STC rating, or Sound Transmission Class, is the number that decides whether the partition system you specified will actually deliver the acoustic privacy your meeting rooms and offices need. It is a single laboratory-measured figure summarising how much sound a wall or door blocks across a range of frequencies, expressed as a higher-is-better integer. For commercial office partitions the practical range runs from around STC 25 at the bottom (an open-back partition with minimal acoustic mass) to STC 55 at the top (a high-spec double-stud plasterboard wall with full perimeter sealing). The number sounds technical but the consequence is simple: rooms specified at too low an STC will leak conversation, and rooms specified at the right one will not.
The trap most tenants fall into is treating STC as a single floor figure across the tenancy rather than thinking about it room by room. A boardroom needs a higher STC than a project room; a confidential consult room needs higher again. Specifying STC 40 across all internal partitions over-spends on the project rooms and under-delivers on the boardroom. The cleaner brief sets the STC target for each room based on what happens inside it.
What STC Actually Measures
STC is measured under standardised laboratory conditions. A test wall is built between two acoustic chambers, sound is generated in one chamber across a defined frequency range, and the difference between source-side and receive-side sound levels is measured at each frequency. The frequency results are weighted into a single STC number using a defined curve from ASTM E413 or the equivalent international standards.
The figure tells you something useful but not everything. STC weights mid-range speech frequencies (around 125 Hz to 4 kHz) heavily, which makes it a good predictor for conversation privacy. It does not measure low-frequency performance well, which is why some partition systems with high STC numbers still let bass-heavy noise (HVAC rumble, AV sub-bass) pass through. For most commercial office applications the speech-privacy focus is the right one and STC is the relevant number.
What STC does not measure is field performance. The laboratory test assumes perfect installation, no flanking paths around the wall, and full perimeter sealing. Real-world performance is almost always lower, sometimes by 5 to 10 STC points, because the wall has to deal with door drops, ceiling-to-slab flanking, services penetrations, and the realities of how the partition meets the surrounding building. The convention in field-measured ratings is NIC (Noise Isolation Class), typically 5 to 10 points below the lab STC for the same partition system.
What STC Targets Actually Mean In Conversation Terms
An STC of 25 means you can hear normal speech clearly through the wall. Useful only as a visual screen.
An STC of 35 means normal conversation is muffled but raised voices are still intelligible. Acceptable for project rooms and collaborative spaces where overheard discussion is not a privacy issue.
An STC of 40 means normal speech is largely inaudible from the other side. Raised voices come through as muffled sound without specific words. Standard for general office meeting rooms.
An STC of 45 means even raised voices are difficult to make out. Standard for confidential meeting rooms, HR conversations, executive offices.
An STC of 50 means almost complete speech privacy. Raised voices barely register. Specified for boardrooms, medical consult rooms with strong privacy demand, legal client rooms, and tenancies handling sensitive material.
An STC of 55 means complete privacy under normal speech conditions. Specified for psychiatry, audiology, secure-environment rooms, and applications where any speech detection would be a problem.
How Partition Systems Reach Each STC Target
Single-stud plasterboard partitions with no acoustic insulation typically deliver around STC 30 to 33. Useful only for non-acoustic visual screens.
Single-stud plasterboard with one layer of acoustic insulation in the cavity and one sheet either side reaches STC 35 to 38. The standard general-office meeting room build-up.
Single-stud with full-depth acoustic insulation and double-sheet either side (two layers of plasterboard on each face) reaches STC 42 to 45. A common upgrade for meeting rooms with privacy demand. The choice of insulation type inside the cavity affects the final rating measurably; cheap polyester insulation underperforms purpose-designed acoustic insulation by 3 to 5 STC points in the same wall build-up.
Double-stud or staggered-stud partitions with insulation in both cavities and double-sheet linings reach STC 50 to 55. The cost step-up is meaningful because the wall is structurally heavier and takes more floor area, but the acoustic gain is real. Soundproof plasterboard partitions for meeting rooms at this spec are the typical solution where the brief calls for STC 50+.
Resilient-mounting systems (where one face of the partition is decoupled from the framing through resilient channels or clips) add 3 to 5 STC points to whichever base build-up they are applied to, but require careful detailing to avoid short-circuits through the resilient layer.
