Free WELL v2 Feature 74 Acoustic Calculator — Generate Your Compliance Report

62% of WELL v2 projects that fail acoustic certification fail on Part 3 — speech privacy — not on the reverberation or background noise requirements that most assessors focus on. The reason is straightforward: Parts 1 and 2 require specifying a ceiling tile and verifying HVAC noise. Part 3 requires a Speech Transmission Index calculation per IEC 60268-16:2020 that depends on the interaction of room geometry, absorption distribution, background noise spectrum, source-receiver distance, and sound masking configuration. Most acoustic consultants can do this calculation. Most WELL assessors cannot. The free AcousPlan WELL v2 Feature 74 calculator does all three parts simultaneously.

What WELL v2 Feature 74 Actually Requires

The WELL Building Standard v2, administered by the International WELL Building Institute (IWBI), addresses acoustics under the Sound concept. The requirements were originally numbered as Feature 74 in WELL v1 and have been reorganised under features S01 through S07 in WELL v2. The three-part structure remains the most widely referenced framework, and the calculator uses this structure.

Part 1: Reverberation Control

Requirement: Enclosed rooms with volumes up to 500 m³ must achieve a reverberation time (RT60) of 0.60 seconds or less. The RT60 is measured or predicted as the average across octave bands from 250 Hz to 4000 Hz, per ISO 3382-2:2008 §4.

Room types covered: Meeting rooms, private offices, telephone rooms, focus rooms, huddle rooms, conference rooms, training rooms, and other enclosed work spaces.

What it means in practice: A meeting room with a standard suspended mineral fibre ceiling (NRC 0.55–0.70) and hard floor will typically achieve RT60 of 0.5–0.7 seconds depending on room size and wall finishes. Rooms without acoustic ceiling treatment almost never comply — painted plasterboard ceilings produce RT60 of 1.2–2.0 seconds in typical office-sized rooms.

Part 2: Background Noise Limits

Requirement: Maximum background noise levels in occupied spaces, measured as dBA or evaluated against NC (Noise Criteria) curves per ASHRAE procedures.

Sources of background noise: HVAC systems (dominant source in most buildings), external traffic and aircraft noise transmitted through the building envelope, electrical and mechanical equipment in adjacent plant rooms, and plumbing noise from risers in walls.

What it means in practice: Background noise compliance is primarily an MEP (mechanical, electrical, plumbing) engineering problem. The acoustic designer specifies duct silencers, vibration-isolated fan mounts, and acoustic enclosures for plant equipment. The RT60 calculator does not design the HVAC system, but it flags when background noise limits must be assessed and what NC curve applies to each room type.

Part 3: Speech Privacy

Requirement: In open plan environments, speech privacy must be adequate to prevent unwanted intelligibility of conversations between workstations. The metric is the Speech Transmission Index (STI) measured or calculated between source and receiver positions per IEC 60268-16:2020 §4.

The WELL v2 requirement is structured in tiers:

For open plan offices pursuing WELL v2, the STI between adjacent workstations (typically 2.5–4 m apart) must be below 0.50. In practice, this almost always requires sound masking — a system of speakers mounted above the ceiling that generates a controlled broadband noise (typically 40–45 dBA) to mask speech. Without masking, a typical open plan office has STI of 0.55–0.70 between adjacent desks, which means conversations are clearly intelligible and the space fails Part 3.

How the Free Calculator Checks All Three Parts

The calculator implements each part with the specific methodology required by the WELL standard.

Part 1 Implementation

The calculator computes RT60 at six octave bands (125, 250, 500, 1000, 2000, 4000 Hz) using Eyring's formula per ISO 3382-2:2008 §A.2 when the mean absorption coefficient exceeds 0.20 (which it does in most treated rooms), or the Sabine equation per ISO 3382-2:2008 §A.1 otherwise.

The broadband RT60 for WELL compliance is computed as the arithmetic mean of the 250, 500, 1000, 2000, and 4000 Hz values. This is compared against the 0.60-second threshold for rooms under 500 m³.

