The most carefully specified partition system in an open-plan office can be completely undermined by a single-digit ceiling tile specification. This is not a hypothetical: it is the standard result when a building is designed with STC 50 demountable partitions, a shared HVAC plenum, and commodity mineral fibre ceiling tiles with CAC 30. The calculated composite speech privacy between offices with this configuration is approximately 30 dB — equivalent to an STC 30 partition with no ceiling at all.
CAC is the most under-specified acoustic metric in commercial office design. Understanding it properly requires understanding how sound travels in the ceiling plenum — a transmission path that most architects and specifiers do not model.
The Plenum Flanking Path
In most commercial office construction, a continuous ceiling plenum is shared between adjacent rooms. The plenum contains HVAC ducts, electrical conduit, sprinkler pipes, and data cabling — all running continuously from one end of the floor plate to the other, above a dropped ceiling of lay-in tiles.
When two rooms are separated by a partition wall that terminates at the underside of the dropped ceiling (not extending to the structural soffit), sound can travel the following path:
- Sound in Room A hits the ceiling tile and transmits through it into the plenum
- Sound propagates through the plenum air space (with minimal attenuation — the plenum is acoustically reverberant)
- Sound reaches the ceiling tile of Room B and transmits down through it
- Listeners in Room B hear speech from Room A as if through a direct air path
For adjacent rooms sharing a plenum, the transmission loss along the flanking path is approximately:
TL_flanking ≈ CAC_A + CAC_B + 10 × log₁₀(D) – correction
Where D is the distance (in metres) between room centres in the plenum and the correction accounts for plenum geometry. For adjacent rooms with shared 600 × 600 mm tile grid, D ≈ 2–5 m, contributing 3–7 dB additional attenuation. More practically: the sum of both ceiling tile traverses (CAC_A + CAC_B) dominates, and is the primary design parameter.
ASTM E1414: The CAC Test Method
CAC is measured according to ASTM E1414 (Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum). The test configuration:
- Two test rooms are constructed side-by-side, separated by a partition that terminates at the ceiling grid underside (not extending to the structural soffit)
- The same ceiling tile system is installed in both rooms, with a common plenum above
- A calibrated loudspeaker in Room 1 generates pink noise
- Sound levels are measured in both rooms across 1/3-octave bands from 200 Hz to 2500 Hz
- The normalised level difference between rooms is compared to a standard contour
- CAC is reported as a single number following the same contour-fitting procedure used for STC (ASTM E413)
CAC Ratings for Common Ceiling Systems
Standard Mineral Fibre Tiles
The workhorse of commercial ceiling specification — Armstrong, USG, Rockfon, and equivalent brands. Standard products:
| Product Category | Typical NRC | Typical CAC | Notes |
|---|---|---|---|
| Standard mineral fibre (16 mm) | 0.55–0.65 | 30–35 | Tegular or square edge |
| Enhanced mineral fibre (19 mm) | 0.70–0.80 | 35–38 | Thicker, denser |
| High-NRC mineral fibre (25 mm) | 0.80–0.90 | 30–35 | Softer face, lower CAC |
| High-CAC sealed mineral fibre | 0.60–0.75 | 40–45 | Sealed face, reduced NRC |
| High-NRC + high-CAC composite | 0.85–0.95 | 40–45 | Premium products |
The trade-off between NRC and CAC is real but not absolute. High-NRC tiles have open, porous faces that allow airflow — which is good for absorption but bad for attenuation. High-CAC tiles have sealed or semi-sealed faces that block airflow — better attenuation but less absorption. Premium products achieve both through a composite construction: absorbent core with a very light perforated face film that is acoustically transparent for the absorption mechanism but provides a barrier at the surface.
Specific product examples at the high-performance end:
- Armstrong Ultima+: NRC 0.90, CAC 44
- Rockfon Sonar: NRC 0.95, CAC 40
- USG Ceilings Frost: NRC 0.70, CAC 42
Metal Pan and Open Grid Systems
Perforated metal pan systems with glass fibre backing: NRC 0.80–0.90 (absorption through perforations), CAC 20–28 (poor — the perforations that enable absorption also allow sound through). Unsuitable for speech privacy applications.
