An open-plan office is acoustically hostile by design: large volume, hard surfaces, and reflective glass on multiple sides. The starting RT60 in an untreated space routinely exceeds 1.2 seconds — more than double the ISO 3382-3 target of 0.3–0.5 s. This article calculates RT60 for a realistic 300 m² open-plan office from first principles, showing every number at each octave band, then demonstrates how treatment brings it into compliance.
The Room
A single-floor open-plan office with these dimensions:
- Plan area: 25 m × 12 m = 300 m²
- Ceiling height: 2.7 m (exposed concrete slab above)
- Volume: 300 × 2.7 = 810 m³
- Total surface area: 2 × (25×12 + 25×2.7 + 12×2.7) = 2 × (300 + 67.5 + 32.4) = 799.8 m² (round to 800 m²)
| Surface | Dimensions | Area (m²) |
|---|---|---|
| Floor | 25 × 12 | 300 |
| Ceiling | 25 × 12 | 300 |
| Long wall A (south) | 25 × 2.7 | 67.5 |
| Long wall B (north) | 25 × 2.7 | 67.5 |
| Short wall A (east) — 60% glazing | 12 × 2.7 | 32.4 |
| Short wall B (west) — solid | 12 × 2.7 | 32.4 |
| Total | 800 m² |
The east wall is 60% glazing (19.4 m²) and 40% plasterboard (13.0 m²).
Sabine's Equation
Per ISO 3382-2:2008 §A.1:
RT60 = 0.161 × V / A
Where A = Σ(Si × αi) is the total absorption area in m² (metric sabins). We calculate A separately for each octave band: 125, 250, 500, 1000, 2000, and 4000 Hz.
Scenario 1 — Bare Office (Untreated)
The untreated office has:
- Floor: polished concrete
- Ceiling: bare concrete slab
- Walls: painted plasterboard (long walls + west short wall)
- East wall: 60% float glass, 40% painted plasterboard
- Furnishings: none (for the bare calculation)
Absorption Coefficients — Untreated Surfaces
All values are ISO 354 random-incidence absorption coefficients:
| Surface | 125 Hz | 250 Hz | 500 Hz | 1000 Hz | 2000 Hz | 4000 Hz |
|---|---|---|---|---|---|---|
| Polished concrete (floor) | 0.01 | 0.01 | 0.02 | 0.02 | 0.02 | 0.02 |
| Bare concrete slab (ceiling) | 0.02 | 0.02 | 0.03 | 0.03 | 0.03 | 0.03 |
| Painted plasterboard on studs | 0.15 | 0.10 | 0.06 | 0.05 | 0.04 | 0.04 |
| Float glass (12 mm) | 0.35 | 0.25 | 0.18 | 0.12 | 0.07 | 0.04 |
Surface Area Breakdown
Before computing absorption, assign each surface its material and area:
| Surface | Material | Area (m²) |
|---|---|---|
| Floor | Polished concrete | 300 |
| Ceiling | Bare concrete slab | 300 |
| Long wall A | Painted plasterboard | 67.5 |
| Long wall B | Painted plasterboard | 67.5 |
| East wall — glass | Float glass | 19.4 |
| East wall — plasterboard | Painted plasterboard | 13.0 |
| West wall | Painted plasterboard | 32.4 |
Total plasterboard area: 67.5 + 67.5 + 13.0 + 32.4 = 180.4 m²
Absorption Calculation at 500 Hz (Key Band)
Working through the 500 Hz band as a demonstration:
| Surface | Area (m²) | α (500 Hz) | Si × αi |
|---|---|---|---|
| Polished concrete floor | 300 | 0.02 | 6.00 |
| Bare concrete ceiling | 300 | 0.03 | 9.00 |
| Painted plasterboard (total) | 180.4 | 0.06 | 10.82 |
| Float glass | 19.4 | 0.18 | 3.49 |
| Total A (500 Hz) | 29.31 m² |
RT60 (500 Hz) = 0.161 × 810 / 29.31 = 130.4 / 29.31 = 4.45 s
That is deeply reverberant — nearly five seconds at 500 Hz.
