8.3 Reverberation Time (S04) & Sound Reducing Surfaces (S05)
Key Takeaways
Both S04 and S05 apply only where the listed space types cumulatively make up at least 10% of occupiable project area.
S04 caps reverberation time at 0.6 seconds for learning and conferencing areas under 10,000 ft3 (280 m3), with wider bands as volume increases.
S04 sets 0.8 to 1.0 seconds for areas for speech, and no more than 1.0 second for dining areas and regularly occupied areas with public address systems.
S05 awards 1 point for minimum NRC of 0.75 across at least 75% of available ceiling area in open workspaces, and 2 points for 0.90 across all available ceiling area.
The S05 2-point tier for open workspaces also requires partial height barriers at least 4 ft (1.2 m) above finished floor with minimum NRC of 0.70 between all opposing workstations.
8.3 Reverberation Time (S04) & Sound Reducing Surfaces (S05)
Core Exam Takeaway: S04 Reverberation Time (2 pt / WELL Core 1 pt) and S05 Sound Reducing Surfaces (2 pt / Core 1 pt) share a crucial gating condition: both apply only where the listed space types cumulatively make up at least 10% of occupiable project area. Memorize S04's 0.6 s cap for small learning and conferencing rooms and S05's NRC 0.75 / 0.90 ladder, plus the 4 ft (1.2 m) / NRC 0.70 partial-height barrier in the 2-point tier.
Reverberation time in one paragraph
Reverberation time (RT, or RT₆₀) is the time for sound pressure to decay by 60 dB after the source stops. It is governed by room volume and total absorption — Sabine's relationship makes RT proportional to volume and inversely proportional to absorption area. Two consequences drive S04's structure: larger rooms reverberate longer for the same finishes, which is why S04's thresholds are banded by volume; and the fix is always absorption, which is why S05 exists as a separate purchasable feature.
Note also that S04 sets ranges, not just ceilings, for several space types. Too little reverberation is also a defect — an anechoic room makes speaking exhausting, because talkers get no acoustic support and unconsciously raise vocal effort. Hence bands like 0.8 ≤ t ≤ 1.0 s for areas for speech.
Feature S04: Reverberation Time — Optimization (2 pt / WELL Core 1 pt)
Intent: design spaces in accordance with comfortable reverberation times that support speech intelligibility, vocal effort and are conducive to concentration.
S04 Part 1 — Achieve Reverberation Time Thresholds
Applicability gate: for projects in which the space types listed cumulatively make up at least 10% of occupiable project area.
Option 1: Reverberation time, design and Option 2: Reverberation time, performance use the same thresholds; the difference is whether compliance is demonstrated by design calculation or by measurement:
| Space type | Space volume, v | Reverberation time, t (seconds) |
|---|---|---|
| Areas for learning and conferencing | v < 10,000 ft³ (280 m³) | t ≤ 0.6 |
| 10,000 ft³ (280 m³) ≤ v ≤ 20,000 ft³ (570 m³) | 0.5 ≤ t ≤ 0.8 | |
| v > 20,000 ft³ (570 m³) | 0.6 ≤ t ≤ 1.0 | |
| Areas for speech | n/a | 0.8 ≤ t ≤ 1.0 |
| Areas for dining; regularly occupied areas with public address systems | n/a | t ≤ 1.0 |
| Areas for fitness | v < 10,000 ft³ (280 m³) | 0.7 ≤ t ≤ 0.8 |
| 10,000 ft³ (280 m³) ≤ v ≤ 20,000 ft³ (570 m³) | 0.8 ≤ t ≤ 1.1 | |
| v > 20,000 ft³ (570 m³) | 1.0 ≤ t ≤ 1.8 | |
| Areas for music rehearsal | v < 10,000 ft³ (280 m³) | t ≤ 1.1 |
| 10,000 ft³ (280 m³) ≤ v ≤ 20,000 ft³ (570 m³) | 1.0 ≤ t ≤ 1.4 |
+-----------------------------------------------------------------------------+
| S04: HOW THE THRESHOLDS MOVE, AND WHY |
| |
| As VOLUME rises, the permitted RT band rises -- Sabine's relationship |
| makes short RT in a large room economically brutal. |
| |
| As the room's PURPOSE shifts from LISTENING to MAKING SOUND, |
| the band rises too: |
| |
| learning / conferencing t <= 0.6 s (small) <-- intelligibility |
| areas for speech 0.8 - 1.0 s <-- support the talker |
| dining / PA systems t <= 1.0 s <-- keep announcements |
| intelligible |
| fitness (small) 0.7 - 0.8 s |
| music rehearsal (small) t <= 1.1 s <-- musicians need decay |
| |
| Note the LOWER bounds: too dead a room is also a failure. |
+-----------------------------------------------------------------------------+
Important
The 10% applicability gate is the most commonly missed element of both S04 and S05. A project whose learning, conferencing, speech, dining, fitness and music-rehearsal spaces together occupy less than 10% of occupiable area is not required to meet the tables in order to earn the feature. This is a scoping rule that keeps the feature proportionate — an office with one small huddle room is not asked to acoustically engineer it.
