WELL v2 Acoustic Requirements — 3 Features Most Projects Fail | Free Checklist | AcousPlan

# WELL v2 Feature 74: The 3 Acoustic Requirements Most Projects Fail

Here is a number that should concern every project aiming for WELL certification: fewer than 40% of WELL v2 projects that attempt Feature 74 earn all available acoustic points. The remainder either skip the harder parts of the standard or submit documentation that does not satisfy the requirement on review. The acoustic section is the second-most-failed feature in WELL v2 after the thermal comfort section — and unlike thermal comfort, acoustic failures are almost always preventable with correct design methodology.

Feature 74 'Sound' has seven distinct parts (L01 through L07). Most project teams read L01 and L02, see "background noise" and "HVAC noise," assume their mechanical engineer has it covered, and move on. They do not. L03 (Sound Isolation), L05 (Room Acoustics), and L07 (Sound Mapping) require specific acoustic design input that the mechanical engineer cannot provide, and they are the three requirements where projects fail most often.

This article dissects each of the seven parts, explains exactly why the three critical ones fail, and shows you what correct compliance looks like with real numbers.

Feature 74 Structure: What You Are Actually Being Scored On

WELL v2 Feature 74 is organised into preconditions and optimisations. The structure matters because you can fail an optimisation and still get partial credit, but failing a precondition fails the entire feature.

Projects pursuing WELL certification typically aim for all four optional points from Feature 74. In practice, most achieve 1–2.

L01: Sound Comfort — The Precondition You Think Is Easy

L01 requires maximum background noise levels in occupied spaces. The limits vary by space type:

These are occupied background noise levels — the level present when the space is in normal operation with HVAC running but no occupant activity. This matters because many project teams measure empty spaces at night and call it compliance.

The precondition is measured using a Type 1 integrating sound level meter (IEC 61672 Class 1), A-weighted, slow time-weighting, during business hours with all mechanical systems at design operating conditions. Not at maximum system capacity, not at minimum — at the designed operating condition for normal occupation.

Where L01 most often fails: the HVAC system is designed for NC 35 or better, commissioning confirms NC compliance, but the LAeq measurement during daytime hours includes nearby traffic, building mechanical plant on the roof, elevator machinery, and other building services that the acoustic analysis did not account for. Background noise at 38–42 dBA is common in city-centre office buildings with inadequate building envelope isolation. The project team assumed their mechanical engineer controlled all background noise sources. He controlled one of them.

L02: Noise Mapping — The Documentation Requirement

L02 requires a noise map of the project space showing background noise levels across the floor plate. This is a documentation requirement rather than a performance requirement — you need to demonstrate that you have characterised the acoustic environment spatially, not just at a single measurement point.

Most project teams satisfy L02 by submitting a floor plan with measurement points annotated with dBA readings. The WELL verifier wants to see that measurement coverage is adequate — typically one measurement point per 50 m² in open-plan areas, with at least one measurement in each enclosed room or zone.

L02 rarely fails on its own. It usually fails as a consequence of L01 failure — if you fail to meet the background noise limits, the noise map becomes evidence of non-compliance rather than compliance documentation.

L03: Sound Isolation — Failure Mode 1

This is where most projects lose points without expecting it. L03 requires minimum sound isolation performance between spaces:

These are field-measured values — not laboratory STC ratings. This distinction is critical. A partition system rated STC 50 in a laboratory will typically achieve STC 42–47 in the field due to flanking transmission through structure, unsealed penetrations, and imperfect edge conditions. The WELL standard requires the in-field performance to meet these limits.

The typical project failure: Conference rooms are specified with STC 50 partitions. The partition manufacturer's data shows STC 50 in laboratory conditions. Nobody calculates field performance. The partitions are installed with HVAC penetrations that have 25mm unsupported gaps around the ductwork. The measured field STC is 39. The project fails L03.

The field STC drop from penetrations follows a predictable relationship. A single 50mm × 50mm gap in a 10 m² STC 50 partition reduces effective isolation to approximately STC 37. The formula for partition with openings uses the transmission loss of each element:

TL_composite = -10 log₁₀(Σ(τᵢ × Sᵢ) / S_total)

where τᵢ = 10^(-TLᵢ/10) is the transmission coefficient of each element.

