Wood Wool Acoustic Performance: What NRC, Rw, and CAC Actually Tell You

Three metrics. Three different things. ‍ Here’s how to know which one is governing your project and how to specify Wood Wool to ensure that metric is being delivered.‍

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Walk through the acoustic ceiling category of any product database and you'll find NRC ratings front and centre. Marketing materials lead with them, product data sheets are structured around them, and specification clauses reference them as if they're the whole story. ‍

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For many project types, NRC is exactly the right metric to be leading with. But it's not the only acoustic metric that governs a ceiling specification. Understanding what NRC does and doesn't tell you is the difference between a ceiling that genuinely performs where you need it to.

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MBS Architectural Wood Wool ceiling panels are NRC-rated. That's the tested, documented performance data we can stand behind, and for the majority of commercial, workplace, and hospitality applications, it's the metric that matters most. But architects specifying Wood Wool on projects with acoustic separation requirements or speech privacy considerations need to understand where the NRC rating ends and where the broader assembly design begins.

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This guide explains all three acoustic metrics you'll encounter in ceiling specifications  (NRC, Rw & CAC), what each one measures, which project types they govern, and how to approach a Wood Wool specification with clear clarity about what the panel delivers and what needs to be addressed through the assembly, the consultant, and the broader ceiling design.

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‍The Three Metrics and What They Actually Measure

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Before we get into Wood Wool specifically, it's worth establishing what each metric tells you. They're not interchangeable, and the confusion between them causes real problems on site.

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Noise Reduction Coefficient (NRC)

Measures how much sound a surface absorbs. Relevant to room acoustics; reverberation, echo, speech intelligibility within a space. Ranges from 0 (total reflection) to 1.0 (total absorption). Does not measure sound transmission between spaces.

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Weighted Sound Reduction Index (Rw)

Measures how much airborne sound a building element blocks from transmitting between spaces. The NCC compliance metric for sound separation between rooms. Higher is better. Governed by the complete assembly, not the ceiling product (Wood Wool) alone.

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Ceiling Attenuation Class (CAC)

Measures how well a suspended ceiling system attenuates sound transmitted laterally through the plenum from one room to an adjacent one. Critical in open-plan offices and wherever rooms share a common ceiling plenum. Often overlooked entirely.

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The simplest way to keep these straight:

⦿ NRC governs what happens to sound within a room.

⦿ Rw governs what happens to sound trying to get through a building element.

⦿ CAC governs what happens to sound trying to travel around a partition via the ceiling plenum.

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Only NRC is a rated property of the MBS Wood Wool panel itself. Rw and CAC are properties of the complete ceiling assembly and require consultant-led design that goes beyond the panel selection.

NRC: What It Tells You About Wood Wool

Wood Wool is an excellent sound absorber, and this is demonstrated in its high NRC rating. The fibrous, open structure of the material (organically bound Wood Wool strands with significant air void content) is highly effective at converting sound energy into heat, which is exactly what absorption requires. Depending on panel thickness, backing configuration, and plenum depth, Wood Wool ceiling systems typically achieve NRC values in the range of 0.50 to 1.00.

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Apollo 25mm thick Wood Wool panels achieve an NRC of 0.70 when installed with a 175mm air cavity. NRC performance can be further improved by adding acoustic insulation to the rear of our Apollo Wood Wool panels.

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That's a strong performance number, and it translates to real outcomes in the spaces where absorption matters most: reducing reverberation in open-plan workplaces, improving speech intelligibility in classrooms, controlling echo in hospitality environments. If the acoustic brief for your project is about how a space feels to be in, whether it's calm, focused, comfortable, NRC is the right starting point.

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What Affects NRC in a Wood Wool System

Panel thickness is the primary variable. Thicker panels absorb more sound at lower frequencies, which matters significantly in spaces with music, mechanical noise, or a lot of low-frequency speech. A 25mm Wood Wool panel performs meaningfully differently to a 35mm panel.

