Reverberation in Rooms: Causes, Effects and Practical Control Methods

Have you ever struggled to understand a speaker in a large hall, or found a restaurant so loud you had to shout across the table? The problem is often not the volume of the sound itself but how long it lingers. That lingering is reverberation.

Reverberation is one of the most important factors in room acoustics. A little gives a room warmth and life; too much turns speech into a blur and raises noise levels. For mosques, classrooms, offices, restaurants, hotels, healthcare facilities, auditoriums and studios across Saudi Arabia, controlling reverberation is often the single biggest acoustic improvement available.

AcouStick controls reverberation through acoustic modelling, RT60 prediction and absorption sizing, using KSA-manufactured panels, ceilings, baffles, plasters and diffusers.

This guide explains what reverberation is, what causes it, how it affects people, how it is measured and predicted, and the practical methods used to control it.

What Is Reverberation?

Reverberation is the persistence of sound in a room after the source has stopped, caused by many reflections from walls, floors, ceilings and furniture combining and slowly decaying.

It is measured as reverberation time. ISO 3382-2 defines reverberation time as the time required for the space-averaged sound energy in an enclosure to decrease by 60 dB after the source stops — often written as RT60.

• T20 — evaluated from 5 dB to 25 dB below the initial level and extrapolated to 60 dB.

• T30 — evaluated from 5 dB to 35 dB below the initial level and extrapolated to 60 dB.

What Causes Excessive Reverberation?

1. Large Room Volume

The Sabine equation, derived by Wallace Sabine from measurements at Harvard University, gives T = 0.161V / A in metric units. Reverberation time increases with room volume (V).

2. Too Little Absorption

In the same equation, reverberation time falls as total absorption (A) increases. Rooms with few absorptive surfaces reverberate for longer.

3. Hard, Reflective Finishes

Concrete, stone, marble, tiles, glass and exposed soffits reflect most of the sound that reaches them.

4. Parallel Surfaces

Acoustic Sciences Corporation describes flutter echo as a pulse reflecting rapidly back and forth between parallel walls — a distinctive form of reverberant ringing.

5. Domes and Curved Surfaces

Concave shapes can concentrate reflections, adding to reverberant problems in large architectural spaces.

6. Minimal Furnishing

Empty or sparsely furnished rooms have less incidental absorption from furniture, curtains and people.

Effects of Excessive Reverberation

• Poor speech clarity — late reflections overlap with new syllables, making words harder to distinguish.

• Higher noise levels — reverberant sound builds up; NRC Canada notes that reverberant level falls by 3 dB for each doubling of absorption, so under-treated rooms run louder.

• The "café effect" — as background rises, people speak louder, raising the level further.

• Listening fatigue — more effort is needed to follow conversation.

• Poor music quality — notes blur together in rooms with excessive reverberation for their use.

• Learning difficulties — classroom acoustics standards set strict reverberation limits for this reason.

Is Reverberation Always Bad?

No. The right amount depends on the room's purpose. Published standards show the range:

• Classrooms under 10,000 ft³ (≈ 283 m³) — maximum 0.6 s, unoccupied and furnished (ANSI/ASA S12.60).

• Classrooms of 10,000–20,000 ft³ — maximum 0.7 s (ANSI/ASA S12.60).

• Professional listening and control rooms — a mean of 0.2–0.4 s (EBU Tech 3276).

Rooms for music, speech and critical listening are designed to different targets, and over-treating a room can make it feel dead and unnatural.

How Reverberation Is Predicted

The Sabine equation is the starting point for design:

1. Calculate room volume — length × width × height for simple rooms.

2. Estimate existing absorption — surface areas multiplied by their absorption coefficients.

3. Calculate reverberation time — T = 0.161V / A.

4. Compare with the target — from the relevant standard or design guide.

5. Size the treatment — add absorption until the target is met.

ScienceDirect's overview notes the Sabine equation works best in relatively live rooms with low average absorption; more detailed methods may be used for highly absorptive or irregular spaces.

