Plenum acoustic barrier: securing the DnT,A of your enclosed offices

02

the framework

The technical plenum, that 30 to 80 cm volume between the removable suspended ceiling and the top slab, is an acoustic highway from one room to the next as soon as the partition stops at the level of the modular ceiling.

The deviations measured in situ on our projects are systematic: the laboratory Rw value declared on the BA13 data sheet reaches 40 to 44 dB according to NF EN ISO 10140-2 testing, while the DnT,A measured in situ without plenum treatment drops to 28 to 32 dB. The loss attributable to the acoustic bridge through the plenum therefore amounts to 12 to 16 dB.

The acoustic reference framework applicable to offices retains three enforceable levels: standard (Rw≥32 dB), high-performance (≥38 dB, standard confidentiality), very high-performance (≥44 dB, enhanced confidentiality). Without a plenum barrier, only the standard level holds, and tenuously.

In practice at Kytom, the plenum barrier is not an execution matter raised during pre-handover inspections: it is an upstream legal matter. The 12 to 16 dB delta between the catalogue Rw and the on-site DnT,A, if it is not neutralised by a handover clause based on a measured DnT,A threshold, exposes the client to an R4213-5 grievance on premises where confidentiality is required (HR, legal, management, medical). The plenum barrier is first and foremost an evidentiary device.

03

what holds up in court

For legal teams and acousticians: what is enforceable, and the rest

For a legal director or an expert acoustician, the right question is not the thickness of mineral wool, but what will hold up in the event of a dispute and which clause secures the works contract.

What is enforceable

• The DnT,A measured in situ according to the standardised method for acoustic measurement of buildings in the presence of an independent third-party acoustician, using pink noise.
• The threshold contractualised in the CCTP, room by room and not by generic typology.
• Article R4213-5 of the Labour Code for premises whose activity requires confidentiality (HR, medical, legal).
• The STI (Speech Transmission Index) measured according to the standardised method applicable to open-plan spaces when the contract targets a speech intelligibility requirement.

What is not

• The laboratory Rw of manufacturer data sheets, measured in a test cell according to the standardised laboratory testing method for construction elements: it qualifies an assembly in a test cell, not a partition on site.
• A generic mention of « acoustic performance compliant with standard practice » in the CCTP: insufficient to ground a claim.
• A test report without a third party: the standardised in situ measurement procedure provides for an auditable protocol.

Recommended standard clause for confidential premises: « The contractor guarantees a DnT,A≥38 dB isolation measured in situ according to the standardised method applicable to acoustic measurements in buildings, between [room A] and [room B] in the presence of a third-party acoustician, plenum cavity treatment included. Handover is subject to the measurement report. » Without this clause, the plenum barrier remains a best-efforts commitment, not an obligation of result, a distinction that judicial expert assessments in acoustic matters regularly confirm.

04

your gains

Three quantified and enforceable acoustic benefits

Our delivered operations provide three benefits documented by handover measurement, based on 34 projects with standardised acoustic measurement and application of the CERFF Partitions reference framework.

• Spoken confidentiality maintained: DnT,A≥38 dB on management offices and HR rooms, the usage condition for the high-performance level for spoken confidentiality. Without this threshold, the conversation remains intelligible in the adjacent room less than 2 metres from the partition (the CERFF Partitions reference framework requires an acoustic reduction of glazed partitions ≥ 38 dB).
• Ambient level controlled: combined with the absorbent treatment of the suspended ceiling (αw 0.90 measured in a standardised reverberation chamber), the plenum barrier makes it possible to hold a LAeq<45 dB(A) in an enclosed office, a level qualified as high-performance for this type of commercial use.
• Contractualised handover: DnT,A report by a third-party acoustician integrated into the DOE, enforceable in the event of a subsequent dispute over R4213-5 compliance.

This triptych turns a manufacturer’s promise into a guarantee of result, within the average Kytom project lead time of 12 weeks.

05

playing fair

When the plenum barrier is not the right answer

Commercial honesty requires us to say when our approach ceases to be cost-effective. Three configurations identified in our portfolio:

1. Floor<300 m² with fewer than 2 critical partitions. A conventional full-height partition costs less than removing and refitting the suspended ceiling and installing the double lining in the plenum. We recommend the continuous partition.
2. Free plenum height<25 cm. This clearance prevents the clean installation of the double lining and the SNJF sealing on the underside of the slab. In this case, we recommend removing the suspended ceiling and switching to a continuous ceiling on confidential premises.
3. Pure open space with no confidential premises. The standard-level acoustic target for offices is met by the bare partition; the budget is better invested in absorbent acoustic correction (αw≥0.90) to bring down the ambient LAeq.

Below these thresholds, the plenum barrier adds cost without a usage gain enforceable at the handover measurement. We tell you this before the quote, not after the measurement report.

06

Method

1. Plenum survey
   Our technicians map the volume above the suspended ceiling: HVAC runs, high and low current cabling, sprinklers, free slab/suspended ceiling height, position of beams and composite decking. The survey identifies the 4 to 6 critical partitions of the floor (meeting rooms, management, HR, video conference, legal-finance) and qualifies the feasibility of the double lining on the underside of the slab. Deliverable: dimensioned plenum plan and list of singular points to be treated.
2. Acoustic barrier design
   We size a double BA13-lined barrier on a 70 mm frame with 70 mm mineral wool at a density of 40 kg/m³, fitted up to the underside of the top slab. The perimeter sealing is carried out with SNJF acoustic sealant on the three slab/partition sides. The unavoidable HVAC penetrations incorporate baffle silencers Dn,e,w≥45 dB so as not to cancel out the gain. Deliverable: CCTP per room with a contractualised DnT,A threshold.
3. Execution and handover measurement
   We orchestrate the execution with removal of the suspended ceiling over the target zones, installation of the barrier, visually inspected sealing before refitting. A third-party acoustician carries out the DnT,A measurement using pink noise between rooms, according to the current normative protocol for standardised weighted acoustic isolation. The report is attached to the DOE and makes handover enforceable under Article R4213-5. Lead time integrated into the average Kytom project of 12 weeks.