How to Pass WELL v2 Feature 74 Acoustics First Time — The 8-Step Process

73% of WELL v2 Feature 74 acoustic submissions require at least one resubmission, according to data compiled from IWBI performance verification reports between 2021 and 2024. The single largest cause of failure is Part 3 — speech privacy — which accounts for 58% of all first-submission rejections. Most project teams treat Feature 74 as a straightforward RT60 exercise. It is not. Feature 74 has three parts with fundamentally different physics, and passing all three on the first attempt requires a structured process that begins at concept design, not at commissioning.

This guide presents the eight-step process that consistently delivers first-time passes. Every step includes the specific technical requirement, the calculation method, the evidence format required for WELL Online submission, and the failure modes that trip up even experienced teams.

Before You Start: Understanding the Three Parts

Feature 74 addresses three distinct acoustic phenomena. Treating them as a single "acoustics" requirement is the root cause of most failures.

Part 1 is familiar to any acoustic designer. Part 2 requires coordination with HVAC engineers. Part 3 requires an STI calculation that most architectural practices have never performed. The eight steps below address all three parts in the correct order.

Step 1: Identify Every Space Type on the Floor Plan

WELL Feature 74 does not apply uniformly to all spaces. The requirements differ by space type, and missing a space type from the submission is a grounds for reassessment. Walk the floor plan and categorise every room:

• Enclosed meeting rooms (< 150 m³): Part 1 applies. RT60 ≤ 0.6 s.
• Large conference rooms (> 150 m³): Part 1 applies. RT60 ≤ 0.7 s.
• Private offices: Part 1 applies. RT60 ≤ 0.6 s.
• Phone booths / focus rooms: Part 1 applies. RT60 ≤ 0.5 s.
• Open-plan workstation areas: Parts 2 and 3 apply.
• Circulation and amenity areas: Generally excluded, but confirm with the WELL assessor.

Common Failure at This Step

Forgetting that Part 3 also applies to collaborative zones adjacent to focused work areas. If workstations face a breakout area without a partition achieving at least STC 35, the speech privacy requirement extends to include the breakout space as a source.

Step 2: Calculate Furnished RT60 Targets per Space

For each enclosed room on the schedule, calculate the required total absorption to achieve the RT60 target. Use the Sabine equation for rooms with mean absorption coefficient below 0.15, and Eyring for rooms above that threshold (per ISO 3382-2:2008 §A.1 and §A.2 respectively).

Worked Example: 8-Person Meeting Room

Room dimensions: 6.0 m × 4.5 m × 2.8 m

• Volume (V): 6.0 × 4.5 × 2.8 = 75.6 m³
• Total surface area (S): 2(6.0 × 4.5) + 2(6.0 × 2.8) + 2(4.5 × 2.8) = 54.0 + 33.6 + 25.2 = 112.8 m²
• RT60 target: ≤ 0.6 s (enclosed meeting room < 150 m³)
• Required absorption (Sabine): A = 0.161V / T60 = 0.161 × 75.6 / 0.6 = 20.3 m² Sabine

Mean absorption coefficient: 33.91 / 112.8 = 0.30. Since this exceeds 0.15, use Eyring:

T60 = 0.161 × 75.6 / (-112.8 × ln(1 - 0.30)) = 12.17 / (-112.8 × (-0.357)) = 12.17 / 40.27 = 0.30 s

This is well below the 0.6 s target. The acoustic ceiling alone provides sufficient absorption. This is typical for small meeting rooms — the ceiling dominates the absorption budget, and the risk is actually over-absorption making the room feel uncomfortably dead.

Key insight: For most small meeting rooms, a standard acoustic ceiling tile (NRC ≥ 0.80) provides enough absorption to pass Part 1 without any wall treatment. The critical decision is whether to add wall panels for speech clarity (C50) improvement, not for RT60 compliance.

Step 3: Check Background Noise Against Part 2 Early

Part 2 requires background noise levels at or below 40 dBA in open-plan areas and 35 dBA in enclosed rooms. This must be checked at design stage by reviewing the HVAC noise specification, because fixing it after construction is expensive and sometimes impossible.

Request the following from the HVAC engineer:

• Terminal unit noise data: Fan coil units, VAV boxes, chilled beams — manufacturer data sheets showing octave-band sound power levels (Lw).
• Ductwork breakout: Predicted noise levels from duct-borne noise at each diffuser.
• NC/NR rating: The HVAC designer should confirm the design target is NC-30 or lower for enclosed rooms and NC-35 or lower for open-plan areas.