How Reverberation Is Measured

In existing rooms, reverberation time is measured to ISO 3382-2 using a sound source and microphone positions. The standard defines survey, engineering and precision methods with increasing numbers of source–microphone combinations and accuracy.

Practical Methods to Control Reverberation

1. Add Absorptive Ceiling Treatment

The ceiling is often the largest surface available. The GSA recommends a minimum NRC of 0.9 across open plan office ceilings.

2. Add Wall Panels

Fabric, PET, wooden and perforated panels on walls control reflections and add absorption at listening height.

3. Use Suspended Baffles and Clouds

Ideal for high or exposed ceilings, atriums, sports halls and industrial spaces.

4. Use Seamless Acoustic Plaster or Spray

Where architecture requires a continuous finish.

5. Treat Parallel Surfaces

Treating one of two facing hard surfaces with absorption or scattering breaks up flutter echo.

6. Add Low-Frequency Absorption

Thin panels have limited effect at low frequencies; membrane absorbers near corners, as recommended in Sound On Sound's control room guide, or thicker absorbers address bass reverberation.

7. Balance With Diffusion

Diffusers scatter reflections rather than removing them. Cox and D'Antonio describe how Schroeder diffusers, invented in the 1970s, allow scattering surfaces with defined performance — useful where a room should stay lively.

8. Consider Furnishings

Carpets, curtains and upholstered furniture add incidental absorption.

Choosing Absorptive Products

• Performance — NRC, SAA or αw from ASTM C423 or ISO 11654.

• Mounting — ASTM E795 notes absorption depends on how the material is mounted.

• Fire classification — ASTM E84 Class A is a flame spread index of 0–25 (Class B 26–75, Class C 76–200).

• Durability and hygiene — important in healthcare, education and hospitality.

• Appearance — fabric, PET, timber, perforated metal, plaster.

Reverberation by Building Type

• Mosques — large, reflective prayer halls; clear speech is the priority.

• Classrooms — strict limits for learning.

• Offices and meeting rooms — speech clarity and video calls.

• Restaurants — comfortable conversation levels.

• Healthcare — comfort with cleanable finishes.

• Auditoriums — balance of clarity and fullness.

• Sports halls and atriums — very large volumes needing baffles.

• Studios — short, controlled reverberation.

Reverberation in Saudi Buildings

Saudi architecture frequently features large volumes, domes, high ceilings and hard finishes such as stone and marble — beautiful, but highly reverberant. Integrating absorption into ceilings, walls and decorative features from the design stage allows the right reverberation time without compromising architectural character.

Common Mistakes

Mistake 1: Treating Without a Target

Without a reverberation target, rooms end up under- or over-treated.

Mistake 2: Only Treating High Frequencies

Thin panels can leave bass reverberation untouched.

Mistake 3: Concentrating Treatment in One Place

Distribution matters; parallel surfaces need attention.

Mistake 4: Expecting Absorption to Soundproof

Reverberation control does not stop sound passing between rooms.

Mistake 5: Ignoring Fire Requirements

Acoustic finishes must meet the space's fire classification.

Checklist

• What is the room used for?

• What is the target reverberation time?

• What is the current reverberation time (calculated or measured)?

• How much absorption is needed, and where?

• Are low frequencies addressed?

• Do products meet fire and durability needs?

• Will the room be measured after treatment?

How AcouStick Controls Reverberation

AcouStick's engineering workflow includes RT60 prediction and absorption sizing, then specifications, shop drawings and site support. Its KSA-manufactured range includes:

• Wall treatment — fabric, PET, wooden and perforated panels

• Ceilings — acoustic tiles, clouds and baffles

• Seamless finishes — acoustic sprays and plasters

• High-performance absorption — wool panels

• Diffusion — wooden diffusers

Frequently Asked Questions

1. What is reverberation?

The persistence of sound in a room after the source stops, caused by multiple reflections.

