What Is Reverberation Time? RT60 Explained

A room can feel loud, hollow or tiring even when no single machine is unusually noisy. The problem may be reverberation time: sound reflects repeatedly from hard surfaces and remains in the room after the original source has stopped. Those overlapping reflections can blur speech, build up activity noise and create the “sound tail” people often describe as echo.

RT60 turns that experience into a measurable room-acoustics parameter. It connects four practical questions: How large is the room? How much sound absorption already exists? How effective are the proposed materials? And how much additional absorption may be needed?

Key takeaways

• Reverberation time is the time required for the sound level in a room to decay by 60 dB after the source stops.
• A larger room volume generally increases RT60 when the amount of absorption stays unchanged.
• More equivalent sound absorption area generally reduces RT60.
• Material performance is frequency-dependent, so one NRC or αw rating does not tell the whole design story.
• The Sabine formula is useful for early estimates, but a professional project should also consider geometry, frequency bands, occupancy, background noise and field measurement.

Contents

1. What is reverberation time?
2. Why does a room sound loud, empty or echoey?
3. What does RT60 mean?
4. How the Sabine formula works
5. How absorption coefficients affect RT60
6. How to reduce reverberation
7. Room-by-room design priorities
8. When a calculation is not enough
9. Frequently asked questions

What Is Reverberation Time?

Reverberation time is the time, in seconds, required for sound in an enclosed room to decay by 60 decibels after the sound source stops. It is commonly written as RT60 or T60. A longer value means sound energy remains in the room for longer; a shorter value means it decays more quickly.

The ISO 3382-2 standard covers methods for measuring reverberation time in ordinary rooms. ISO 3382-1 addresses performance spaces and other room-acoustic parameters as well as reverberation time.

Reverberation is not automatically bad. Music may benefit from a degree of persistence and spaciousness, while speech-focused rooms usually need enough decay control to prevent one syllable from masking the next. The correct result therefore depends on the room’s use, volume, occupancy and applicable project criteria—not on one universal RT60 target.

Why Does a Room Sound Loud, Empty or Echoey?

When a person speaks or a loudspeaker operates, the listener receives direct sound first. Sound then reaches the floor, ceiling, walls, glazing, furniture and other surfaces. Part of that energy is absorbed, part is scattered and part is reflected back into the room.

Rooms with extensive glass, concrete, plaster, tile or other reflective finishes can retain more reflected energy. Several symptoms may appear:

• speech sounds blurred at a distance;
• group conversations build into a general roar;
• claps, footsteps or announcements produce a noticeable tail;
• an unoccupied room sounds hard or “empty”;
• users raise their voices to compete with the existing sound field.

People often call all of these effects “echo,” but echo and reverberation are not identical. An echo is a reflection heard as a distinct repetition. Reverberation is the dense, continuing decay produced by many reflections arriving close together. A room can have excessive reverberation without one obvious discrete echo.

What Does RT60 Mean?

The “60” in RT60 refers to a 60 dB decay, not a target sound level and not complete silence. For example, if the reverberant sound field begins at 95 dB, an ideal 60 dB decay would reach 35 dB.

In real rooms, background noise often prevents a clean 60 dB decay from being captured directly. Measurements may therefore use a shorter portion of the decay curve:

• T20 is calculated from a 20 dB decay range and extrapolated to 60 dB.
• T30 is calculated from a 30 dB decay range and extrapolated to 60 dB.

The exact evaluation procedure belongs in the applicable measurement standard and test plan. As a practical illustration, MathWorks’ RT60 measurement documentation explains why T20 or T30 may be used when the available dynamic range is limited by the noise floor.

RT60 is also frequency-dependent. A report may show results at octave-band center frequencies such as 125, 250, 500, 1,000, 2,000 and 4,000 Hz. Two rooms with the same mid-frequency average can still sound different if one retains much more low-frequency energy.

How the Sabine Formula Connects Volume and Absorption

For an initial estimate in SI units, the Sabine formula is commonly written as:

RT60 = 0.161V / A

where:

• RT60 = reverberation time in seconds;
• V = room volume in cubic metres (m³);
• A = total equivalent sound absorption area in square metres, often described as metric sabins.

