WELL v2 Feature 74 (Sound) is the most comprehensive acoustic requirement in any commercial wellness certification. It has four sub-parts — L01 through L07 (skipping L02 and L04 which were renumbered) — each addressing a different aspect of acoustic performance. This article works through all four parts for a three-storey commercial office building seeking WELL v2 Gold certification.
Building Description
Building: 3-storey corporate headquarters, 4,500 m² total floor area Floors 1–3: Open office, enclosed offices, conference rooms, focus rooms, break rooms WELL v2 target: Gold certification (requires Precondition + both Optimization points for Sound)
Typical floor plan (Floor 2, representative):
| Space | Dimensions | Area | Ceiling Height |
|---|---|---|---|
| Open office | 30 m × 15 m | 450 m² | 2.9 m |
| Enclosed offices (×6) | 4.5 m × 3.5 m each | 94.5 m² | 2.7 m |
| Large conference room | 9.0 m × 6.5 m | 58.5 m² | 2.9 m |
| Small conference room (×2) | 5.0 m × 4.0 m each | 40 m² | 2.7 m |
| Focus rooms (×4) | 2.5 m × 2.0 m each | 20 m² | 2.7 m |
| Break room / kitchen | 8.0 m × 5.0 m | 40 m² | 2.9 m |
Part 1 — L01: Sound Mapping (Precondition)
Requirement: A sound mapping exercise must be conducted to identify acoustic zones, classify each space by use category, and document intended acoustic performance targets.
This is primarily a process requirement, but the mapping must include:
- Floor plan with all acoustic zones delineated
- Each zone classified as one of: Open Collaboration, Private Communication, Focus Work, Support Space
- Minimum performance targets assigned to each zone pair boundary
| Space | WELL Acoustic Zone |
|---|---|
| Open office | Open Collaboration |
| Enclosed offices | Private Communication |
| Large conference room | Private Communication |
| Small conference rooms | Private Communication |
| Focus rooms | Focus Work |
| Break room | Support Space |
Documentation required:
- Scale floor plan with zone boundaries marked
- Partition legend showing assembly type and rated STC/FSTC
- Compliance schedule cross-referencing each boundary to L03 requirements
Part 2 — L03: Minimum STC Requirements (Precondition)
Requirement: Minimum STC (lab) or FSTC (field) values must be achieved between specific space pairings.
Key boundaries on Floor 2 and required STC:
| Boundary | Space A | Space B | Required STC |
|---|---|---|---|
| Enclosed office / open office | Enclosed office | Open office | 45 |
| Conference / conference | Large conference | Small conference | 50 |
| Conference / open office | Large conference | Open office | 50 |
| Focus room / open office | Focus room | Open office | 45 |
| Break room / enclosed office | Break room | Enclosed office | 45 |
Enclosed Office / Open Office Partition
Specified assembly: 90 mm steel stud, single layer 15 mm gypsum board each side, no resilient bar, no acoustic batt. Lab STC: 38.
STC 38 < Required STC 45 → FAIL
Required revision: Upgrade to 90 mm stud, dual layer 15 mm gypsum one side, single 15 mm gypsum other side, 50 mm acoustic batt infill. Lab STC: 46. PASS.
Conference Room Partition
Specified assembly: 90 mm steel stud, dual layer 15 mm fire-rated gypsum each side, resilient bar one side, 75 mm mineral wool infill. Lab STC: 55.
Required STC: 50. STC 55 ≥ 50 → PASS
However, the ceiling plenum above the conference room is open (uninterrupted mineral fibre tile). Predicted FSTC (using flanking method from ISO 15712):
Ceiling flanking TL at 500 Hz: 2 × TL_tile + 5 dB = 2 × 25 + 5 = 55 dB. At 125 Hz: 2 × 12 + 5 = 29 dB.
The combined FSTC with open plenum flanking is predicted at approximately 41–44, which is below the required field target of 50.
Required fix: Install plenum barrier — continuation of the partition assembly above the ceiling tile plane to the structural soffit, or at minimum a 90 mm mineral wool batt in the plenum above the partition line + 1 layer gypsum board on each side of the partition extension. This recovers approximately 15 dB at 125 Hz through the plenum.
Revised predicted FSTC: ~50. Marginal pass — verify by measurement at practical completion.
Focus Room / Open Office
Specified assembly: Single stud, single layer gypsum each side. STC 32. Required: 45. → FAIL by 13 dB.
Required revision: Double-stud wall (or staggered stud), dual layer gypsum each side, resilient mounting. Lab STC: 48 → PASS
L03 Summary:
| Boundary | Original STC | Pass? | Revised STC | Pass? |
|---|---|---|---|---|
| Enclosed office / open office | 38 | FAIL | 46 | PASS |
| Conference / conference | 55 (FSTC ~42) | FAIL (field) | 55 + plenum barrier (FSTC ~50) | PASS |
| Focus room / open office | 32 | FAIL | 48 | PASS |
| Break room / enclosed office | 42 | FAIL | 48 | PASS |
L03 Status: PASS after design revisions.