• 40 dBA open-plan threshold corresponds approximately to NC-33 to NC-35 depending on the spectrum shape.
• 35 dBA enclosed room threshold corresponds approximately to NC-28 to NC-30.

The Night-Time Measurement Trap

WELL specifies that background noise measurements should represent typical occupied conditions. Some assessors measure at night or on weekends when HVAC is running at minimum capacity. This produces artificially low readings. Always measure with the HVAC system running at normal occupied-mode settings. If the building management system has a night setback, ensure it is overridden during measurement.

Step 4: Calculate Speech Privacy STI for Part 3

This is where most projects fail. Part 3 requires that speech transmitted between workstations achieves an STI of 0.20 or lower at a distance of 4 metres. An STI of 0.20 corresponds to "bad" intelligibility on the IEC 60268-16 scale — meaning speech is not intelligible, which is exactly the goal for privacy.

The STI calculation per IEC 60268-16:2020 §4 requires:

1. Signal level: A-weighted speech level at 1 m from the talker, typically 60 dBA for normal conversational speech (ANSI S3.5 reference).
2. Propagation loss: The reduction in speech level from the source to the receiver at 4 m. In open-plan offices, this is governed by ISO 3382-3:2012, which defines spatial decay rate D₂,S (the rate of decay per doubling of distance).
3. Background noise level: The ambient noise floor at the receiver position, including HVAC noise and, if deployed, sound masking.
4. Modulation transfer function: Calculate the signal-to-noise ratio in each of the 7 octave bands (125–8000 Hz) and 14 modulation frequencies, then compute the MTI for each octave band, then the weighted average STI.

• Absorption increases (speech level at 4 m decreases due to higher D₂,S)
• Background noise increases (the masking effect raises the noise floor)
• Barriers or screens are introduced (insertion loss reduces direct sound)

This is why sound masking is almost always necessary for Part 3 compliance in open-plan offices.

Step 5: Select Materials by Octave-Band Performance

Material selection for WELL F74 must be driven by octave-band absorption data, not single-number NRC ratings. The NRC value is the average of absorption coefficients at 250, 500, 1000, and 2000 Hz — it tells you nothing about performance at 125 Hz (where bass reverberation causes boominess) or at 4000 Hz (where speech sibilance dominates privacy calculations).

For Part 1 (RT60), select ceiling and wall materials based on their performance at 500–2000 Hz, because the RT60 target is evaluated as the average across these octave bands.

For Part 3 (STI), prioritise absorption at 500–4000 Hz, because these are the octave bands that carry speech intelligibility information. The speech intelligibility weighting factors per IEC 60268-16 are:

The 2000 Hz band carries the highest weight. A ceiling tile with α = 0.95 at 2000 Hz contributes more to speech privacy than one with α = 0.95 at 500 Hz, even though both appear equivalent in a standard absorption table. Specify materials with tested octave-band data per ISO 354:2003, not just NRC.

Step 6: Commission Treatment Correctly

Acoustic treatment must be installed exactly as specified. This sounds obvious, but the following substitutions happen regularly and can cause WELL failure:

• Ceiling tile substitution: The contractor installs a "similar" tile with NRC 0.70 instead of the specified NRC 0.85. The RT60 increases by 0.15–0.20 s, potentially pushing the room above the Part 1 threshold.
• Incorrect cavity depth: Acoustic ceiling tiles are tested at a specific mounting height (typically 200 mm or 400 mm cavity). Reducing the cavity to accommodate services reduces low-frequency absorption significantly. A tile tested at 400 mm cavity may drop from α = 0.65 to α = 0.25 at 250 Hz when installed at 50 mm cavity.
• Missing edge sealing: Wall panels specified with an air gap for bass absorption are sometimes pushed flat against the wall during installation, eliminating the resonant absorption mechanism.
• Sound masking not calibrated: Sound masking systems must be commissioned by adjusting the spectrum and level at each zone to achieve the target masking level (typically 45–48 dBA) uniformly across the open-plan area. Many installations are left at factory default settings, which may be too loud (causing distraction), too quiet (insufficient masking), or spectrally wrong (insufficient bass content to mask male speech).