The total equivalent sound absorption area is estimated from all relevant surfaces and objects:

A = Σ(Sᵢ × αᵢ)

where Sᵢ is the area of each surface and αᵢ is its sound absorption coefficient at the frequency being assessed.

The relationship is straightforward:

Design change	If everything else stays the same	Expected RT60 effect
Increase room volume	Sound energy has a larger enclosure in which to persist	RT60 increases
Increase effective absorption	More reflected energy is removed at each interaction	RT60 decreases
Use a higher-performing absorber over the same area	Equivalent absorption area increases	RT60 decreases
Treat too little surface area	Total absorption changes only slightly	RT60 may remain too long

A Simple RT60 Calculation

Consider a meeting room with a volume of 240 m³ and an estimated existing equivalent absorption area of 32 m² sabins at the frequency of interest.

Existing RT60 = 0.161 × 240 / 32 = 1.21 seconds

Now assume the design adds 40 m² of treatment with an absorption coefficient of 0.75 at that frequency. For a simplified illustration, that adds:

40 × 0.75 = 30 m² sabins

The estimated total absorption becomes 62 m² sabins, giving:

Estimated treated RT60 = 0.161 × 240 / 62 = 0.62 seconds

This is a teaching example, not a project specification. A detailed calculation should account for the finish displaced by the treatment, frequency-specific test data, mounting method, air gaps, furnishings, people and the validity limits of the chosen room model.

How Material Absorption Affects Reverberation Time

An absorption coefficient describes the fraction of incident sound energy treated as absorbed under the relevant test method. A coefficient of 0.75 means that one square metre of that tested assembly contributes approximately 0.75 m² of equivalent absorption area under the calculation assumptions.

The words “tested assembly” matter. Performance can change with:

• material thickness and density;
• perforation or slat geometry;
• backing material;
• the depth of the air cavity;
• mounting type and edge exposure;
• frequency;
• laboratory method and specimen arrangement.

ISO 354 specifies reverberation-room measurement of sound absorption for wall and ceiling treatments and the equivalent absorption area of objects. ASTM International’s ASTM C423 overview likewise warns that laboratory coefficients require judgement when applied to practical rooms because real sound fields and installation areas differ from test conditions.

For procurement, do not ask only, “What is the NRC?” Ask:

1. Which test standard was used?
2. What exact product build-up was tested?
3. What mounting method and air cavity were used?
4. What are the octave- or one-third-octave-band results?
5. Does the proposed site installation match the tested assembly?

Review LEEYIN’s available acoustic test reports before selecting a product value for a calculation. Use the report that matches the proposed construction rather than borrowing a rating from a visually similar panel.

How to Reduce Reverberation

Reducing reverberation is not simply a matter of buying more panels. A reliable process moves from diagnosis to design and then verification.

1. Define the Room and Its Use

Record the room length, width, height, geometry, finishes, glazing, doors, seating, occupancy and major equipment. Then define what happens there: conversation, teaching, amplified speech, music, sport, recording or several activities.

2. Measure the Existing Condition

An impulse-response measurement can show the decay by frequency band and at multiple receiver positions. Measurement also helps separate a long reverberant decay from distinct flutter echoes, sound-system problems or noise entering from another space.

3. Set an Appropriate Target

The target should come from the room function, applicable regulations or standards, client requirements and the acoustic consultant’s design criteria. Avoid copying a number from an unrelated room type. A classroom, open office, theatre and sports arena do not serve the same acoustic purpose.

4. Estimate the Required Additional Absorption

For a preliminary Sabine-based calculation:

Required total absorption A = 0.161V / target RT60

Subtract the room’s estimated existing equivalent absorption to find the additional amount required. Then divide that requirement among suitable ceiling, wall, suspended or freestanding treatments, using frequency-specific test data.

5. Choose the Assembly, Not Just the Finish

Match the design to verified test reports, fire requirements, durability, cleanability, impact risk, humidity, appearance and installation constraints. A material that works in a protected office ceiling may not be suitable at ball-impact height in a gymnasium.

6. Place Absorption Where It Can Work

Coverage and distribution both matter. Ceiling treatment can provide broad coverage in offices and classrooms. Wall treatment may control strong lateral reflections or flutter between parallel surfaces. Suspended absorbers can add effective area where walls are interrupted or protected.