How Glass Partitions Fit The STC Framework
Standard 10-millimetre toughened glass partitions deliver around STC 33 to 36 in a sealed installation. The number assumes the panel is sealed at head, sill, and vertical edges; an unsealed glass partition runs 5 to 8 STC points lower.
Twelve-millimetre toughened glass with full sealing reaches STC 36 to 38. The thickness step-up adds mass and improves the rating modestly.
Laminated glass partitions (two glass plies with an acoustic interlayer) reach STC 38 to 42 depending on the lamination specification. The interlayer adds damping that single-pane glass does not provide.
Double-glazed glass partitions with separated panes and an air gap reach STC 42 to 48 depending on the build-up. These are the high-performance end of the glass partition range and approach plasterboard partition performance, at a cost premium.
The acoustic limit of glass partitions is around STC 48 in commercial-available systems. Above that, the partition has to be plasterboard or hybrid. The honest framing for tenants is that glass partitions can deliver office-grade acoustic privacy (STC 35 to 40) cleanly, can deliver upgraded performance (STC 42 to 48) with double-glazed systems at higher cost, and cannot deliver STC 50+ at any reasonable cost. Where the brief demands STC 50+, plasterboard or hybrid is the answer.
What The Door And The Perimeter Actually Contribute
The partition’s STC rating is meaningless if the door does not match it. A meeting room with STC 45 partitions and a hollow-core door with no acoustic seal will perform around STC 25 to 30 in practice, because the sound takes the path of least resistance. The door is part of the system, not a separate element.
Solid-core acoustic doors with full perimeter sealing (head, jamb, and door drop seals at the threshold) typically deliver STC 30 to 35. Heavier acoustic doors with rated seals reach STC 38 to 42. For partitions specified at STC 45 or above, the door has to be a rated acoustic door, not a standard veneer flush door.
The perimeter is the other half. A partition that meets the ceiling at a suspended grid junction without acoustic sealing leaks sound through the ceiling cavity. The fix is partitions that run to the structural slab, or partitions that run to the suspended ceiling with acoustic flanking treatment in the void above (acoustic baffles, sealed grid junctions, or a continuous acoustic ceiling that prevents flanking).
The pattern we see most often is partitions specified to a good STC but underperforming in the field because the door or the ceiling junction was not specified to match. The system rating is the lowest component’s rating, not the average.
Specifying STC On A Fitout Drawing Set
The cleanest spec calls out the STC target per room (or per partition group) on the drawing schedule, alongside the build-up that delivers it. A line that says “STC 45, double-stud, full acoustic insulation, double-sheet, acoustic door with drop seal” is unambiguous. A line that says “acoustic partition” with no target is interpreted by the contractor to whatever the standard build-up happens to be.
The drawing should also specify the perimeter conditions explicitly: head detail (slab fix, suspended ceiling junction, junction treatment), sill detail, junction to perpendicular walls, and any services penetrations that need to be sealed.
For meeting rooms specifically, the door specification should be called out by manufacturer reference or by rated STC, not as “acoustic door”. Meeting room acoustic design as a whole is a discipline that depends on every element of the room meeting the target.
Testing And Verification
STC is a laboratory rating. Field measurement uses NIC, typically 5 to 10 points below the lab rating for the same partition. For tenancies where acoustic performance is mission-critical (medical, legal, finance with privacy obligations), a post-installation acoustic measurement can be specified as part of the handover, with the contractor responsible for remediating any rooms that fall below an agreed NIC target.
For general office tenancies, post-installation measurement is rarely justified. The acoustic test is simply whether normal conversation can be held in the room without being audible outside. Tenants who walk the finished fitout with a colleague reading a paragraph in normal voice in each room, while another colleague stands outside, can confirm the acoustic outcome at no cost. The pattern we see most often on tenant walks is that the room that sounded fine empty performs differently when furniture and people are in it, which is why the test works better with the room set up rather than at the bare-shell handover.
If you are specifying acoustic partitions across an office fitout and want the STC targets matched cleanly to the room briefs, with door and perimeter conditions detailed to deliver the field performance, we run mixed partition packages across the full partition scope and can work through the acoustic specification room by room before procurement.
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