Tip
The 0.6-second cap for small learning and conferencing spaces is the single most quotable number in S04. It reflects a large speech-intelligibility literature showing that classroom comprehension — especially for children, second-language listeners and anyone with hearing loss — degrades sharply above roughly 0.6 s in typically sized rooms.
Feature S05: Sound Reducing Surfaces — Optimization (2 pt / WELL Core 1 pt)
Intent: design spaces with sound reducing surfaces to minimize the buildup of speech or other unwanted sound.
S05 Part 1 — Implement Sound Reducing Surfaces
Same applicability gate: the listed space types cumulatively make up at least 10% of occupiable project area.
| Space type | Metric | 1 point (Core 0.5) | 2 points (Core 1) |
|---|---|---|---|
| Open workspaces | Minimum NRC/ | 0.75 for at least 75% of available ceiling area | 0.90 for all available ceiling area |
| Minimum furniture height and NRC/ | n/a | Partial height barriers with a minimum height of 4 ft (1.2 m) above finished floor and a minimum NRC/ of 0.70 between all opposing workstations | |
| Areas for conferencing and learning | Minimum NRC/ at ceilings | 0.75 for at least 50% of available ceiling area | 0.90 for all available ceiling area |
| Minimum NRC/ at walls | 0.75 on at least 25% of two walls | 0.80 on at least 25% of two perpendicular walls | |
| Areas for dining | Minimum NRC/ at ceilings | 0.75 for at least 50% of available ceiling area | 0.90 for all available ceiling area |
WELL Core guidance: non-leased spaces; an additional point for also meeting requirements in leased spaces.
Important
Open workspaces have a stricter 1-point ceiling requirement than conferencing, learning and dining areas — 75% of available ceiling area versus 50%. Open plans have no walls to stop speech, so the ceiling is the only large absorptive surface available, and it has to work harder.
Tip
The 2-point tier for open workspaces adds a second, independent requirement. It is not enough to upgrade the ceiling to NRC 0.90 across all available area; the project must also install partial height barriers at least 4 ft (1.2 m) above finished floor with NRC of at least 0.70 between all opposing workstations. Ceiling absorption handles reflected sound; the barrier interrupts the direct line-of-sight path between facing occupants, which absorption alone cannot address.
Tip
Note "two perpendicular walls" in the 2-point conferencing tier. Treating two parallel walls leaves the perpendicular pair to sustain a flutter echo between them; treating perpendicular surfaces breaks reflection paths in both room axes. The upgrade from 1 point to 2 points here is a geometry change as much as a performance change — and the wall coverage requirement drops from NRC 0.75 to 0.80 while the geometric constraint tightens.
NRC and
NRC (Noise Reduction Coefficient) is the arithmetic average of a material's absorption coefficients at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05, on a 0-to-1 scale. (weighted sound absorption coefficient) is the ISO equivalent. An NRC of 0.90 means the surface absorbs roughly 90% of incident sound energy in the speech-relevant bands — which is why standard mineral-fibre acoustic tile typically sits around 0.70 and reaching 0.90 usually requires a high-performance tile, a suspended baffle array, or an exposed-structure ceiling with a dedicated absorptive layer.
How S04, S05 and S06 combine
These three features solve the same problem — speech that should not be intelligible — by three different physical mechanisms, and a project usually needs at least two of them:
| Mechanism | Feature | What it does to a distant conversation |
|---|---|---|
| Absorb | S05 | Removes reflected energy, so only the direct path survives |
| Shorten decay | S04 | Stops syllables smearing into one another across the room |
| Mask | S06 | Raises the noise floor so the surviving speech is unintelligible |
S06 Part 2 pays an extra point precisely for combining masking with either S03 isolation or S05 absorption — WELL's acknowledgement that no single mechanism is sufficient in an open plan.
What condition must be met before Features S04 and S05 apply to a project's listed space types?
At least one acoustic zone must be labeled as a Quiet zone
The project must first achieve S01 Sound Mapping Part 2
The listed space types must cumulatively make up at least 10% of occupiable project area
The project must exceed 20,000 ft³ (570 m³) in total volume
A conference room with a volume of 8,000 ft³ (225 m³) is pursuing S04. What reverberation time must it achieve?
0.8 to 1.0 seconds
0.5 to 0.8 seconds
0.6 to 1.0 seconds
t no more than 0.6 seconds
To earn the 2-point tier of S05 for open workspaces, what must a project provide?
Minimum NRC of 0.90 across all available ceiling area only
Full height partitions between all workstations with minimum STC of 40
Minimum NRC of 0.75 across 75% of available ceiling area plus wall treatment on two perpendicular walls
Minimum NRC of 0.90 across all available ceiling area, plus partial height barriers at least 4 ft (1.2 m) above finished floor with minimum NRC of 0.70 between all opposing workstations
Why does S05 require a higher ceiling coverage percentage in open workspaces than in conferencing, learning and dining areas at the 1-point tier?
Because open workspaces have higher ceilings on average
Because dining areas are exempt from ceiling requirements
Because open workspaces have no walls to interrupt speech, making the ceiling the only large absorptive surface available
Because conferencing rooms rely on sound masking instead
Sections you finish are checked off in the contents.