Every Material Choice Is an Acoustic Decision
A single specification change — replacing carpet with polished concrete across a 200 m² office floor — removes approximately 60–80 m² Sabine of acoustic absorption and can increase reverberation time by 0.3–0.5 seconds. That change alone can push a room from "comfortable for conversation" to "fatiguing after thirty minutes." Interior designers make dozens of these decisions on every project, usually without realising that each one has a measurable acoustic consequence.
This is not about adding acoustic panels to fix a problem. It is about understanding that every surface in a room — floor, ceiling, walls, furniture, curtains, partitions — has an absorption coefficient, and the sum of those coefficients determines whether the room sounds good or sounds terrible. Interior designers are acoustic designers whether they know it or not.
How Sound Behaves in Interior Spaces
When sound is produced in a room — speech, music, a phone ringing — it radiates outward and hits every surface. At each surface, three things happen:
- Absorption: Some energy is converted to heat within the material. Soft, porous, fibrous materials absorb more. Hard, smooth, dense materials absorb less.
- Reflection: The remaining energy bounces back into the room. Flat, hard surfaces create strong specular reflections. Curved or irregular surfaces scatter reflections.
- Transmission: A small amount of energy passes through the surface to the other side. This is the sound insulation dimension.
RT60 — the reverberation time — depends on the total absorption in the room. Per the Sabine equation (ISO 3382-2:2008 §A.1):
RT60 = 0.161 × V / A
Where V is room volume (m³) and A is total absorption (m² Sabine). More absorption means shorter reverberation. The interior designer's job is ensuring that A is large enough to achieve the target RT60 while maintaining the aesthetic vision.
Material-by-Material Acoustic Impact
Flooring: The Largest Horizontal Surface
The floor is typically the largest single surface in a room (equal to the ceiling area). Its acoustic impact depends on both absorption (how much airborne sound it absorbs) and impact isolation (how much footfall noise it transmits to spaces below).
| Flooring Type | NRC | 500 Hz α | 1 kHz α | Impact Notes |
|---|---|---|---|---|
| Polished concrete | 0.02 | 0.02 | 0.02 | Maximum reflection, maximum footfall noise |
| Porcelain tile | 0.02 | 0.01 | 0.02 | Similar to concrete acoustically |
| Hardwood (solid on slab) | 0.08 | 0.07 | 0.10 | Slight improvement, still highly reflective |
| Engineered timber on underlay | 0.12 | 0.10 | 0.15 | Underlay adds low-frequency absorption |
| Vinyl (luxury vinyl tile, LVT) | 0.05 | 0.03 | 0.05 | Minimal absorption, moderate impact improvement |
| Carpet tile (standard, 6mm) | 0.25 | 0.20 | 0.35 | Good high-frequency absorption |
| Carpet (cut pile, 10mm on underlay) | 0.40 | 0.30 | 0.55 | Excellent high-frequency, poor low-frequency |
| Rubber (8mm sport/acoustic) | 0.10 | 0.08 | 0.12 | Good impact isolation, modest airborne absorption |
The acoustic difference between carpet and hard flooring is dramatic. In a 200 m² room, carpet (NRC 0.35) provides approximately 70 m² Sabine of absorption. Replacing it with polished concrete (NRC 0.02) removes 66 m² Sabine. For a 3.0 m ceiling height (600 m³ volume), this changes the RT60 calculation significantly — potentially from 0.6 s to over 1.0 s if no other absorption is added.
Seating: Fabric vs Leather vs Vinyl
Seating is one of the most underestimated acoustic elements in interior design. Upholstered furniture is a highly effective absorber, particularly at mid and high frequencies.
| Seating Type | Absorption per Seat (m² Sabine) | Notes |
|---|---|---|
| Fabric upholstery (deep padding) | 0.50–0.70 | Best acoustic performer |
| Fabric upholstery (thin padding) | 0.30–0.45 | Standard commercial seating |
| Leather upholstery | 0.15–0.25 | Surface reflects; only foam core absorbs |
| Vinyl/faux leather | 0.10–0.20 | Air-impermeable surface limits absorption |
| Timber/metal (hard seat) | 0.02–0.05 | Negligible absorption |
| Mesh (task chair) | 0.35–0.50 | Acoustically transparent — air flows through |
In a boardroom with 14 seats, the difference between leather chairs (14 × 0.20 = 2.8 m² Sabine) and fabric-upholstered chairs (14 × 0.55 = 7.7 m² Sabine) is 4.9 m² Sabine — equivalent to approximately 6 m² of wall-mounted acoustic panel. For a 100 m³ boardroom, this difference alone changes RT60 by approximately 0.08 seconds.