7. Verify the Completed Room

After installation, repeat measurements under a clearly documented room condition. Compare the results with the agreed criteria and record the source positions, receiver positions, occupancy state and frequency bands. Product laboratory data supports design; it does not replace field verification of the whole room.

How Reverberation Control Changes by Room Type

Auditoriums and Performance Spaces

An auditorium must balance clarity, coverage, early reflections, reverberant support, background noise and the sound system. One average RT value cannot describe all of those conditions. See the Guangdong Baiyun University Auditorium acoustic design case for an example of design coordination followed by multi-position testing.

Offices and Meeting Areas

In offices, excessive reverberation can increase activity-noise build-up and reduce conversational comfort. Absorption should be coordinated with workstation layout, speech privacy, partitions, background sound and building services. The Foshan Lishui Service Center office acoustics case shows why public counters, training rooms and quiet work areas need different acoustic priorities.

Schools and Learning Spaces

Teaching spaces normally prioritize clear speech, but the appropriate criterion depends on room type and learner needs. The UK Department for Education’s BB93 guidance is one example of a framework that treats reverberation time as one part of school acoustic performance. Local requirements must govern the actual project.

Gymnasiums and Sports Arenas

Large volumes, hard impact-resistant finishes, long throw distances and loud crowds make sports venues challenging. Absorption often has to be integrated overhead or outside impact zones while coordinating structure, lighting, scoreboards and fire safety. The Shenzhen Bay Gymnasium acoustic treatment case illustrates an overhead spatial-absorber strategy for a large multipurpose venue.

When the Sabine Formula Is Not Enough

The Sabine formula is valuable because it makes the relationship between volume and total absorption easy to understand. It is still an estimate based on simplifying assumptions.

Its accuracy can reduce when:

• the room is very small, irregular or strongly coupled to another volume;
• absorption is highly uneven or concentrated on one surface;
• the average absorption is high;
• the sound field is not sufficiently diffuse;
• strong discrete reflections, flutter echoes or focusing dominate;
• low-frequency room modes are important;
• occupancy and movable elements change substantially;
• background noise limits the usable decay range.

In these conditions, an acoustic consultant may use additional prediction methods, room simulation, impulse-response analysis and site measurements. The goal is not to produce the most complicated model; it is to use a method appropriate to the decision and risk.

What to Provide for a Project Acoustic Assessment

To make an initial review more useful, prepare:

• architectural plans, sections and room dimensions;
• intended uses and occupancy;
• existing and proposed surface finishes;
• photos or a 3D model;
• the current acoustic complaint;
• available RT60, T20 or T30 measurements;
• target criteria or applicable standard;
• MEP and background-noise information;
• fire, impact, cleaning and appearance requirements;
• programme, access and installation constraints.

With these inputs, the discussion can move from “the room sounds too echoey” to a defined assessment of room volume, frequency-dependent decay, existing absorption and the additional treatment required.

Frequently Asked Questions

Turn an “Echoey” Room Into a Measurable Brief

Reverberation time gives a shared language to clients, designers, contractors and acoustic specialists. RT60 explains the symptom; the Sabine formula shows why room volume and equivalent sound absorption area matter; material test reports help quantify a proposed assembly; and field testing confirms how the completed room performs.

If you are planning an auditorium, office, school, gymnasium or other reverberant space, request a project acoustic assessment with the room dimensions, intended use, finishes and any available measurements. LEEYIN can help review the acoustic problem, material evidence, treatment strategy and verification plan before the installation is finalized.

---

Sources

• International Organization for Standardization. ISO 3382-1:2009—Measurement of room acoustic parameters in performance spaces. Current edition status confirmed by ISO; accessed August 3, 2026.
• International Organization for Standardization. ISO 3382-2:2008—Reverberation time in ordinary rooms. Current edition status confirmed by ISO; accessed August 3, 2026.
• International Organization for Standardization. ISO 354:2003—Measurement of sound absorption in a reverberation room. Edition confirmed in 2024; accessed August 3, 2026.
• ASTM International. ASTM C423-23e1—Sound absorption and sound absorption coefficients by the reverberation room method. Accessed August 3, 2026.
• MathWorks. RT60: Estimate decay time of room impulse response. Accessed August 3, 2026.
• UK Department for Education. BB93: Acoustic design of schools—performance standards. Accessed August 3, 2026.