Reverberation Time
The time in seconds for sound in a room to decay by 60 dB after the source stops. Longer times mean more echo; more absorbing surfaces shorten it.
Reverberation time, often written RT60, is the time in seconds that sound takes to fall by 60 decibels after the source has stopped. It is the standard single-number measure of how long a room sustains sound, and it explains why a bare, hard-surfaced room echoes. In Slovak professional texts the term is doba dozvuku.
What is reverberation time, and how is it defined?
Reverberation time is the rate at which sound energy decays in an enclosed space. When a source stops, the sound does not vanish at once. It reflects off walls, ceiling, floor and furniture, and each reflection loses part of its energy to absorption. The decay is tracked as a drop in level, and the benchmark is the time for the level to fall by 60 dB, which corresponds to a millionfold drop in sound intensity. That benchmark is where the symbol RT60 comes from.
It is calculated per frequency band, because absorbers work differently on low and high pitches, and designers focus on the middle bands where speech sits.
How is reverberation time calculated with the Sabine formula?
The classic estimate is Sabine's formula, named after Wallace Clement Sabine, who worked on room acoustics at the end of the nineteenth century. In metric units it reads:
RT60 = 0.161 × V / A
V is the room volume in m3. A is the total absorption in metric sabins, where one sabin is one square metre of a perfectly absorbing surface. Total absorption is the sum, over every surface and object in the room, of area multiplied by absorption coefficient. The formula is statistical: it assumes a reasonably diffuse sound field and evenly spread absorption, so it is weaker for rooms with a few very strong absorbers or unusual shapes.
| Element | What it means | Effect on reverberation time |
|---|---|---|
| Volume (V) | Air space of the room, in m3 | Larger volume gives a longer decay |
| Absorption coefficient | Share of incident sound a material absorbs, from 0 to 1 | Higher values give a shorter decay |
| Surface area | Floor, walls, ceiling, glazing and objects, each in m2 | More absorbing area gives a shorter decay |
| Total absorption (A) | Sum of area times coefficient for all surfaces and objects | Higher totals give a shorter decay |
The formula explains why two rooms of the same size can sound very different. A room with a hard floor, glass and bare concrete has a small total absorption A. The same room with carpet, upholstered seating and curtains has a much larger A, so its decay is much shorter.
Why do minimalist open-plan living rooms with hard surfaces sound echoey?
Minimalist interiors tend to rely on the materials that absorb least: polished or ceramic floors, large glazed walls, bare concrete ceilings and painted plaster. Each has a low absorption coefficient, so the total absorption A stays small. Open plan makes it worse. Kitchen, dining and living zones merge into one large volume V, and the ratio of volume to absorption rises. Sound bounces around the space for longer, and each new syllable lands on the tail of the one before. Speech becomes harder to follow, even at modest volume.
This is the same mechanism described in room acoustics. The fix is more absorbing area, not more mass: weight does not shorten decay.
How is reverberation time different from sound absorption and sound insulation?
Three terms are easily confused, and each describes a different thing.
- The absorption coefficient describes one material, from 0 to 1.
- Reverberation time describes the whole room, in seconds, from all its surfaces and objects together.
- Airborne sound insulation, given as Rw, describes how much a wall, floor or door blocks sound passing into the next space.
Adding absorbers shortens reverberation time, but it does nothing to stop a neighbour's music from entering the flat. For that you need mass, decoupling and sealed junctions, which are covered under airborne sound insulation. Sound can also bypass a partition through other elements, known as flanking transmission.
How is reverberation time measured in a finished room?
On site, a loud, short impulse or noise signal is produced, the decay is recorded with a microphone, and the time for a set level drop is calculated. Because the lowest part of the decay curve is often masked by background noise, practitioners usually extrapolate from the early part of the curve, a method known as T20 or T30 depending on the range used. Measurements are taken at several frequencies and from several source and microphone positions. Standardised test procedures, such as the ISO 3382 series, set out how the measurement should be carried out.
Which measures shorten reverberation time in a home?
Absorption can be added without changing the structure. The table below lists common options and the limits of each. Place absorbing area where reflections are strongest.
| Measure | How it works | Limits |
|---|---|---|
| Soft furnishings (upholstered sofa and armchairs) | Add absorbing fabric and foam inside the room | Helpful mainly at middle and high frequencies; a few pieces do little in a very large room |
| Curtains and heavy drapes | Cover reflective glazing with a large fabric area | Need some depth and folds; flat fabric against glass does little |
| Rugs and carpets | Absorb sound on the floor, usually the largest hard surface | A thin rug on a hard floor gives limited gain; thicker pile helps more |
| Bookshelves and stored objects | Irregular, porous surfaces scatter and absorb sound | Glass-fronted cabinets reflect; open shelving with books works better |
| Acoustic panels on walls | Engineered absorbers, often mineral wool or fabric covered | Placement matters; panels hidden behind furniture contribute less |
| Ceiling absorbers (baffles, clouds, perforated panels) | Cover large ceiling area, where reflections from floor and walls converge | Often the most effective single addition in a hard open room, though it affects the look and the lighting plan |
Several moderate measures usually work better than one large one. Check manufacturer absorption data against a recognised test before relying on it.
What reverberation time should a home be designed for?
This article gives no target value, and none should be copied from a generic source. The right figure depends on the room's volume, its use, and the frequency band in question. A fully dead room is not the goal either, since it feels unnatural to speak in. In Slovakia, acoustic requirements come from the current building act and its technical standards, which should be read in their current form before any figure is quoted to a client. For a specific room, an acoustic specialist can measure it and propose targeted measures.
Frequently asked questions
- Does reverberation time mean how long an echo lasts?
- Not exactly. An echo is a distinct reflection heard separately from the original sound, while reverberation time describes the smooth overall decay of sound in a room. Large hard rooms often show both, but they have different causes and different fixes.
- Why does my quiet minimalist living room still feel echoey?
- Loudness and reverberation are separate properties. Hard floors, glass and concrete keep reflections strong and the decay slow, so speech overlaps even at low volume. Speaking more quietly does not fix this.
- Will a rug and curtains fix the echo in an open-plan room?
- They help, especially if the room has none now, but they rarely solve the problem alone in a very large hard space. Combining soft furnishings with wall or ceiling absorbers usually gives a more reliable result.
- Is reverberation time the same as sound insulation between flats?
- No. Reverberation time describes decay inside one room, while airborne sound insulation describes how much sound a wall or floor blocks between spaces. Absorbing materials in your own room do little against noise arriving from the flat next door.
- Do I need measurements to know whether a room's reverberation time is too long?
- A simple listening test can reveal a problem, but measurement provides the figures a design needs. An acoustic specialist can estimate the value from the room's surfaces or measure it on site, then recommend targeted absorption.