Walls: Hard Finishes and Their Alternatives
Wall finishes are where interior design aesthetics most often conflict with acoustic requirements. Feature walls in concrete, tile, or stone create strong reflections that increase reverberation and can cause flutter echo (rapid repetitive reflections between parallel hard surfaces).
| Wall Finish | NRC | Acoustic Character |
|---|---|---|
| Painted plasterboard | 0.05 | Reflective — standard baseline |
| Exposed brick | 0.04 | Very reflective, scatters slightly |
| Timber panelling (solid) | 0.10 | Low-frequency panel resonance possible |
| Fabric wall covering (3mm) | 0.15 | Minimal absorption, primarily decorative |
| Fabric-wrapped panel (50mm absorber) | 0.85 | High-performance absorber |
| Felt wall panel (9mm) | 0.30 | Moderate absorption, design-forward |
| Perforated timber over 50mm cavity | 0.65 | Concealed absorption behind architectural finish |
| Acoustic plaster (20mm on substrate) | 0.60 | Seamless visual finish, good performance |
| Cork wall tile (6mm) | 0.15 | Modest absorption, natural aesthetic |
| Green wall (living plants, 150mm depth) | 0.40 | Variable performance, biophilic benefit |
The key insight for interior designers is that absorption can be concealed. Microperforated timber, acoustic plaster, stretched fabric systems, and perforated metal panels all provide absorption while maintaining a refined visual finish. The days when "acoustic treatment" meant ugly foam wedges are long past.
Partitions and Glass
Glazed partitions are ubiquitous in modern interior design — they admit light, create visual connection, and feel contemporary. Acoustically, they present two issues: low absorption (glass is highly reflective, NRC 0.05–0.08) and limited sound insulation.
| Partition Type | NRC | STC/Rw | Notes |
|---|---|---|---|
| Single glazed (10mm) | 0.05 | STC 31 | Poor insulation, high reflection |
| Double glazed (6-12-6, sealed) | 0.08 | STC 35 | Moderate improvement |
| Acoustic double glazed (10-50-6) | 0.08 | STC 42 | Wide air gap critical |
| Laminated acoustic glass (10.8mm) | 0.06 | STC 38 | PVB interlayer adds mass |
| Solid plasterboard (2×12.5mm, 90mm stud) | 0.05 | STC 44 | Standard commercial partition |
| Solid partition with acoustic insulation | 0.05 | STC 52 | Insulated cavity, resilient channels |
For interior designers specifying glazed partitions to meeting rooms, the critical question is whether speech privacy is required. If a meeting room shares a glazed partition with an open plan area, STC 35 (standard single glazing) means conversations are audible as muffled but partially intelligible speech. STC 42 (acoustic double glazing) reduces intelligibility significantly. STC 48+ is needed for true speech privacy per WELL v2 Feature 74 requirements.
WELL v2 Feature 74 for Interior Designers
WELL v2 Feature 74 directly impacts interior design decisions. The three parts create specific material requirements:
Part 1: Enclosed Room Acoustics
- RT60 ≤ 0.6 s for meeting rooms under 150 m³ (per ISO 3382-2:2008 §A.1)
- Background noise ≤ 35 dBA in enclosed spaces
Part 2: Open Plan Noise Levels
- Background noise ≤ 45 dBA overall, ≤ 35 dBA from mechanical systems
- Sound masking at 40–45 dBA if installed
Part 3: Speech Privacy
- STC requirements between enclosed rooms and adjacent spaces
- STI targets that depend on background noise and absorption
Worked Example: 200 m² Co-Working Space Interior Fitout
A co-working operator commissions an interior fitout of a 200 m² shell-and-core commercial space. Ceiling height is 3.2 m (total volume: 640 m³). The space must accommodate:
- Hot desking zone (100 m²): 24 desks, open plan
- Focus pods (2 × 8 m² = 16 m²): enclosed, single-person quiet work
- Meeting room (20 m²): 8-person capacity, glazed on one side
- Phone booths (3 × 2 m² = 6 m²): single-person calls
- Social/kitchen area (40 m²): informal meetings, coffee, lunch
- Circulation (18 m²)
The Acoustic Challenge
The design brief creates a conflict. Exposed ceiling services eliminate the most effective absorption surface. Polished concrete eliminates the second most effective absorption surface. The RT60 of the raw shell will be approximately:
Shell surfaces: 200 m² concrete floor (NRC 0.02), 200 m² concrete ceiling (NRC 0.02), approximately 160 m² perimeter walls with windows (average NRC 0.05). Total absorption: 200×0.02 + 200×0.02 + 160×0.05 = 16.0 m² Sabine. RT60 = 0.161 × 640 / 16.0 = 6.4 seconds — a concrete box.
