Sound absorption · Glass wool
SCG Cylence Zoftone
- Thickness
- 25–50 mm
- Density
- 32–64 kg/m³
- Sound absorption (NRC)
- up to 1.00
What are reverberation and reverberation time (RT)?
In short
Reverberation is the mass of reflections from walls, ceiling and floor that overlap so the sound lingers in the room after the source has stopped. Reverberation time (RT or RT60) is the time it takes for the sound level in the room to fall by 60 dB after the source stops. Large rooms with hard surfaces have a long RT; rooms with plenty of absorption have a short RT.
Reverberation vs echo
- Reverberation — many reflections arrive so close together that the ear can't separate them, so sound hangs and trails, and speech becomes unclear
- Echo — a distinct repeat you hear separately, usually when sound returns from a distant hard surface late enough for the ear to pick it out
- Flutter echo — sound bouncing back and forth between two parallel hard walls, heard as a rapid rattling repeat
In everyday Thai all of this is called "ห้องก้อง" (an echoey room), but the fix can differ slightly depending on what you hear.
Why is a room echoey?
Reverberation increases when:
- The room is large — sound travels further between surfaces, so it lingers longer
- Surfaces are hard and smooth — glass, concrete, tiles and plain gypsum reflect almost all sound
- There is little in the room — no curtains, upholstered furniture or people
How reverberation time is measured
RT is the time in seconds for the level to drop 60 dB after the source stops. In practice, measurements to ISO 3382-2 usually take a 20 dB or 30 dB portion of the decay and extrapolate to 60 dB (called T20 and T30), and they are made per frequency band, because a room can be more reverberant in the bass than in the treble.
When reading a report, check which frequency bands were used and which bands any single summary figure averages.
Room size and absorption
Sabine's formula estimates RT well in rooms where absorption is spread fairly evenly:
RT ≈ 0.16 × V ÷ A
- V = room volume (m³)
- A = total absorption area (m²) = the sum of each surface's area × its sound absorption coefficient (α)
What it tells you: a bigger room has a longer RT, and doubling the total absorption area roughly halves the RT. So both the absorption of the material and the area installed matter, not just the figure on the datasheet.
Sabine's formula suits a first estimate. For unusual room shapes, absorption concentrated on one surface, or when accuracy matters, have an acoustic consultant calculate or measure.
What RT is right?
The right value depends on the use and size of the room. Rooms for clear speech, such as classrooms and meeting rooms, usually need a shorter RT than rooms for some kinds of music. One published example is the US classroom standard ANSI/ASA S12.60, which sets a maximum RT of 0.6 seconds for unoccupied, furnished classrooms up to about 283 m³, and 0.7 seconds for classrooms of about 283–566 m³.
These figures are criteria for classrooms under that standard, not a value for every room. If your project has its own requirement, follow it.
How to reduce reverberation
- Add sound-absorbing material to walls or ceiling — see How to choose sound-absorbing material and What is NRC
- Treat the problem surfaces, such as two parallel hard walls or a large ceiling
- Check the bass too — if the room booms at low frequencies, thin material fixed flat to the wall may help little
- Furnishings help a little — heavy curtains, carpets, upholstered furniture
Less reverberation is not sound insulation
Reducing RT makes sound inside the room clearer, but it does not stop sound passing through the walls to other rooms. See Do sound absorbing panels stop noise leaving a room?
See solution: Echoey rooms
Sources
- ISO 3382-2 — Acoustics — Measurement of room acoustic parameters — Part 2: Reverberation time in ordinary rooms
- ISO 354 / ASTM C423 — measurement of sound absorption in a reverberation room (uses the Sabine relationship between RT, volume and absorption area)
- ANSI/ASA S12.60 — Acoustical Performance Criteria, Design Requirements and Guidelines for Schools (cited via Armstrong World Industries)
- ROCKWOOL International — 5 facts about Acoustics and stone wool (2020): definition of reverberation time
Related products
Categories and example products for this topic
- SCG Cylence Zoftone — Wall panel that adds absorbing area; NRC 0.85–1.00 at 50 mm depending on grade
- SCG Cylence Zandera — Fabric-wrapped decorative panel in 9 colours, NRC 0.75 at 25 mm, fixed with adhesive
- M-Board — Glues onto an existing smooth ceiling; NRC 0.53 measured by Get Best Sound (unpainted)
- SCG Cylence Armax — White-vinyl glass wool ceiling tile for T-bar grids, replacing hard tiles to cut echo
Sound absorption · Glass wool
SCG Cylence Zandera
- Thickness
- 25 mm
- Density
- 100 kg/m³
- Sound absorption (NRC)
- 0.75