Part 3 — L05: Background Noise Levels (Optimization, 1 point)
Requirement: HVAC and building mechanical noise must not exceed:
- Open office: NC 40
- Private offices, conference rooms: NC 35
- Focus rooms, executive offices: NC 30
Open Office (NC 40 target)
HVAC background noise measured at occupied zone (1.2 m height):
| Octave Band (Hz) | 63 | 125 | 250 | 500 | 1k | 2k | 4k |
|---|---|---|---|---|---|---|---|
| Measured L (dB) | 62 | 52 | 46 | 40 | 35 | 30 | 26 |
NC curve determination: the NC value equals the NC curve that the spectrum just touches (from above). Plotting against NC curves:
| Band | Measured | NC 40 limit | NC 45 limit |
|---|---|---|---|
| 63 | 62 | 71 | 74 |
| 125 | 52 | 64 | 67 |
| 250 | 46 | 57 | 60 |
| 500 | 40 | 50 | 53 |
| 1k | 35 | 45 | 48 |
| 2k | 30 | 41 | 44 |
| 4k | 26 | 39 | 42 |
The highest NC-curve tangency occurs at 1k Hz: 35 dB is at NC 35. However the tangency at 63 Hz (62 dB vs NC 40 = 71 dB limit) gives headroom.
Open office NC rating: NC 35. Below NC 40 target → PASS with margin.
Conference Room (NC 35 target)
Supply air diffuser in small conference room creates a jet noise source. Measured:
| Band (Hz) | 63 | 125 | 250 | 500 | 1k | 2k | 4k |
|---|---|---|---|---|---|---|---|
| Measured L (dB) | 58 | 51 | 48 | 44 | 41 | 36 | 30 |
NC tangency at 500 Hz: 44 dB. NC 40 curve at 500 Hz = 50 dB (headroom). NC 35 curve at 500 Hz = 47 dB. Tangency at 1k Hz: 41 dB against NC 35 (1k) = 45 dB. Tangency at 250 Hz: 48 dB against NC 35 (250 Hz) = 56 dB.
Conference room NC rating: NC 40. Exceeds NC 35 target by 5 NC → FAIL
Cause: Diffuser velocity too high (supply velocity 4.2 m/s; maximum for NC 35 is 2.0–2.5 m/s at duct exit).
Fix: Replace 300 × 150 mm diffuser with 600 × 300 mm diffuser (4× area). Velocity reduces from 4.2 to approximately 1.05 m/s. NC contribution from diffuser drops by approximately 12 dB → conference room NC reduces to approximately NC 30. PASS.
Focus Rooms (NC 30 target)
Focus rooms have no dedicated supply — ambient air from corridor under door. Measured:
| Band (Hz) | 63 | 125 | 250 | 500 | 1k | 2k | 4k |
|---|---|---|---|---|---|---|---|
| Measured L (dB) | 48 | 40 | 36 | 30 | 25 | 20 | 18 |
NC tangency at 125 Hz: 40 dB. NC 30 curve at 125 Hz = 57 dB. At 500 Hz: 30 dB vs NC 30 (500 Hz) = 45 dB. At 1k Hz: 25 dB vs NC 30 (1k Hz) = 35 dB.
Focus room NC rating: NC 25. Below NC 30 target → PASS with 5 NC margin.
L05 Summary:
| Space | Measured NC | Target | Pass? |
|---|---|---|---|
| Open office | NC 35 | NC 40 | PASS |
| Conference rooms (after fix) | NC 30 | NC 35 | PASS |
| Focus rooms | NC 25 | NC 30 | PASS |
L05 Status: PASS after HVAC diffuser upgrade → 1 point earned
Part 4 — L07: Reverberation Time (Optimization, 1 point)
Requirement (WELL v2 Feature 74 L07):
- Open office: RT60 at 500–2000 Hz average ≤ 0.6 s
- Conference rooms ≤ 800 m²: 0.4–0.8 s (500–2000 Hz)
- Focus rooms: RT60 ≤ 0.6 s
Open Office RT60 Calculation
Dimensions: 30 m × 15 m × 2.9 m = 1305 m³ Total surface area: 2(30×15 + 30×2.9 + 15×2.9) = 2(450 + 87 + 43.5) = 1161 m²
Surface absorption (Sabine):
| Surface | Area (m²) | Material | α_500 | A_500 |
|---|---|---|---|---|
| Floor | 450 | Carpet tiles | 0.20 | 90.0 |
| Ceiling | 450 | Mineral fibre tile NRC 0.75 | 0.75 | 337.5 |
| Walls | 261 | Painted gypsum board | 0.05 | 13.1 |
| Total | 440.6 m² |
RT60_500 = 0.161 × 1305 / 440.6 = 210.1 / 440.6 = 0.477 s
Octave band calculation:
| Band (Hz) | A_floor | A_ceiling (α: 0.18/0.50/0.75/0.85/0.80/0.65) | A_walls (α: 0.15/0.10/0.05/0.05/0.04/0.04) | A_total | RT60 (s) |
|---|---|---|---|---|---|
| 125 | 450×0.02=9 | 450×0.18=81 | 261×0.15=39.2 | 129.2 | 1.62 |
| 250 | 450×0.06=27 | 450×0.50=225 | 261×0.10=26.1 | 278.1 | 0.75 |
| 500 | 450×0.20=90 | 450×0.75=337.5 | 261×0.05=13.1 | 440.6 | 0.477 |
| 1k | 450×0.30=135 | 450×0.85=382.5 | 261×0.05=13.1 | 530.6 | 0.397 |
| 2k | 450×0.50=225 | 450×0.80=360 | 261×0.04=10.4 | 595.4 | 0.354 |
| 4k | 450×0.60=270 | 450×0.65=292.5 | 261×0.04=10.4 | 572.9 | 0.368 |
RT60 average at 500–2000 Hz = (0.477 + 0.397 + 0.354) / 3 = 0.409 s
Target: ≤ 0.6 s → PASS with significant margin.