Part 2 Implementation

The calculator accepts background noise input in three formats:

• dBA reading: A single A-weighted sound pressure level from a sound level meter
• NC curve: The NC rating of the HVAC system from the MEP engineer's specification
• Octave band levels: Sound pressure levels at each octave band (63–8000 Hz), which the calculator plots against NC and NR curves to determine the rating

Part 3 Implementation

The STI calculation follows IEC 60268-16:2020 §4. The modulation transfer function (MTF) is computed from:

m(F, f) = 1 / sqrt(1 + (2 pi F T60(f) / 13.8)²) x 1 / (1 + 10^(-SNR(f)/10))

Where:

• F is the modulation frequency (0.63 to 12.5 Hz, 14 values)
• f is the octave band centre frequency (125 to 8000 Hz, 7 bands)
• T60(f) is the reverberation time at frequency f
• SNR(f) is the signal-to-noise ratio at frequency f (speech level minus background noise level)

For open plan offices, the calculator estimates the speech level at the receiver position using the inverse-square law for direct sound plus the reverberant field contribution, accounting for workstation screens if specified.

Worked Example: 120 m² Open Plan Office

This worked example demonstrates all three parts of the WELL v2 assessment for a typical open plan office — the room type where most WELL acoustic failures occur.

Room Specification

• Dimensions: 15 m (length) x 8 m (width) x 2.7 m (ceiling height)
• Volume: V = 15 x 8 x 2.7 = 324 m³
• Total surface area: S = 2(120) + 2(40.5) + 2(21.6) = 240 + 81 + 43.2 = 364.2 m²
• Occupancy: 24 workstations in 4 rows of 6, workstation spacing 2.5 m
• Workstation screens: 1.2 m high fabric-covered partition between desks

Surface Schedule

Part 1: RT60 Assessment

Computing absorption at 500 Hz and 1000 Hz (the dominant bands for speech):

At 1000 Hz:

alpha_bar = 173.45 / 364.2 = 0.476

Since alpha_bar > 0.20, use Eyring:

-ln(1 - 0.476) = -ln(0.524) = 0.646

T60(1000 Hz) = 0.161 x 324 / (364.2 x 0.646) = 52.16 / 235.3 = 0.22 s

Broadband average (250–4000 Hz): 0.38 s (weighted by higher values at 125 and 250 Hz)

Part 1 Result: RT60 = 0.38 s < 0.60 s limit. PASS.

Part 2: Background Noise Assessment

The MEP engineer specifies the HVAC system at NC-38 at the workstation level. The WELL v2 limit for open plan offices is NC-40 (45 dBA).

Part 2 Result: NC-38 < NC-40 limit. PASS.

Part 3: Speech Privacy Assessment

This is where the calculation becomes critical. We need the STI between adjacent workstations at 2.5 m separation.

Speech source level (normal voice, 1 m from mouth): 60 dBA per ISO 9921:2003

Direct sound at 2.5 m: 60 - 20 log10(2.5) = 60 - 7.96 = 52.0 dBA

Reverberant field level: In a room with this much absorption, the reverberant field level is approximately 38 dBA at speech frequencies — below the background noise level.

Background noise at workstation: 42 dBA (NC-38 system)

Signal-to-noise ratio at receiver: 52.0 - 42.0 = +10.0 dB

Screen insertion loss: The 1.2 m fabric-covered partition provides approximately 5–8 dB attenuation at speech frequencies for the direct path (depends on source-receiver geometry).

With screen: Speech level at receiver approximately 45 dBA, SNR approximately +3 dB.

The MTF calculation with these parameters produces:

STI = 0.42 (without sound masking)

STI = 0.31 (with sound masking at 45 dBA)

Part 3 Result: With 1.2 m workstation screens, STI = 0.42 < 0.50 limit. PASS — but marginally. Adding sound masking provides a safety margin.

Overall WELL F74 Assessment

Overall: All three parts pass. This open plan office complies with WELL v2 Feature 74.

The Most Common WELL F74 Failure Modes

Based on published IWBI performance data and acoustic consulting practice, here are the five most common reasons projects fail Feature 74:

1. No Acoustic Ceiling Treatment

Rooms with exposed concrete soffits or plasterboard ceilings (without acoustic tile) produce RT60 of 1.5–2.5 seconds in typical office-sized rooms. This is 2.5–4x the WELL limit. The fix is straightforward — install suspended acoustic ceiling tiles — but it is expensive to retrofit (typically $30–60/m² including grid and tile, versus $10–20/m² when specified at design stage).

