In 2024 a Plantronics study of 2,000 office workers found that 60% rated poor acoustics as the primary barrier to effective meetings — ranking above poor internet connectivity and inadequate display technology. The statistic is credible because it is physically inevitable: a conference room with an RT60 of 0.9 seconds and no acoustic treatment will produce a Speech Transmission Index (STI) of approximately 0.40 at the far end of a 6-metre table, placing speech intelligibility squarely in the "Poor" category under IEC 60268-16. Every word said at that table is understood with effort, not clarity.
This guide provides the acoustic design framework for conference rooms — from the standard targets, through the physics of what goes wrong, to the treatment strategy that fixes it.
The Standards Framework
Three documents govern conference room acoustics in most markets:
IEC 60268-16:2020 defines the Speech Transmission Index (STI) and its rapid measurement variant STIPA. STI is the primary intelligibility metric. For conference rooms, STI ≥ 0.60 is classified "Good." STI ≥ 0.75 is "Excellent" — appropriate for executive boardrooms and teleconference suites. STI below 0.45 is "Poor" and speech effort increases significantly.
ANSI/ASA S12.60-2010/Part 2 extends classroom acoustic requirements to conferencing and multi-use spaces. Section 4.1 specifies a maximum RT60 of 0.6 seconds for rooms up to 283 m³ and background noise of NC 30 or lower.
ITU-T P.800 / ISO 29661 governs teleconferencing audio quality, defining MOS-LQO (Mean Opinion Score — Listening Quality Objective) targets of ≥ 3.5 for acceptable voice quality in hybrid meetings. Room acoustics directly feed into MOS through the Noise and Reverberation impairment factors.
RT60 Targets by Room Size
RT60 is not a single target — it scales with room volume because the ear integrates reverberation against the direct sound from the nearest speaker. A longer RT60 is perceptually tolerable in a large room because the initial time delay gap (the time between the direct sound and the first reflection) is greater, giving the auditory system time to process the direct signal before the reverberant tail arrives.
| Room Volume | Recommended RT60 (500 Hz) | Occupancy |
|---|---|---|
| < 30 m³ | 0.30–0.40 s | 2–4 person huddle |
| 30–100 m³ | 0.40–0.55 s | 6–12 person meeting |
| 100–300 m³ | 0.50–0.65 s | Boardroom, training room |
| 300–700 m³ | 0.60–0.80 s | Large conference, AV-assisted |
These targets apply at mid-frequencies (500 Hz and 1000 Hz octave bands). Low-frequency RT60 (125 Hz and 250 Hz) is typically 20–40% longer than mid-frequency in concrete-and-glass construction — this is acceptable acoustically provided the low-frequency background noise spectrum is also low (the NC 30 criterion includes the 125 Hz band).
Speech Transmission Index Requirements
STI is the metric that ties RT60, background noise, and system gain together into a single intelligibility number. It ranges from 0.0 (unintelligible) to 1.0 (perfect), and is calculated by measuring how much the room degrades the modulation of speech across seven octave bands from 125 Hz to 8000 Hz.
For conference rooms, the two primary STI degradation paths are:
Reverberation: High RT60 smears successive syllables. The critical interference occurs when the reverberation tail from one syllable overlaps the direct sound of the next. At normal conversational rates (3–4 syllables per second), reverberation lasting more than 0.6–0.7 seconds begins to cause measurable masking.
Background noise: HVAC noise, computer fan noise, and noise bleed from adjacent spaces raise the noise floor against which speech must compete. The Signal-to-Noise Ratio (SNR) must exceed +15 dB for STI ≥ 0.60. At NC 30, the A-weighted noise floor is approximately 35–38 dBA; speech at the near end of a 6-metre table is approximately 60–65 dBA at the listener — giving SNR of 22–30 dB, which is adequate. Problems arise when NC levels creep to 40+ (HVAC undersized for the added load of AV equipment) or when speech levels are low (soft-spoken remote participants).
