnishings, occupancy assumptions, or non-diffuse room behavior.

For preliminary quantity planning, see How Many Acoustic Panels Do I Need?. For projects involving guestrooms and public spaces, How Can Hotels Control Noise in Guest Rooms and Public Areas? helps distinguish reverberation from sound insulation and impact noise.

Project Measurement and Procurement Checklist

Room information

• Plans, sections, room dimensions, and total volume;
• Intended use, occupancy, seating, and operational layouts;
• Existing and proposed surface areas and finishes;
• Curtains, movable partitions, retractable seating, stage shells, and other variable elements;
• HVAC and background-noise conditions;
• Photos, 3D model, and description of the acoustic complaint.

Acoustic criteria

• Governing standard or client brief;
• Target bands and permitted averaging;
• Empty, occupied, or other reference condition;
• Required parameters: EDT, T20, T30, RT, STI, clarity, or others;
• Number and distribution of source/receiver positions;
• Acceptance tolerance and post-installation test procedure.

Product evidence

• Complete laboratory report and test standard;
• Product name, thickness, density, finish, pattern, and batch-relevant construction;
• Backing, air cavity, frame, suspension, and edge condition;
• Octave- or one-third-octave-band absorption data;
• Fire, emissions, durability, impact, moisture, cleaning, and installation documentation;
• Confirmation that the proposed build-up matches the tested assembly.

Leeyin’s test-report library can support product comparison, but the selected report must match the exact system being proposed.

Common Reverberation-Time Mistakes

Mistake 1: Using one “ideal RT” for every room

Room purpose, volume, occupancy, frequency, and applicable standards determine the target. A value appropriate for a classroom may be unsuitable for opera or symphonic music.

Mistake 2: Treating a phone clap test as final evidence

A rough recording may reveal an obvious tail, but compliance requires suitable excitation, calibration, band analysis, positions, dynamic range, and documentation.

Mistake 3: Calculating with NRC as if it were every band coefficient

NRC averages four mid-frequency absorption coefficients. It cannot represent 125 Hz performance or substitute for the full spectrum in a band-by-band RT calculation.

Mistake 4: Forgetting the original surface

Covering a hard finish adds the difference between new and old absorption, not the new coefficient multiplied by area without adjustment.

Mistake 5: Ignoring mounting conditions

A panel tested with an air gap or backing may not perform identically when directly bonded to a wall. Use the tested construction or obtain relevant evidence.

Mistake 6: Expecting absorption to solve sound transmission

Acoustic panels can reduce reflected sound in the treated room. They do not automatically stop voices, music, traffic, or footsteps passing through walls and floors.

Mistake 7: Verifying only one position or frequency

Local variations and unbalanced low-frequency decay can disappear inside one average. Review representative positions and the bands that matter to the activity.

Frequently Asked Questions

Turn an Echoey Room Into a Measurable Acoustic Brief

Reverberation time gives clients, architects, contractors, and acoustic specialists a common language for sound decay. RT60 defines the outcome; EDT, T20, and T30 describe how decay can be evaluated; the Sabine formula connects room volume with equivalent absorption; and field measurement verifies the completed space.

The most important decision is not “Which panel has the highest NRC?” It is “What decay does this room need, at which frequencies and condition, and how much verified absorption must the complete design add?”

To compare acoustic wall and ceiling products, send Leeyin Acoustic the room dimensions, intended use, existing finishes, target reverberation time, frequency requirements, proposed mounting, and available measurements through the project contact page.

Authoritative Sources

• Acoustical Society of America. Reverberation time terminology definition. Accessed September 2, 2026.
• Harvard University Department of Physics. History of the Department. Wallace C. Sabine’s 1895 Fogg lecture-room assignment; accessed September 2, 2026.
• International Organization for Standardization. ISO 3382-1:2009—Measurement of room acoustic parameters in performance spaces. Current edition status accessed September 2, 2026.
• International Organization for Standardization. ISO 3382-2:2008—Reverberation time in ordinary rooms. Confirmed current in 2022; accessed September 2, 2026.
• International Organization for Standardization. ISO 3382-3:2022—Open-plan offices. Accessed September 2, 2026.
• International Organization for Standardization. ISO 23591:2021—Acoustic quality criteria for music rehearsal rooms and spaces. Accessed September 2, 2026.
• UK Department for Education. Building Bulletin 93: Acoustic design of schools—performance standards. Accessed September 2, 2026.
• International Organization for Standardization. ISO 354:2003—Measurement of sound absorption in a reverberation room. Accessed September 2, 2026.
• ASTM International. ASTM C423-23e1—Sound absorption and sound absorption coefficients by the reverberation room method. Accessed September 2, 2026.