Short answer: Reverberation time (RT60) is how long sound takes to fade by 60 decibels after it stops. Speech clarity is often rated with the Speech Transmission Index (STI) from 0 to 1. Halls are specified by setting target values for both, measured to ISO 3382 and IEC 60268-16, before and after the work.
Committees often describe a hall as “echoey” or say “the back rows cannot hear”. Acoustic designers turn those complaints into two measurable numbers: reverberation time and speech clarity. Knowing what they mean helps you brief consultants, compare bids and check the finished work.
Our team has worked in acoustics since 2021, with 50 projects for 49 organisations across Pakistan, from government departments and universities to broadcasters.
What is reverberation time (RT60)?
When a sound stops, it keeps bouncing around the room for a while. Reverberation time is the time it takes for that sound to fall by 60 decibels. It is written as RT60 or T60 and given in seconds.
- A long RT60 makes speech blur, as each word overlaps the last.
- A very short RT60 can make music sound dry and lifeless.
- The right value depends on the room volume and its main use.
ISO 3382-1 covers measurement in performance spaces such as auditoriums. ISO 3382-2 covers ordinary rooms such as classrooms and offices.
Why the right value depends on use
| Main use | General aim |
|---|---|
| Speech: lectures, conferences, debates | Shorter RT60 for clear words |
| Mixed use: convocations, events, speeches with music | A balanced value, often helped by the sound system |
| Music: concerts, qawwali, choirs | Longer RT60 for warmth and fullness |
| Recording studios and control rooms | Short and even across frequencies |
Ask your acoustic designer to set a target value and tolerance for each frequency band, based on the room’s volume.
What is speech clarity (STI)?
The Speech Transmission Index (STI) rates how well speech is understood, on a scale from 0 (unintelligible) to 1 (excellent). It is defined in IEC 60268-16. STI takes account of echo, background noise and the sound system together. It can be measured with or without the sound system running.
Other clarity measures in ISO 3382-1, such as C50 and D50, compare early and late sound energy. They are useful for detailed design.
How the numbers are measured
- A test signal is played from the stage or the loudspeakers.
- Microphones record the response at several seats across the hall.
- Software calculates RT60, STI and other values for each position.
- Results are averaged and compared with the targets.
Measurements are usually taken in an unoccupied hall, with the result for an occupied hall estimated, because people absorb sound.
What changes the numbers
- Room volume: bigger, taller rooms tend to have longer RT60.
- Surfaces: marble, tiles, glass and bare plaster reflect sound. Panels, seats and carpets absorb it.
- Seats: upholstered seats keep the hall consistent whether full or half empty. See our auditorium chairs page.
- Background noise: AC, fans and generators lower STI.
- Sound system: well-aimed loudspeakers improve STI. Badly placed ones excite the echo.
How to write it into a specification
A clear clause might read: “The hall shall achieve a mid-frequency reverberation time within the target range set by the designer, measured unoccupied to ISO 3382-1, and an STI of the agreed value at all listed seats with the sound system operating, measured to IEC 60268-16.” Add the list of measurement positions and link part of the payment to passing the test. Our acoustic tender specification guide explains more, and our tender specifications page has sample clauses.
Common myths
- “A bigger sound system will fix the echo.” More power often excites more echo. Room treatment and well-aimed speakers matter more.
- “Carpet alone solves it.” Carpet absorbs mainly high frequencies. Walls and ceilings usually need treatment too.
- “Acoustic foam is the same as acoustic treatment.” Thin foam reduces some high-frequency reflections but does little for a large hall.
- “Acoustics is only for concert halls.” Council chambers, classrooms, conference halls and masjids all depend on clear speech.
- “It can be fixed after the interiors are done.” Planning acoustics early is easier and cheaper than reworking finished surfaces.
How halls are treated
- Absorptive panels on the rear wall and parts of the side walls
- Ceiling treatment or clouds above the audience
- Diffusers to keep sound even without over-deadening
- Sound insulation to keep outside noise out
- A sound system designed for the treated room
See our auditorium renovation service and acoustic consultancy.
Studios and critical listening rooms
Recording studios need short, even reverberation and very low background noise. Treatment is chosen for recording and critical listening, not just to reduce echo.
Related reading
- Auditorium sound system cost factors: what shapes your quote
- Lecture hall acoustics: a specification guide for universities
- Auditorium renovation checklist for public buildings
Related projects
Auditorium acoustic treatment and optimisation at Mukabbir University of Science & Technology, Lahore (2025-26), is one of the hall projects on our client list.
Frequently asked questions
What is a good reverberation time for an auditorium?
It depends on the hall volume and its main use. Speech halls need a shorter RT60 than music halls. Your acoustic designer should set a target range and measure it to ISO 3382-1.
What is STI?
The Speech Transmission Index rates speech intelligibility from 0 to 1, where higher is clearer. It is defined in IEC 60268-16 and accounts for echo, background noise and the sound system.
Can a sound system fix a hall with long reverberation?
Only partly. Directional loudspeakers help, but in a very reverberant hall, acoustic treatment is usually needed for clear speech.
Can you arrange RT60 and STI measurements in Lahore, Karachi and Islamabad?
Yes. We arrange measurements as part of a paid site survey or a handover test, in Lahore, Karachi, Islamabad and other cities.
How do we start?
Call or WhatsApp +92 300 8467101 for a free phone consultation. For measurements and a design, book a paid site survey.
Talk to Sound Krafted
Call or WhatsApp +92 300 8467101 for a free phone consultation. For an accurate design, BOQ and quotation, book a paid site survey.
About the author. This article was written by the Sound Krafted acoustics team. Our team has worked in acoustics since 2021 and has delivered 50 projects for 49 organisations across Pakistan, including government institutions, universities, broadcasters and corporate organisations. We are based in Lahore and are a project of Media K Solutions (Pvt) Ltd.. About us