Full Octave-Band Calculation — Untreated
| Band (Hz) | A_concrete floor | A_concrete ceiling | A_plasterboard | A_glass | Total A (m²) | RT60 (s) |
|---|---|---|---|---|---|---|
| 125 | 3.00 | 6.00 | 27.06 | 6.79 | 42.85 | 3.04 |
| 250 | 3.00 | 6.00 | 18.04 | 4.85 | 31.89 | 4.09 |
| 500 | 6.00 | 9.00 | 10.82 | 3.49 | 29.31 | 4.45 |
| 1000 | 6.00 | 9.00 | 9.02 | 2.33 | 26.35 | 4.95 |
| 2000 | 6.00 | 9.00 | 7.22 | 1.36 | 23.58 | 5.53 |
| 4000 | 6.00 | 9.00 | 7.22 | 0.78 | 23.00 | 5.67 |
The mid-frequency RT60 (average of 500 and 1000 Hz) = (4.45 + 4.95) / 2 = 4.70 s. This is approximately ten times the ISO 3382-3 target.
Scenario 2 — Partially Treated (Typical Fit-Out)
A standard commercial fit-out adds:
- Floor: carpet tiles (full area, 300 m²)
- Ceiling: 600×600 mm suspended acoustic ceiling tiles (T-bar grid, full area, 300 m²)
- Walls: unchanged painted plasterboard
- Glass: unchanged
- Furniture: 30 open-plan workstations with upholstered partitions (1.5 m height, 1.5 m wide × 2 sides × 30 = 90 m² of upholstered surface)
New Absorption Coefficients
| Surface | 125 Hz | 250 Hz | 500 Hz | 1000 Hz | 2000 Hz | 4000 Hz |
|---|---|---|---|---|---|---|
| Carpet tiles on concrete | 0.02 | 0.06 | 0.14 | 0.37 | 0.60 | 0.65 |
| Acoustic ceiling tile (NRC 0.85) | 0.25 | 0.50 | 0.80 | 0.95 | 0.95 | 0.90 |
| Upholstered office partitions | 0.05 | 0.15 | 0.35 | 0.55 | 0.60 | 0.60 |
| Painted plasterboard on studs | 0.15 | 0.10 | 0.06 | 0.05 | 0.04 | 0.04 |
| Float glass (12 mm) | 0.35 | 0.25 | 0.18 | 0.12 | 0.07 | 0.04 |
Full Octave-Band Calculation — Partially Treated
Computing each term:
A_carpet = 300 × α_carpet (each band) A_ceiling = 300 × α_ceiling A_plasterboard = 180.4 × α_pb A_glass = 19.4 × α_glass A_partitions = 90 × α_partitions
| Band (Hz) | A_carpet | A_ceiling | A_plasterboard | A_glass | A_partitions | Total A | RT60 (s) |
|---|---|---|---|---|---|---|---|
| 125 | 6.00 | 75.00 | 27.06 | 6.79 | 4.50 | 119.35 | 1.09 |
| 250 | 18.00 | 150.00 | 18.04 | 4.85 | 13.50 | 204.39 | 0.64 |
| 500 | 42.00 | 240.00 | 10.82 | 3.49 | 31.50 | 327.81 | 0.40 |
| 1000 | 111.00 | 285.00 | 9.02 | 2.33 | 49.50 | 456.85 | 0.29 |
| 2000 | 180.00 | 285.00 | 7.22 | 1.36 | 54.00 | 527.58 | 0.25 |
| 4000 | 195.00 | 270.00 | 7.22 | 0.78 | 54.00 | 527.00 | 0.25 |
Mid-frequency RT60 = (0.40 + 0.29) / 2 = 0.35 s. This is within the ISO 3382-3 target range of 0.3–0.5 s.
However, the 125 Hz band at 1.09 s is elevated — a common problem in open-plan offices where carpet and ceiling tiles provide excellent mid/high frequency absorption but little bass control.