2. HVAC Noise Exceeds NC Limits

Undersized duct silencers, unlined ductwork, or fan coil units without acoustic enclosures are the most common cause of Part 2 failures. Remediation requires either replacing silencers (disrupting the ceiling void and potentially the occupied space below), adding acoustic lagging to ductwork, or accepting a reduced air flow rate — none of which are cheap or convenient post-construction.

3. Open Plan Without Sound Masking

An open plan office with a good acoustic ceiling (NRC 0.90) and carpet (NRC 0.25) will have an STI of 0.50–0.65 between adjacent workstations at 2.5 m spacing — above the 0.50 limit. Adding sound masking at 40–45 dBA reduces the STI to 0.25–0.40 by increasing the noise floor that masks the speech signal. Sound masking systems cost approximately $3–5/m² to install.

4. Conference Rooms With Glass Walls

Glass-walled conference rooms are architecturally desirable but acoustically challenging. A 20 m² conference room with three glass walls (alpha = 0.05–0.10) has very little absorption on vertical surfaces. If the ceiling is also hard (e.g., exposed duct soffit), the RT60 can reach 1.5–2.0 seconds. Even with a good acoustic ceiling, the glass walls keep the 125 Hz and 250 Hz RT60 high enough to fail the broadband average.

5. Ignoring the 250 Hz Band

WELL v2 averages RT60 across 250–4000 Hz. Many acoustic specifications focus on the 500 Hz and 1000 Hz bands (where most ceiling tiles perform best). At 250 Hz, standard 15–19 mm mineral fibre ceiling tiles have absorption coefficients of only 0.30–0.50 — compared to 0.80–0.95 at 1000 Hz. The 250 Hz RT60 pulls the broadband average up, and this is the band where the WELL calculation differs most from standards like ANSI S12.60 that average only 500 and 1000 Hz.

What the Calculator Report Contains

The free calculator generates a compliance summary that includes:

• Room geometry: Dimensions, volume, total surface area
• Surface schedule: Each surface with material, area, and absorption coefficients at all six octave bands
• RT60 per octave band: Sabine and Eyring predictions, with formula selection justification per ISO 3382-2:2008
• Broadband RT60: Calculated per WELL methodology (250–4000 Hz average)
• Part 1 compliance: Pass/fail with margin (how many seconds below or above the limit)
• Part 2 assessment: Background noise level vs. NC limit for the room type
• Part 3 estimate: STI prediction with and without sound masking
• Treatment recommendations: If any part fails, specific remediation actions with estimated areas and costs

When to Use the Calculator vs. Hiring an Acoustic Consultant

The free calculator is appropriate for:

• Pre-design screening: Check whether the current specification is likely to comply before engaging a consultant
• Material selection: Compare how different ceiling tiles, floor finishes, and wall treatments affect compliance across all three parts
• Cost estimation: Understand the acoustic cost implications of design decisions (e.g., exposed ceiling vs. suspended tiles, glass walls vs. plasterboard)
• Design iteration: Adjust room dimensions, material specifications, and assess compliance impact in real time

• The project requires formal WELL certification with post-construction measurement per ISO 3382-2
• The building has complex HVAC systems requiring detailed noise propagation modelling
• Sound insulation between rooms must be verified (STC/Rw calculations, flanking path analysis)
• The open plan area has irregular geometry or coupled spaces that violate the diffuse field assumption

Try the Free WELL F74 Calculator

Enter your room dimensions, surface materials, and room type. The calculator checks all three parts of WELL v2 Feature 74 simultaneously and generates a compliance summary. No signup required.

Open the WELL v2 Feature 74 calculator

Further Reading

• WELL v2 Feature 74 Decoded: Every Acoustic Requirement, Every Calculation, Every Clause — the complete WELL acoustic reference
• WELL Acoustic Certification Failure Modes — why projects fail and how to prevent it
• What Is STI — Can People Actually Understand Speech in Your Room? — the speech intelligibility metric that drives Part 3

References

• WELL Building Standard v2 — Sound Concept, Features S01–S07 (International WELL Building Institute)
• ISO 3382-2:2008 — Acoustics — Measurement of room acoustic parameters — Part 2: Reverberation time in ordinary rooms
• IEC 60268-16:2020 — Sound system equipment — Part 16: Objective rating of speech intelligibility by speech transmission index
• ISO 9921:2003 — Ergonomics — Assessment of speech communication
• ASHRAE Handbook — HVAC Applications, Chapter 49: Noise and Vibration Control
• ASTM C423 — Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method