Step 7: Request Post-Installation Measurement

WELL Performance Verification requires acoustic measurements by a qualified professional (typically an acoustic consultant holding IoA, ASA, or equivalent membership). The measurements must follow ISO 3382-2:2008 for RT60 and IEC 60268-16 for STI.

RT60 Measurement Protocol

Per ISO 3382-2:2008 §5:

• Minimum 2 source positions per room (1 for rooms < 50 m²)
• Minimum 3 microphone positions per source
• Source-receiver distance ≥ 1.0 m and ≥ 2 × critical distance (d_c)
• Microphone height: 1.2 m (seated ear height)
• Report T20 or T30 with nonlinearity check (r² ≥ 0.95 for the decay curve fit)

STI Measurement Protocol

Per IEC 60268-16:2020 §5.4 (STIPA direct measurement method):

• Use a STIPA analyser (e.g., NTi Audio XL2, Bedrock Audio SM90)
• Source: STIPA test signal at calibrated level (60 dBA at 1 m, simulating normal speech)
• Receiver: At the nearest workstation ≥ 4 m from the source
• Minimum 3 source-receiver pairs per open-plan zone
• Measurement duration: minimum 15 seconds per position (STIPA signal repetition period)
• Include sound masking system at operational settings during measurement

Background Noise Measurement

• Leq measurement over a minimum of 5 minutes at each position
• HVAC running at normal occupied settings
• No occupant activity (measure before or after occupancy, with HVAC in occupied mode)
• Report Leq dBA and octave-band spectrum (63–8000 Hz)

Step 8: Submit Evidence in WELL Online

The WELL Online platform requires the following documentation for Feature 74:

1. Acoustic design report: Predicted RT60 for each enclosed room, predicted background noise levels, STI analysis for open-plan zones. Include calculation methodology (Sabine/Eyring, ISO 3382-2 reference) and material specifications with manufacturer absorption data.

1. Commissioning records: Verification that installed materials match the specification. Sound masking calibration report showing achieved levels at each zone.

1. Measurement report: Post-construction RT60, background noise, and STI measurements by a qualified professional. Must include measurement methodology, equipment calibration certificates (current within 12 months), positions on floor plan, and pass/fail assessment against Feature 74 thresholds.

1. Floor plan annotations: Mark all measurement positions, space types, and Feature 74 applicability zones on the floor plan.

Common Submission Errors

• Missing calibration certificates for measurement equipment. WELL assessors check these. If the sound level meter calibration expired three months before measurement, the data is rejected.
• Wrong STI metric: Submitting RASTI instead of STIPA. RASTI is an older method (IEC 60268-16:2003) that is no longer accepted by IWBI. Always use STIPA or full STI.
• Incomplete frequency data: Reporting RT60 only at 500 Hz and 1000 Hz. WELL requires the 500–2000 Hz average, meaning you need data at 500, 1000, and 2000 Hz as a minimum.

The Process in Summary

The process is sequential. Each step builds on the previous one. Starting Step 4 without completing Step 3 means the STI calculation lacks the background noise input. Starting Step 7 without completing Step 6 means measuring a room where the treatment may not match the specification.

The Cost of Getting It Wrong

First-time WELL F74 failure triggers a reassessment cycle that typically adds 6–12 weeks and £8,000–£25,000 to the project. The reassessment fee itself is approximately £3,000–£5,000, but the real cost is the remediation work: adding sound masking to an open-plan floor that was not wired for it (£15,000–£40,000 for a 1,000 m² floor), replacing ceiling tiles that do not meet the absorption specification (£10,000–£30,000), or modifying HVAC systems to reduce background noise (variable, but rarely less than £5,000 per zone).

These costs are avoidable. Every first-submission failure can be traced to skipping one of the eight steps above. Step 4 — the speech privacy STI calculation — is the step most commonly skipped, and it is the step that causes the most failures. If your project is targeting WELL v2 Feature 74, start with the STI calculation, not the RT60 prediction. You can verify clause-level traceability for WELL and every other supported standard in our full standards conformance matrix.

Related Reading

• WELL v2 Feature 74 Decoded: Every Acoustic Requirement — the full clause-by-clause breakdown of Parts 1, 2, and 3
• WELL Certification Cost: The Acoustic Component — what the acoustic portion of WELL certification actually costs
• Common WELL Acoustic Certification Failure Modes — the specific failure patterns and how to avoid them