Scenario 3 — Optimised Treatment (Bass Extended)
To address the 125 Hz excess, we add:
- Wall panels: 30 × 600×1200 mm fabric-wrapped panels with 75 mm mineral wool, placed on long walls at 900 mm centres. Area = 30 × (0.6 × 1.2) = 21.6 m². These panels replace plasterboard on the long walls.
| 125 Hz | 250 Hz | 500 Hz | 1000 Hz | 2000 Hz | 4000 Hz |
|---|---|---|---|---|---|
| 0.60 | 0.85 | 0.95 | 0.95 | 0.90 | 0.85 |
The remaining plasterboard on long walls = (67.5 + 67.5) - 21.6 = 113.4 m²
Recalculating at 125 Hz:
| Surface | Area (m²) | α (125 Hz) | Si × αi |
|---|---|---|---|
| Carpet floor | 300 | 0.02 | 6.00 |
| Acoustic ceiling | 300 | 0.25 | 75.00 |
| Mineral wool panels | 21.6 | 0.60 | 12.96 |
| Remaining plasterboard (long walls) | 113.4 | 0.15 | 17.01 |
| East/west plasterboard | 45.4 | 0.15 | 6.81 |
| Float glass | 19.4 | 0.35 | 6.79 |
| Upholstered partitions | 90 | 0.05 | 4.50 |
| Total A (125 Hz) | 129.07 m² |
RT60 (125 Hz) = 0.161 × 810 / 129.07 = 130.4 / 129.07 = 1.01 s
Better, but still elevated. Adding two more rows of 75 mm panels (42 panels total, 30.24 m²) achieves:
A (125 Hz) with 30.24 m² panels = 75.0 + 6.0 + (30.24 × 0.60) + (104.8 × 0.15) + (45.4 × 0.15) + 6.79 + 4.50 = 75.0 + 6.0 + 18.14 + 15.72 + 6.81 + 6.79 + 4.50 = 132.96 m²
RT60 (125 Hz) = 0.161 × 810 / 132.96 = 0.98 s
In practice, achieving sub-0.5 s at 125 Hz in a concrete-slab office requires either deep absorbers (150 mm+ mineral wool) or resonant panel absorbers tuned to 125 Hz. The optimised scheme is a reasonable compromise.
Before and After Summary
| Frequency (Hz) | Bare RT60 (s) | Fit-Out RT60 (s) | Optimised RT60 (s) | ISO 3382-3 Target |
|---|---|---|---|---|
| 125 | 3.04 | 1.09 | 0.98 | < 0.8 s (advisory) |
| 250 | 4.09 | 0.64 | ~0.50 | — |
| 500 | 4.45 | 0.40 | ~0.38 | 0.3–0.5 s |
| 1000 | 4.95 | 0.29 | ~0.28 | 0.3–0.5 s |
| 2000 | 5.53 | 0.25 | ~0.25 | — |
| 4000 | 5.67 | 0.25 | ~0.25 | — |
The standard fit-out (carpet + acoustic ceiling + workstation partitions) reduces mid-frequency RT60 from 4.70 s to 0.35 s — an extraordinary improvement that demonstrates how dominant these three treatment types are in open-plan acoustics.
Key Lessons
1. Ceiling is the most powerful surface. At 500–1000 Hz, the suspended acoustic ceiling contributes 240–285 m² of absorption from a single surface. Nothing else comes close.
2. Carpet unlocks high-frequency clarity. Carpet shifts the RT60 profile from flat (all frequencies reverberant) to high-pass (only low frequencies reverberant). Combined with acoustic ceiling tiles, the two surfaces together deliver 282–480 m² of absorption across the critical speech bands.
3. Bass is always the residual problem. After carpet and ceiling tiles, the 125 Hz band remains at 1.09 s while 500 Hz has dropped to 0.40 s. Correcting this imbalance requires purpose-built low-frequency absorbers.
4. Furniture matters. The 90 m² of upholstered partitions contribute 31.5 m² of absorption at 500 Hz — approximately equivalent to a 10% improvement in ceiling tile area. In densely furnished offices this effect is larger.
5. Glass is a persistent reflector. The 19.4 m² of glazing contributes only 3.49 m² of absorption at 500 Hz, while reflecting 96.5% of incident sound energy back into the room. Full-height glazed facades are acoustically problematic and should be treated with soft furnishings adjacent to the glass.
For any open-plan project, run this calculation at the earliest schematic design stage — ceiling height, floor area, and glazing ratio are all set before the acoustic consultant is typically engaged, yet they determine whether the ISO 3382-3 targets are even achievable with standard treatment.