Note: 125 Hz RT60 = 1.62 s. This is not part of the WELL L07 requirement but would cause speech clarity issues. Specify a minimum of 25% of wall area as fabric-wrapped panels or consider ceiling tile upgrade to 25 mm perforated panels.
Large Conference Room RT60
Dimensions: 9.0 m × 6.5 m × 2.9 m = 170 m³ Surface area: 2(9×6.5 + 9×2.9 + 6.5×2.9) = 2(58.5 + 26.1 + 18.85) = 206.9 m²
| Surface | Area (m²) | α_500 | A_500 |
|---|---|---|---|
| Floor | 58.5 | Carpet 0.20 | 11.7 |
| Ceiling | 58.5 | Mineral fibre NRC 0.85 | 49.7 |
| Walls | 89.9 | 30% fabric panels (α 0.90) + 70% gypsum (α 0.05) | 26.9+3.1=30.0 |
| Total | 91.4 |
RT60_500 = 0.161 × 170 / 91.4 = 27.37 / 91.4 = 0.299 s
Target: 0.4–0.8 s → FAIL (too low, 0.3 s — room is over-treated and will sound "dead")
Fix: Reduce fabric panel area from 30% to 15% of wall area.
| Surface | Area (m²) | α_500 | A_500 |
|---|---|---|---|
| Floor | 58.5 | 0.20 | 11.7 |
| Ceiling | 58.5 | 0.85 | 49.7 |
| Walls (revised) | 89.9 | 15% fabric (α0.90) + 85% gypsum (α0.05) | 12.1+3.8=15.9 |
| Total | 77.3 |
RT60_500 (revised) = 0.161 × 170 / 77.3 = 0.354 s
Still below the 0.4 s target. The high-NRC ceiling (0.85) is over-absorbing for this small room. Consider reducing ceiling tile absorption to NRC 0.65 product, and remove all fabric panels.
With NRC 0.65 ceiling (α_500 = 0.70): A_ceiling = 58.5 × 0.70 = 40.95
A_total = 11.7 + 40.95 + 89.9×0.05 = 11.7 + 40.95 + 4.5 = 57.15
RT60_500 = 0.161 × 170 / 57.15 = 0.479 s → PASS (within 0.4–0.8 s)
Focus Room RT60
Dimensions: 2.5 m × 2.0 m × 2.7 m = 13.5 m³
At this room volume, Sabine is inaccurate (highly non-diffuse). Use Eyring equation:
Surfaces: Floor 5 m² carpet (α 0.30); Ceiling 5 m² acoustic tile (α 0.75); Walls 24.1 m² gypsum (α 0.05).
Total S = 5+5+24.1 = 34.1 m² A_500 = 5×0.30 + 5×0.75 + 24.1×0.05 = 1.5 + 3.75 + 1.2 = 6.45 ᾱ_500 = A/S = 6.45/34.1 = 0.189
RT60 (Eyring) = 0.161 × 13.5 / (−34.1 × ln(1−0.189)) = 2.174 / (−34.1 × (−0.209)) = 2.174 / 7.13 = 0.305 s
Target: ≤ 0.6 s → PASS
L07 Summary:
| Space | RT60 (500–2k avg) | Target | Pass? |
|---|---|---|---|
| Open office | 0.41 s | ≤ 0.60 s | PASS |
| Large conference (revised) | 0.48 s | 0.4–0.8 s | PASS |
| Focus rooms | 0.31 s | ≤ 0.60 s | PASS |
L07 Status: PASS after conference room ceiling revision → 1 point earned
Final WELL v2 Feature 74 Score
| Part | Requirement | Status | Points |
|---|---|---|---|
| L01 Sound Mapping | Precondition | PASS | — |
| L03 Minimum STC | Precondition | PASS (after design revisions) | — |
| L05 Background Noise | Optimization | PASS (after diffuser upgrade) | +1 |
| L07 Reverberation Time | Optimization | PASS (after ceiling revision) | +1 |
| Feature 74 total | 2 / 2 points |
All four parts are achievable for this building, but none of the original specifications passed without revision. The most common design issues were: under-specified conference room partitions, HVAC diffuser velocities too high, and conference room acoustic treatment that overshot the RT60 target.
Run your WELL Feature 74 compliance calculations using AcousPlan's WELL v2 calculator, which checks all four parts simultaneously and flags the specific deficiencies requiring design attention.