Metal Stud Wall Acoustic Performance: Achieving Rw 40-55dB with Stud Partition Systems

The acoustic performance of a metal stud partition wall is determined by three main variables: stud width, the type and density of cavity fill, and the number and type of board layers on each face. A basic 70mm stud partition with 50mm mineral wool and single board on each face will typically achieve around Rw 40dB. Add a second board layer, increase the cavity fill density, or move to a wider stud, and performance climbs toward Rw 50dB and above. Understanding how each variable contributes – and where the diminishing returns start – allows contractors and specifiers to hit acoustic targets without over-engineering the build-up.

How Acoustic Performance Is Measured

Rw (weighted sound reduction index) is the laboratory measurement of how much sound a partition reduces across the frequency range, expressed as a single number in decibels. It’s the figure quoted in manufacturer system data and used as a reference point for comparing partition build-ups.

In-situ performance – what the partition actually achieves once built into a real building – is expressed as DnTw, which accounts for flanking transmission through the surrounding structure. DnTw is typically 3-8dB lower than the laboratory Rw figure, which is why a partition with a published Rw of 45dB might achieve DnTw 40dB or slightly below in practice. This gap is important when specifying to meet a regulatory target – building in headroom above the minimum is standard practice rather than over-specification.

For Part E compliance on conversion projects, the target is DnTw + Ctr of 43dB for separating walls. For new build residential, the target is 45dB. These are in-situ measurements, so the laboratory Rw of the specified system needs to be comfortably above these figures to account for the lab-to-field reduction.

The Effect of Stud Width on Acoustic Performance

Stud width affects acoustic performance primarily through the air gap it creates between the two board faces. A wider stud means a deeper cavity and a greater separation between the mass layers on each face of the partition. This mass-air-mass relationship is one of the fundamental principles of acoustic partition design – the two board faces act as independent mass elements, and the air gap between them resists the transmission of sound energy.

Increasing stud width from 70mm to 92mm typically adds around 3-5dB to the Rw of a comparable build-up, all else being equal. Moving from 92mm to 146mm adds a similar increment. The gains are real but not dramatic on their own – stud width is one variable in a system where board configuration and cavity fill matter equally.

Where stud width has the most significant effect is at low frequencies, where the mass-air-mass resonance frequency of the partition falls within the audible range. A deeper cavity pushes the resonance frequency lower, away from the speech frequencies where acoustic complaints are most common. This is why wider studs are specified on high-performance acoustic partitions even when the headline Rw figure might be achievable with a narrower stud and additional board layers.

The Effect of Cavity Fill on Acoustic Performance

Mineral wool in the stud cavity addresses sound energy travelling through the air within the cavity itself. Without fill, the air gap acts as a resonant cavity that can actually amplify certain frequencies. Mineral wool absorbs this energy and flattens the frequency response of the partition, improving mid and high frequency performance in particular.

The density of the mineral wool affects performance across the frequency range. Standard acoustic mineral wool slab at 10kg/m³ is adequate for most domestic and light commercial applications. Increasing density to 23kg/m³ or above improves performance across the mid-frequency range and is worth specifying where targets are above Rw 45dB or where low-frequency sound – music, bass, mechanical plant – is the primary concern.

Partially filling the cavity rather than filling it completely is sometimes specified on wider stud systems. The principle is to leave an air gap on one side of the mineral wool to maintain some mass-air-mass effect while still absorbing cavity resonance. In practice, for most standard partition applications, full cavity fill with standard density mineral wool is the default and performs well across the target frequency range.

The Effect of Board Layers on Acoustic Performance

Plasterboard mass is the most direct way to improve acoustic performance within a given stud width. Each additional board layer adds mass to the face of the partition, and mass is one of the most reliable predictors of sound reduction across the mid and high frequency range.

A single layer of 12.5mm standard board on each face contributes around Rw 38-40dB on a 70mm stud with mineral wool. Adding a second layer of 12.5mm board on each face typically pushes this to Rw 43-45dB. Substituting one of the standard board layers for a 15mm acoustic board (which is denser than standard board of the same thickness) adds a further 2-3dB without increasing the overall thickness of the partition significantly.

Beyond two layers per face, the returns diminish. A third board layer adds mass but the improvement in Rw is smaller than the step from one to two layers, and the cost and weight implications become more significant. For partitions targeting above Rw 50dB, additional strategies – wider studs, isolated double frames, or resilient bar decoupling – are more effective than continuing to add board layers.

Acoustic Build-Up Comparison Table

Figures are indicative based on published system data across the main UK manufacturers. Actual performance will vary by specific product, fixing configuration, and site conditions.

Where Resilient Bars Fit Into the Picture

For partitions where the target Rw is above what standard stud framing and board layering can reliably achieve – or where flanking transmission through the structure is a significant concern – adding resilient bars to the build-up introduces decoupling that addresses the structural vibration path alongside the airborne path.

A resilient bar system on a 70mm metal stud partition can push performance above Rw 50dB without needing to move to a wider stud or a double-frame system. The bars decouple the board from the stud, reducing the transmission of structural vibration in the same way they do in ceiling applications. This is particularly effective in timber frame construction where the structural path is more significant than in masonry or concrete frame buildings.

The trade-off is installation complexity and the additional fixing discipline required to avoid short-circuiting the resilient layer – the same flanking risks that apply to ceiling applications apply equally to walls. But where the acoustic target requires it, the combination of metal stud framing and resilient bar decoupling is one of the most versatile routes to high performance in a relatively slim overall build-up.

Specifying to Meet Part E

For domestic separating walls on conversion projects, the practical starting point is a 70mm stud with full cavity fill at 10kg/m³ and two layers of 12.5mm board on each face. This build-up, correctly installed with proper perimeter isolation, should comfortably achieve DnTw 43dB in most scenarios. Where the existing wall provides some additional mass – for example, where the metal stud partition is built against a brick or blockwork party wall rather than as a freestanding element – performance will typically be higher.

For new build residential targets of DnTw 45dB, a 92mm stud with 75mm mineral wool and two board layers is a more reliable starting point, providing headroom above the target to account for flanking and in-situ variability.

For commercial applications above DnTw 45dB, 92mm or 146mm stud with denser mineral wool and additional board layers is the standard approach, with resilient bar decoupling added where targets push above Rw 50dB.The full metal stud and track range is available at Online Insulation across all standard stud widths. For decoupled wall and ceiling applications, the resilient bars range covers the additional components needed for higher performance builds. The MF ceiling system and wall lining system ranges complete the drylining system for projects requiring ceiling and wall treatment alongside partition work.