The Solution: Compensating Absorption Strategy
Ceiling: Install acoustic rafts (suspended horizontal panels, 1200×600mm, 40mm mineral wool, fabric-faced) covering 40% of the ceiling area. 80 m² at NRC 0.90 = 72.0 m² Sabine. The remaining 120 m² of exposed services contributes approximately 0.10 NRC = 12.0 m² Sabine (pipes, ducts, and cables provide modest scattering absorption).
Walls: Fabric-wrapped acoustic panels on 30% of the perimeter walls. 48 m² at NRC 0.85 = 40.8 m² Sabine. Remaining 112 m² at NRC 0.05 = 5.6 m² Sabine.
Floor: Polished concrete throughout (as per brief). 200 m² at NRC 0.02 = 4.0 m² Sabine.
Furniture: 24 upholstered task chairs at 0.45 m² Sabine each = 10.8 m² Sabine. 2 sofas in social area at 3.5 m² Sabine each = 7.0 m² Sabine.
Total absorption: 72.0 + 12.0 + 40.8 + 5.6 + 4.0 + 10.8 + 7.0 = 152.2 m² Sabine
Resulting RT60: 0.161 × 640 / 152.2 = 0.68 seconds
This is within the WELL v2 target for open plan areas (0.6–0.8 s) and very close to the 0.6 s target for the meeting room. The meeting room, being a smaller enclosed space (20 m² × 3.2 m = 64 m³), will have its own acoustic environment. With an absorptive ceiling raft above the meeting room (NRC 0.90) and fabric-wrapped panels on one wall, the meeting room can independently achieve RT60 ≤ 0.6 s.
Partition Specifications
- Meeting room glazed wall: Acoustic double glazed (10-50-6mm configuration), STC 42, full-height to soffit with acoustic seals at all edges
- Focus pods: Solid partition with acoustic insulation, STC 45, solid-core door STC 33
- Phone booths: Proprietary acoustic booth system, minimum STC 35 composite (including door)
Cost Impact
The acoustic treatment adds approximately £120–£180 per m² to the fitout cost (2026 UK pricing):
| Element | Area | Unit Cost | Total |
|---|---|---|---|
| Acoustic ceiling rafts | 80 m² | £85–£120/m² | £6,800–£9,600 |
| Fabric wall panels | 48 m² | £95–£140/m² | £4,560–£6,720 |
| Acoustic glazing upgrade (meeting room) | 12 m² | £180–£250/m² | £2,160–£3,000 |
| Phone booth pods (3 units) | — | £3,500–£5,000 each | £10,500–£15,000 |
| Total acoustic treatment | £24,020–£34,320 | ||
| Per m² of gross floor area | 200 m² | £120–£172/m² |
This represents approximately 8–12% of a typical commercial fitout cost of £1,200–£1,500/m². Without this investment, the space will fail WELL certification, generate complaints from every tenant who tries to make a phone call, and require retrospective treatment that costs 50–100% more than doing it right during the fitout.
Five Rules for Interior Designers
- Compensate aggressively: Every hard surface you specify requires compensating absorption elsewhere. If the floor is polished concrete and the feature wall is exposed brick, the ceiling and remaining walls must work much harder.
- Ceiling first: If you can only absorb on one surface, make it the ceiling. Ceiling absorption controls RT60 more effectively than any other surface because sound hits the ceiling on nearly every reflection path.
- Concealed absorption exists: Microperforated timber, acoustic plaster, stretched fabric systems, and perforated metal panels provide NRC 0.50–0.85 while maintaining a refined visual finish. You do not need to compromise the design.
- Doors kill partitions: An STC 45 wall with an STC 20 hollow-core door delivers a composite rating of approximately STC 27. Every acoustically-rated partition requires a matching door specification. Solid-core doors with perimeter seals and drop-bottom seals achieve STC 33–38.
- Check with a calculator: Use AcousPlan's RT60 calculator to verify that your material selections meet the RT60 target before finalising the specification. It takes five minutes and prevents expensive post-occupancy failures.
Further Reading
- WELL v2 Feature 74 Decoded — complete technical breakdown of every WELL acoustic requirement
- How Acoustic Panels Work: The Physics Explained — understand absorption mechanisms behind the products
- Guide to Acoustic Materials — comprehensive material reference with absorption data