Open grid systems (exposed cross-tee with batt insulation): CAC < 15. These systems provide essentially no plenum attenuation and should never be used in office environments requiring speech privacy.
Concealed Suspension Systems (Monolithic Ceilings)
Veneer plaster and gypsum board ceilings on concealed suspension: NRC depends on surface treatment (0.05–0.20 for hard plaster), but more importantly the ceiling may extend to the structural soffit (no plenum), eliminating the flanking path entirely. If there is still a plenum for HVAC, gypsum board ceiling achieves high CAC (45–55) due to mass, but NRC is typically low.
Composite Partition Performance
The combined effect of wall STC and ceiling CAC can be estimated using the energy-addition formula for parallel transmission paths:
TL_composite = –10 × log₁₀ (10^(-STC/10) + 10^(-CAC_flanking/10))
For a wall with STC 50 and a plenum flanking path with CAC_flanking = 30 (CAC_A + CAC_B combined ≈ 30 dB, neglecting distance):
TL_composite = –10 × log₁₀ (10^(-50/10) + 10^(-30/10)) = –10 × log₁₀ (10^-5 + 10^-3) = –10 × log₁₀ (0.00001 + 0.001) = –10 × log₁₀ (0.00101) = –10 × (–2.995) = 30 dB
The flanking path completely dominates — the composite is essentially equal to the CAC_flanking, regardless of the wall STC. Increasing the wall STC from 50 to 60 would make no perceptible difference. Only improving the ceiling tile CAC improves the composite performance.
For wall STC 50 and ceiling CAC 40:
TL_composite = –10 × log₁₀ (10^-5 + 10^-4) = –10 × log₁₀ (0.00011) = 39.6 dB
Still limited by the flanking path, but meaningfully better — moving from borderline to adequate privacy for most office applications.
For wall STC 50 and ceiling CAC 45:
TL_composite = –10 × log₁₀ (10^-5 + 10^-4.5) = –10 × log₁₀ (0.000042) = 43.8 dB
At CAC 45, the ceiling and wall contributions are nearly balanced — close to the maximum composite performance achievable without the wall and ceiling simultaneously achieving very high ratings.
Plenum Barriers: When Tiles Are Not Enough
For spaces requiring high speech privacy — executive offices, board rooms, therapy rooms, medical consulting rooms, legal interview rooms — even high-CAC ceiling tiles may not provide sufficient composite privacy. The solution is a plenum barrier: a continuous partition extending from the top of the demountable partition up to the structural soffit, sealing the plenum above the room.
A plenum barrier:
- Is typically constructed from 1–2 layers of 12 mm gypsum board on metal stud framing
- Must be continuous and airtight — any gap for HVAC duct, sprinkler pipe, or electrical conduit must be sealed with acoustic mastic or mineral wool packing
- Must address flanking around the barrier (through shared HVAC ducts, which must receive lined duct branches or sound attenuators)
- Extends the room's STC to include the full structural slab-to-slab partition performance — typically adding 15–25 dB above the CAC-limited performance
Design Checklist for Speech Privacy via Ceiling
Before specifying a ceiling tile for an office requiring speech privacy:
- Identify the flanking path: Is there a shared plenum above the partition? If yes, CAC governs.
- Determine the privacy target: Normal office (CAC 30–35 adequate), private office (CAC 35–40), confidential/medical (CAC 40–45 or plenum barrier required).
- Calculate composite performance: Use the energy-addition formula above to check that wall STC + ceiling CAC combination meets the composite target.
- Check HVAC crosstalk: Shared HVAC branch ducts are a separate flanking path. Sound attenuators (25 dB insertion loss) on branch ducts are required if composite partition insulation is > 40 dB.
- Specify tile by CAC, not NRC alone: Many tile data sheets lead with NRC. Always verify CAC is specified and meets the performance brief.
- Verify field installation: Tiles must be fully seated in grid, with no gaps at perimeter. A survey of installed tiles after ceiling completion should confirm all tiles are flush and undamaged.
- Revisit after any ceiling access: Maintenance access panels, IT cabling runs, and sprinkler head replacements all disturb tiles. Post-maintenance checks of tile seating are needed to maintain the design CAC performance over the building's life.
Calculate speech privacy in your space using AcousPlan, including the composite effect of partition STC and ceiling CAC.