For a 10 m² partition (TL = 50 dB, τ = 0.00001) with a 0.0025 m² gap (TL ≈ 0 dB, τ = 1.0):

τ_composite = (0.00001 × 9.9975 + 1.0 × 0.0025) / 10.0
            = (0.0000999 + 0.0025) / 10.0
            = 0.0026 / 10.0
            = 0.00026
TL_composite = -10 log₁₀(0.00026) = 35.8 dB

That single 50mm gap dropped the effective isolation from STC 50 to approximately STC 36. This is not a hypothetical — it happens on every project where HVAC ductwork penetrates partitions without acoustic treatment.

Correct approach for L03 compliance: Specify all partition penetrations to be sealed with acoustic putty or mineral wool collar, minimum 100mm depth. Specify that any ductwork penetrating partitions adjacent to conference rooms shall have flexible connections with acoustic lining for 1000mm either side of the partition. Commission an acoustic measurement after fit-out but before furniture installation to verify compliance. Budget two weeks for remediation if non-compliant.

L04: Mechanical Equipment Noise — The Forgotten Part

L04 requires that mechanical equipment noise in occupied spaces meets NC 35 or NR 35 limits for offices and NC 40 for open-plan. This is different from L01 — L04 is specifically about mechanical equipment contribution in isolation, not total background noise.

The distinction matters in practice. L01 can be passed with a noisy space if external noise is high (the standard requires meeting the limit, but the source attribution is flexible). L04 requires specifically that HVAC and mechanical plant not exceed the NC/NR limits.

HVAC noise from supply diffusers is the primary source. The NC criterion governs: a diffuser generating 200 m³/h at 5 Pa static pressure will typically produce 35–42 dBA at 2m distance depending on diffuser type and duct velocity. High-velocity supply systems (duct velocity > 4 m/s) regularly exceed NC 40 in terminal equipment. Slowing duct velocity to < 3 m/s adds duct cross-section area but is the single most effective way to reduce terminal noise levels.

Most projects that specify NC 35 never verify HVAC equipment noise against the NC curve. The mechanical engineer specifies equipment by capacity and static pressure, the contractor installs it, and nobody measures. L04 can fail at commissioning even when all other parts pass.

L05: Room Acoustics — Failure Mode 2

L05 requires RT60 (reverberation time) compliance in enclosed occupied spaces. WELL v2 Feature 74 Part L05 specifies:

These are 500 Hz octave-band values. Compliance is measured using interrupted noise or backward integration method per ISO 3382-2:2008 §4.4.

Why L05 fails: The acoustic consultant calculates RT60 using Sabine's equation at the design stage, confirms the room will meet 0.6 s, and approves the ceiling tile selection. The ceiling tile is installed. The post-construction RT60 measurement shows 0.82 s.

What happened? The calculation used the tile's NRC value (0.80) as a proxy for α₅₀₀. The actual tile α₅₀₀ is 0.90, so the ceiling is performing correctly. The problem is that the calculation assumed wall finishes would contribute absorption at 500 Hz. They do not — painted gypsum has α₅₀₀ ≈ 0.03, glass has α₅₀₀ ≈ 0.04. The floor is polished concrete at α₅₀₀ ≈ 0.02. The total room absorption at 500 Hz is entirely from the ceiling tile, and the room volume is larger than the absorbed area can accommodate.

For a conference room: 8m × 6m × 3m = 144 m³, ceiling 48 m², walls 84 m², floor 48 m².

A_ceiling = 48 × 0.90 = 43.2 m²  (Ecophon Focus Ds 20mm)
A_walls   = 84 × 0.03 = 2.5 m²   (painted gypsum)
A_floor   = 48 × 0.02 = 1.0 m²   (polished concrete)
A_people  = 8 × 0.45  = 3.6 m²   (8 occupants)
A_total   = 50.3 m²
RT60 = 0.161 × 144 / 50.3 = 0.46 s

That passes. But the room in question has a glazed wall — 24 m² of floor-to-ceiling glass replacing two of the wall panels. Recalculate:

A_walls_gypsum = 60 × 0.03 = 1.8 m²
A_walls_glass  = 24 × 0.04 = 1.0 m²
A_total        = 43.2 + 2.8 + 1.0 + 3.6 = 50.6 m²

Nearly identical — the glazing makes almost no difference because both surfaces are equally poor absorbers. But the room has no furniture, and the space is commissioning-tested without occupants, so:

A_without_people = 46.0 m²
RT60_empty = 0.161 × 144 / 46.0 = 0.50 s

Still passes. The failure typically occurs when designers assume a higher ceiling α value than the installed product delivers. Substituting a cheaper tile at value engineering — switching from 20mm Ecophon Focus Ds (α₅₀₀ = 0.90) to 15mm Armstrong Cortega (α₅₀₀ = 0.75) — changes the result materially:

A_ceiling_revised = 48 × 0.75 = 36.0 m²
A_total_revised   = 36.0 + 2.8 + 1.0 = 39.8 m²
RT60_revised = 0.161 × 144 / 39.8 = 0.58 s  (marginal)

Now, test the room post-construction without furniture and with the actual tile that was installed — perhaps a mix of 15mm and 20mm due to site procurement — and you have an RT60 of 0.65–0.70 s. Certification fails.