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Plenum depth amplifies absorption performance. Wood Wool installed directly to a substrate performs noticeably less well than the same panel suspended with an air gap behind it. The cavity allows lower frequencies to be absorbed that the panel alone would not. This is a detail worth capturing in your specification: nominating a minimum plenum depth is more than a services coordination decision. It makes a dramatic difference to the acoustic performance of your ceiling.

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Backing material also matters. An impermeable backing behind a Wood Wool panel reduces absorption at certain frequencies by absorbing rather than allowing sound to pass through to the plenum. Where maximum absorption is the goal, confirm that your specified configuration (panel, backing, and plenum) has been modelled by your acoustic consultant as an assembly.

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Specifiers Note: NRC values published by manufacturers are typically tested to ISO 354 in a reverberation room. The result reflects the panel in isolation, not the assembled as installed on-site. Your acoustic engineer can be engaged early to model the performance of your full nominated ceiling assembly.

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Rw: What it Means for Your Wood Wool Projects

Rw measures airborne sound transmission loss through a building element. It specifically measures how much of the sound energy hitting one side of a wall, floor, or ceiling is blocked from reaching the other side. The NCC sets minimum Rw requirements for building elements by class and location, and your certifier will be looking for compliance against these requirements. This is the metric that governs acoustic separation between a bedroom and a corridor, between two tenancies, or between a classroom and a hall.

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MBS Architectural Wood Wool panels do not carry a tested Rw rating, and neither does any other Wood Wool panel on the market. This is not a limitation of the product, but more a reflection of how Rw ratings work. This is an important distinction to understand, and one that's worth stating directly rather than leaving to inference. The Wood Wool panel is the visible, absorptive element of the ceiling, but Rw performance is a property of the complete floor/ceiling assembly. This includes the structural slab or floor above, the plenum depth and contents, the insulation within the plenum, and the ceiling system below. No single ceiling panel can be Rw-rated in isolation, because Rw measures the assembly as a whole.

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When Rw Is a Requirement on Your Project

Rw becomes a governing requirement whenever acoustic separation between spaces is mandated by the NCC. Multi-residential buildings, healthcare, education, and projects with tenancy separation requirements. If your project falls into any of these categories and you're considering Wood Wool as the ceiling finish, the right approach is to engage an acoustic consultant who can assess and document the complete ceiling assembly against the required Rw, using the Wood Wool panel as part of a designed and engineered solution.

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In many cases this is entirely achievable. Wood Wool's mass and density can contribute positively to an assembly's Rw performance when the full system is designed correctly. But the claim needs to come from a tested or engineered assembly, not from the panel's NRC rating. If you're in this situation, talk to your acoustic consultant and talk to us. We can provide the panel data that they need to model the assembly performance.

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Specifier’s Note: If your project requires Rw compliance, do not attempt to infer Rw performance from NRC data. They measure different things and the numbers are not transferable. Engage an acoustic consultant early, provide them with the full system configuration and ensure the assembly is assessed and documented before the specification is finalised.

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CAC: An Acoustic Metric That’s Easy to Overlook

CAC is the metric that generates the most blank stares when it comes up in specification conversations. This is a problem. In commercial workplace design it's often the most consequential one to overlook.

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Ceiling Attenuation Class measures how well a suspended ceiling system resists sound travelling laterally through the plenum from one room to an adjacent one. Imagine two meeting rooms separated by a partition that terminates at the underside of a suspended ceiling. Sound from one room travels up through the ceiling, across the shared plenum, and back down through the ceiling into the adjacent room, bypassing the partition entirely. CAC measures how effectively the ceiling system resists this path.

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In an open-plan office where the ceiling is continuous and there are no partitions, CAC is irrelevant. In a workplace with enclosed meeting rooms, private offices, or any spaces where speech privacy is a requirement, CAC can be the single most important acoustic consideration governing the ceiling design.

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CAC and Wood Wool: What You Need to Know

MBS Architectural Wood Wool panels do not carry a tested CAC rating, and it's worth understanding why. Open fibrous ceiling materials are strong sound absorbers precisely because sound passes through them readily into the plenum, which is the opposite of what CAC resistance requires. A Wood Wool panel's acoustic strength (high NRC) and CAC performance are, to a significant degree, in tension with each other by the nature of the material.