AV System Acoustic Interaction
The AV system is not separate from acoustics — it is part of the acoustic chain. Three interactions matter most:
Loudspeaker Directivity and Coverage
A ceiling loudspeaker in a reverberant room creates a transition point at the Critical Distance (Dc) where the reverberant field energy equals the direct field energy. Beyond Dc, STI drops rapidly because the listener is receiving more reverberant energy than direct energy.
Dc = 0.14 × √(Q × V / T60)
Where Q is the loudspeaker directivity (Q-factor), V is room volume (m³), and T60 is the reverberation time (s). For a typical conference room with V = 80 m³ and T60 = 0.5 s, a ceiling speaker with Q = 3 (hemispherical) gives Dc = 0.14 × √(3 × 80 / 0.5) = 0.14 × √480 = 3.1 m. Listeners more than 3.1 m from the ceiling speaker are in the reverberant field. A distributed array with speakers every 2–3 m keeps all positions within Dc and maintains STI without requiring lower RT60.
Echo Cancellation and Beamforming Microphones
Modern conference room DSP processors include Acoustic Echo Cancellation (AEC) with Tail Length settings of 128–512 ms. This suppresses loudspeaker-to-microphone feedback within the cancellation window. However, AEC requires a minimum Direct-to-Reverberant Ratio (DRR) of approximately +6 dB at the microphone to converge reliably. In a room with RT60 = 1.2 s, ceiling-mounted microphones typically receive DRR of 0 to –6 dB, causing AEC instability — characterised by the "underwater" speech artefact that remote participants hear.
Beamforming microphone arrays (e.g., ceiling pucks with 64-microphone arrays) achieve effective gain of 12–18 dB over omnidirectional elements, raising DRR by the same margin. This compensates for moderate over-reverberation, but the compensation is finite and expensive. Fixing the room RT60 is always the more reliable solution.
Hearing Loop Compatibility
Conference rooms in public buildings in the UK (under BS 8300) and many US jurisdictions (ADA Technical Bulletin No. 1) must provide hearing loop (induction loop) coverage. Hearing loop performance is specified by IEC 60118-4 and requires field strength of –16 dBa ± 3 dB across the room footprint. The loop frequency response is flat from 100 Hz to 5000 Hz. Architectural metalwork (raised access floors, aluminium partitions, reinforced concrete) can cause up to 20 dB of loop field attenuation — flagged at design stage, not on commissioning day.
Worked Example: 50 m³ Meeting Room
Room dimensions: 5.0 m × 4.0 m × 2.5 m = 50 m³
Occupancy: 8 people (1.0 m² average seat spacing)
Existing finishes (before treatment):
- Concrete floor with carpet tiles: α₅₀₀ = 0.20
- Plastered concrete walls: α₅₀₀ = 0.04
- Suspended ceiling (mineral fibre, 15 mm): α₅₀₀ = 0.55
- Glass (one 8 m² façade): α₅₀₀ = 0.04
Total absorption A = Σ(Si × αi):
- Floor (20 m²): 20 × 0.20 = 4.0 m²
- Three plaster walls (minus door and glass): (38 – 8 – 2) m² × 0.04 = 1.12 m²
- Glass façade (8 m²): 8 × 0.04 = 0.32 m²
- Door (2 m²): 2 × 0.10 = 0.20 m²
- Ceiling (20 m²): 20 × 0.55 = 11.0 m²
- 8 seated persons: 8 × 0.50 = 4.0 m² (per ISO 354 absorption units)
RT60 = 0.161 × V / A = 0.161 × 50 / 20.64 = 0.39 s
That result looks fine — but it conceals a problem. Mineral fibre ceiling tiles perform well at 500 Hz (α = 0.55–0.65) but perform poorly at 125 Hz (α = 0.15–0.25). Recalculating at 125 Hz:
Total A₁₂₅ = (20 × 0.08) + (28 × 0.02) + (8 × 0.03) + (20 × 0.20) + (8 × 0.25) = 1.6 + 0.56 + 0.24 + 4.0 + 2.0 = 8.4 m²
RT60₁₂₅ = 0.161 × 50 / 8.4 = 0.96 s
The 125 Hz reverberation time is 2.5× longer than the 500 Hz value. This means the low-frequency tail of male voices (fundamental frequency 85–185 Hz, with first formant at 250–800 Hz) will extend far beyond the high-frequency decay. The room will sound "boomy" and speech will lack clarity at the bottom end.