2. What is RT60?

The time for sound energy to decay by 60 dB, as defined in ISO 3382-2.

3. What causes too much reverberation?

Large volume, hard surfaces, too little absorption, parallel walls and domes.

4. How do you reduce reverberation?

Add absorption to ceilings and walls, use baffles in large spaces, treat parallel surfaces and balance with diffusion.

5. What is a good reverberation time?

It depends on use — for example up to 0.6 s for small classrooms (ANSI/ASA S12.60) and 0.2–0.4 s for listening rooms (EBU Tech 3276).

6. Does reducing reverberation make a room quieter?

It lowers reverberant build-up — about 3 dB per doubling of absorption — but not direct sound near the source.

Final Takeaway

Reverberation is caused by volume, hard surfaces and too little absorption, and it affects speech clarity, noise levels and comfort. Setting the right target, predicting with Sabine, measuring with ISO 3382-2 and adding the right absorption in the right places brings it under control.

Is reverberation affecting a room in your Saudi project? Share your drawings with AcouStick for an RT60 assessment and treatment recommendation.

If a room in your project is echoey or noisy, AcouStick can help predict its reverberation and specify the treatment to fix it.

Email: info@acoustick.co | Call: +966 51 130 809 | Web: www.acoustick.co

REFERENCES

• International Organization for Standardization. ISO 3382-2:2008, "Acoustics — Measurement of room acoustic parameters — Part 2: Reverberation time in ordinary rooms" (preview edition, iTeh Standards).

• ScienceDirect Topics. "Sabine Equation" — metric form T = 0.161V/A and the equation's origin: sciencedirect.com/topics/engineering/sabine-equation

• Acoustic Sciences Corporation. "Flutter Echo / Flutter Tones," ASC Newsletter: acousticsciences.com/asc-newsletter/flutter-echo-flutter-tones

• Warnock, A.C.C. "Reverberant Noise Control in Rooms Using Sound Absorbing Materials." Building Research Note, National Research Council Canada, June 1980: nrc-publications.canada.ca

• ANSI/ASA S12.60-2010/Part 1 (R2020), "Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools" — limits as summarised by Armstrong World Industries, "Classroom Acoustics and ANSI Standard S12.60": armstrong.com/en-us/articles/classroom-acoustics-ansi-standard.html

• European Broadcasting Union. EBU Tech 3276, "Listening conditions for the assessment of sound programme material: monophonic and two-channel stereophonic," 2nd edition, May 1998: tech.ebu.ch/docs/tech/tech3276.pdf

• U.S. General Services Administration. "Sound Matters: How to Achieve Acoustic Comfort in the Contemporary Office." GSA Public Buildings Service, December 2011 / January 2012: gsa.gov

• Sound On Sound. "The SOS Guide to Control Room Design" — LEDE (1980), Reflection-Free Zone (D'Antonio & Konnert, 1984), first-reflection mirror method, symmetry and bass trapping: soundonsound.com/techniques/sos-guide-control-room-design

• Cox, T.J., & D'Antonio, P. "Schroeder Diffusers: A Review." Building Acoustics, Vol. 10, No. 1, 2003, DOI 10.1260/135101003765184799.

• ASTM International. ASTM C423, "Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method" — NRC and SAA definitions as summarised by Intertek: intertek.com/building/standards/astm-c423

• International Organization for Standardization. ISO 11654:1997, "Acoustics — Sound absorbers for use in buildings — Rating of sound absorption" — αw derivation and Annex B absorption classes (BS EN ISO 11654:1997 text).

• ASTM International. ASTM E795-16, "Standard Practices for Mounting Test Specimens During Sound Absorption Tests" — scope statement: store.astm.org/e0795-16.html

• ASTM International. ASTM E84, "Standard Test Method for Surface Burning Characteristics of Building Materials" — Class A/B/C thresholds as summarised by Intertek: intertek.com/building/standards/astm-e84