The WELL project documentation review is not forgiving about "close to the limit" submissions. 0.62 s in a space with a 0.6 s limit requires remediation.

Correct approach: Run your calculation with octave-band data, not NRC. Use the WELL F74 calculator to verify compliance before and after any product substitution. Build in a 15% margin — specify to achieve RT60 ≤ 0.51 s to account for measurement uncertainty and product variation.

L06: Acoustic Signage

L06 requires the project to provide acoustic wayfinding — essentially, floor plan documentation identifying acoustic zones and the performance criteria applying to each zone. This is a documentation requirement. Most projects satisfy it with a well-annotated drawing set. It is rarely the cause of failure on its own.

L07: Sound Mapping — Failure Mode 3

L07 requires a sound mapping exercise after occupancy — measuring actual acoustic conditions throughout the space and documenting how they compare to design targets. This is the most underestimated requirement in the entire feature.

The failure mode is simple: L07 requires post-occupancy measurement, and most projects either do not budget for it or do not schedule it correctly. The WELL verifier requires data collected during normal building occupation — not at commissioning, not in an empty building, but with the space in operational use with typical occupancy and activity levels.

Scheduling post-occupancy acoustic measurement requires coordination between the acoustic consultant, the building manager, and the occupants. The measurement must capture representative conditions: a typical working day, HVAC at design operating condition, normal occupancy density. This cannot be done during the fit-out period or at handover. It requires return access 4–8 weeks after occupation.

Most project budgets do not include this return visit. The acoustic consultant completes work at practical completion, invoices are closed, and nobody schedules the L07 measurement. Certification is submitted without L07 documentation. The WELL verifier rejects it.

The measurement protocol for L07: minimum 5 measurement points in spaces > 100 m², minimum 3 points in smaller spaces. Type 1 sound level meter, LAeq over 15-minute periods, documented occupancy count and activities during measurement. WELL requires the sound map data to be included in the performance verification report submitted for certification.

Correct approach: Include L07 measurement explicitly in the acoustic consultant's scope of works. Schedule the measurement return visit in the project programme as a contractual deliverable. Brief the client facilities manager on what is needed and why. Budget for it at the outset — it costs £800–£2,000 depending on space size and is the difference between earning the point and losing it.

Earning All Four Optional Points: A Checklist

A Note on WELL Verifier Expectations

WELL certification is issued by the International WELL Building Institute (IWBI) and verified by third-party assessors. The acoustic section is reviewed by assessors who understand acoustics — do not submit calculations that use NRC as a proxy for octave-band data, and do not submit measurements made at unoccupied times and label them "typical conditions." WELL verifiers have seen these approaches before.

The documentation that passes includes: full octave-band absorption data for all specified materials, RT60 calculations at 500 Hz with a clear material schedule, field measurement reports with calibration data and weather conditions recorded, and sound isolation test reports per ASTM E336 (field measurement standard).

The Backlink-Bait Finding: WELL Acoustics Outperforms LEED

LEED v4 BD+C has no equivalent to WELL Feature 74. The LEED Acoustic Performance pilot credit (EA Pilot 55) was optional and rarely attempted, and LEED v4.1 retains it as a pilot. WELL v2 makes acoustic performance a precondition — you cannot certify without meeting background noise limits.

This means a WELL Gold-certified building has stronger acoustic guarantees than a LEED Platinum building. If you are designing spaces where acoustic performance matters to occupants — healthcare, education, finance, law — WELL certification is a better framework for acoustic compliance than LEED. The two certifications are not equivalent in acoustic scope.

Start Your WELL F74 Compliance Check Now

Use the WELL v2 Feature 74 calculator to check all seven parts for your project before specification closes. The calculator accepts room dimensions, materials, occupancy, and HVAC noise data and outputs pass/fail status against each L-part with margin to requirement.

Cross-check your acoustic ceiling specification against octave-band absorption data for common ceiling products. Verify that your partition system STC ratings include a field performance derating before comparing to L03 limits.

Related reading: BS 8233 vs WELL F74 · Acoustic Certification Comparison · Acoustic Design for Architects