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This doesn't make Wood Wool unsuitable for projects where speech privacy matters, but it does mean that CAC performance cannot be expected from the panel itself, and needs to be addressed through the broader ceiling assembly design. The most common approaches are acoustic barriers at partitions. Conditions that seal the plenum at room boundaries, plenum infill treatments between enclosed rooms, or a hybrid ceiling strategy where Wood Wool is used in open-plan zones and a higher-CAC system is specified at enclosed room locations. An acoustic consultant can model the most appropriate solution for your specific layout.

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A Practical Decision Guide: Which Metric Governs Your Project?

Use this as a starting framework when approaching the acoustic ceiling specification on a new project. The middle column identifies the governing metric; the right column indicates whether Wood Wool's NRC rating is sufficient, or whether additional consultant-led assembly design is required.

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How to write a Better Acoustic Specification for Wood Wool

Armed with a clear understanding of which metrics govern your project, here's how to translate that into a specification that delivers.

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Start by naming the performance requirement, not just the product. A specification that says "supply and install Wood Wool ceiling panels to NRC 0.85" is less useful than one that says "the ceiling assembly shall achieve a minimum NRC of 0.85 when tested in accordance with ISO 354, with a minimum plenum depth of 200mm and acoustic insulation as specified." The first describes a product; the second describes an outcome. That outcome is what matters.

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Where Rw compliance is required, this cannot be addressed by the Wood Wool panel specification alone. The complete assembly needs to be engineered and documented by an acoustic consultant, with the Wood Wool panel positioned as one component of a tested or modelled system. The specification should reference the assembly, the consultant's assessment, and the test or modelling data, not just the ceiling product.

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Where enclosed rooms share a ceiling plenum and CAC is a design consideration, address the plenum boundary conditions explicitly in your documentation. Acoustic barriers at partition heads, plenum infill, or a change of ceiling system at enclosed room locations are all valid approaches, but all of them need to be in the specification, not resolved on site. A Wood Wool ceiling with no plenum treatment at room boundaries will not deliver speech privacy, regardless of how well it performs on NRC.

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Specifier’s Quick List:

⦿ Identify which metric (NRC, RW, or CAC) governs each zone of your project before selecting a system.

⦿ For open-plan and hospitality applications, the MBS Wood Wool NRC rating is your primary specification tool — confirm with panel thickness and plenum depth

⦿ Specify minimum plenum depth as an acoustic requirement, not just a services coordination decision

⦿ For projects with Rw requirements, engage an acoustic consultant to engineer and document the complete assembly — do not infer Rw performance from NRC data

⦿ For projects with enclosed rooms sharing a ceiling plenum, address CAC through partition head details and plenum treatment — not the panel alone

⦿ Engage an acoustic consultant for healthcare, education, performing arts, and any project with acoustic separation or speech privacy requirements

⦿ Ask for frequency-specific absorption data (not just a single-number NRC) on projects where low-frequency performance matters.

⦿ Confirm that plenum insulation type, thickness and installation method are consistent with the acoustic performance target.

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Wood Wool is one of the most effective acoustic ceiling materials available for the project types it's best suited to. Those include open-plan workplaces, hospitality environments & education spaces where reverberation control is the primary goal. Its NRC performance is strong and well-documented and specified correctly it makes a perceptible and lasting difference to how a space sounds and feels.

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Where projects require Rw compliance or CAC performance, Wood Wool remains a viable and often excellent design choice,  but those requirements need to be addressed through the assembly, the consultant, and the broader ceiling design rather than assumed from the panel's NRC rating. Being clear about that distinction from the outset is what allows the design intent to be achieved and the compliance to be demonstrated, without surprises at certification.

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At MBS Architectural, we're happy to talk through the acoustic requirements for any Wood Wool project and provide the product data your acoustic consultant needs to model the assembly. And if you're ready to move into specification, our step-by-step guide to specifying a Wood Wool ceiling system covers everything from substrate to finished surface.

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