8.3 Reverberation Time (S04) & Sound Reducing Surfaces (S05)

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.

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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:

+-----------------------------------------------------------------------------+
|              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.

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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.

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 α_w

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. α_w (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:

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.