Treatment strategy:
Adding two 1200 × 2400 × 100 mm thick fibre glass panels (48 kg/m³) mounted on the rear wall with 50 mm air gap provides:
- At 125 Hz: α = 0.80 per panel → additional A₁₂₅ = 2 × (2.88 × 0.80) = 4.6 m²
- At 500 Hz: α = 0.95 → additional A₅₀₀ = 2 × 2.88 × 0.95 = 5.5 m²
Adding 15 m² of thinner panel (50 mm, NRC 0.90) to wall segments instead gives: Post-treatment RT60₅₀₀ = 0.161 × 50 / (20.64 + 15 × 0.90) = 0.161 × 50 / 34.14 = 0.24 s (too dead)
The practical optimum for this room is approximately 6–8 m² of 100 mm panel at corners plus 6–8 m² of 50 mm panel on the rear wall, targeting RT60 = 0.40–0.45 s across the 500 Hz and 1000 Hz bands.
Treatment Strategies
Ceiling Treatment
The ceiling is acoustically the most efficient surface in a conference room because it covers the largest area and sits in the near-field of ceiling-mounted speakers and microphones. Options in order of acoustic performance:
Suspended baffles (100 mm thick, hung 200–400 mm below structural ceiling): NRC 0.90–1.05 (can exceed 1.0 because absorption occurs on all surfaces). Effective from 125 Hz upward. Ideal for rooms with exposed concrete soffit and HVAC services.
Lay-in ceiling tiles (Armstrong, Rockfon, USG Boral): NRC varies from 0.55 (standard 16 mm mineral fibre) to 0.90 (25 mm perforated metal with glass fibre backing). Higher-performance tiles with NRC 0.85+ are available in standard 600 × 600 mm grid systems.
Acoustic clouds (suspended panels, 50–100 mm, fabric-wrapped): Most aesthetically flexible. Effective when positioned directly above the conference table, where they intercept early ceiling reflections before they blur direct speech.
Wall Treatment
Side wall reflections arrive at the listener 10–30 ms after the direct sound in a 4–6 m wide conference room. Reflections arriving within 35 ms are added to the direct sound by the auditory system (Haas effect) — they reinforce clarity. Reflections arriving after 50 ms degrade STI. Side wall treatment should target the zones where reflections arrive late (rear half of side walls, rear wall), not the front wall where early reflections from the presentation surface are beneficial.
Panel specification: 50 mm glass fibre, 48 kg/m³, fabric-wrapped (Guilford of Maine FR701 or equivalent). Cover 30–40% of side wall and 60–70% of rear wall area. NRC 0.90–1.00 at 500 Hz.
Noise Control
Meeting room HVAC noise is the second acoustic parameter after RT60. NC 30 (equivalent to approximately 35–38 dBA) is the target. Common problems:
- Undersized supply grilles producing velocity noise above 5 m/s (target < 2.5 m/s)
- Duct-borne crosstalk between adjacent rooms via shared HVAC trunk (fix: sound attenuator on branch ducts)
- VRF/FCU fan noise radiating through the ceiling plenum (fix: acoustic lining in plenum above meeting room)
Summary Checklist
A conference room acoustic brief should specify:
- RT60 target band at 500 Hz (typically 0.4–0.6 s for 30–200 m³)
- STI target at the far end of the table (≥ 0.60 Good, ≥ 0.75 Excellent)
- Background noise limit (NC 30 maximum, NC 25 for executive spaces)
- Loudspeaker Critical Distance calculation confirming coverage model
- Treatment area and specification (NRC, thickness, density, installation detail)
- Low-frequency RT60 check at 125 Hz — often 3× the mid-frequency value in untreated rooms
- AEC tail length confirmation (DSP setting matches actual room RT60)