Acoustic separation in curtain-walled buildings can present challenges for both façade designers and engineers. These types of façade system, commonly composed of low mass materials such as glazed panels and lightweight and hollow aluminium profiles, are prone to flanking sound transmission: noise can travel from room to room via a path other than the separating partition. Curtain walls provide multiple paths for internal transfer.
To address this, well thought out acoustic strategies should be adopted early in the project timeframe. They should consider the design of junctions between party walls, floors and the façade, as well as noise transfer through the façade, to ensure adjacent spaces remain quiet, comfortable and private.
Measuring sound
Before discussing the main flanking sound transmission paths in curtain walling systems, it may be useful to define some of the key measurements used in building acoustics. Noise levels are measured in decibels (dB). However, decibel values alone lack context. For instance, a given decibel value could relate to the performance of a single element or an entire detail. It may indicate either airborne or structure-borne noise and may or may not inform us of the time period over which the noise is measured.
To add context and clarification we therefore use ‘acoustic metrics’. These are presented alongside the decibel measurement: for example, 55 dB DnT,w. In the case of curtain wall acoustics, the key metrics applicable are as follows:
dB Rw – Weighted sound reduction index
This value identifies the airborne sound insulation performance of a single proprietary building element. It is used for internal or external walls, ceilings/floors, windows, doors and any other separating element. The higher the Rw value, the better the sound reduction performance of the element.
Since dB Rw is measured in a laboratory, it does not account for site factors such as tolerance or workmanship, nor any flanking noise paths that may occur onsite. This means that the Rw performance of an element in a laboratory setting is typically higher than would be achieved in the context of a full detail.
dB Dnf,w – Weighted normalised flanking level difference
This laboratory-measured value represents the performance of a flanking element alone, such as a curtain wall façade, suspended ceiling, or raised access floor. Noise transmitted between rooms through noise paths other than the separating partitions is flanking noise.
dB DnT,w – Weighted standardised level difference
This is a site-measured value and indicates the total airborne sound insulation between two rooms, accounting for all noise paths – both direct noise through the separating partition and flanking noise via any other path.
The DnT,w test includes an adjustment for reverberation time – or echo – in the rooms being evaluated. This enables acousticians to compare measured sound insulation results between different rooms and at various times, while accounting for the relative room size and any changes made to furnishings when in use (for example, the presence of furniture and soft finishes like carpets and curtains).
Sound paths for noise travel
With these metrics in mind, we can take a closer look at the five primary paths through which noise travels in curtain wall façade systems. The Dn,f,w performance of the façade is influenced by each individual path, and the achievable performance limited by whichever of these noise paths is weakest.
Glazing
Sound waves are incident on a pane of vision glass, causing the glass to vibrate. These vibrations travel through to adjacent rooms via the structure of the glass itself and re-radiate as noise. Noise that travels via solid materials in this manner is called structure-borne noise. Glazing type, spans of glass and spandrel panels, and the number of separating transoms and their stiffness, all influence how sound travels along this path.
Transoms
Most curtain walling systems use aluminium profiles. These framing elements are relatively lightweight and sound energy incident on them may easily break through into adjacent spaces. The height and construction of spandrel panels, as well as the transom design and number of separating transoms, affect how sound travels along this path.
Mullion joint
Sound energy hits the mullion causing it to vibrate. These vibrations (structure-borne noise) travel along the mullion and radiate into adjacent rooms. The design of the mullion joint is the most significant factor in how this noise path performs.
Continuous mullions can be a significant factor* in limiting achievable performance.
*It’s important to note that the overall performance of a curtain wall system is only as good as its weakest link. This is known as the performance-limiting factor (PLF). Addressing the weakest sound transmission path is crucial for achieving substantial improvements in noise control.
Mullion hollow
If sound energy hits the mullion and passes through into the hollow profile, it can travel up and down the building before re-emerging in another space. The hollows of mullions create a hidden pathway for unwanted noise.
Perimeter joint / movement gap / firestop zone
The essential gaps at the joints where different building elements meet (for example, at the slab edge/façade interface) accommodate building movement and fire protection measures, but they can also present a path for flanking sound transmission. Carefully designing and treating these zones with high mass and isolating materials with fire-resistant properties is crucial for achieving good sound insulation, as well as meeting the required fire resistance performance.
Noise mitigation strategies
Designers can employ various measures to reduce noise transmission through these pathways, improving the acoustic properties of curtain-walled buildings while preserving their aesthetic appeal. Here are some examples:
Splitting mullions at floor levels
A significant increase in overall performance may be accomplished by decoupling the mullions at the floor level. This can involve splitting mullions then reconnecting them using joining spigots at the floor line.
Generally, splitting the mullions in the floor zone is necessary to reach flanking performance levels appropriate for residential and commercial projects.
Twin transom design
Single transom designs will limit the possible overall floor-to-floor performance, while twin transom configurations can help reduce noise transmission through these elements.
High-performance glazing
Specifying glass with enhanced acoustic properties is typically intended to mitigate noise transmission from outside the building to the inside. It can also be beneficial for addressing flanking noise from within the building.
High-mass treatments
Incorporating materials with higher mass, such as acoustic matting or panels at the slab edge, can significantly improve floor-to-floor acoustic performance.
Decoupled framing
Physically separating framing elements from surrounding structures can effectively reduce noise transfer through structural connections.
Noise treatment solutions for curtain wall elements
It is essential to invest in acoustic products that demonstrate verified performance. The standalone acoustic performance of materials and products is typically rated in the UK and Europe using dB Rw. Although specific performance requirements and product selection will vary according to project needs, selecting solutions that are purposefully designed, engineered, and tested for noise control in curtain wall systems can help ensure compatibility and achieve compliance with acoustic targets.
Siderise has developed a range of products specifically designed to address the acoustic issues associated with curtain wall systems, as well as ensuring their passive fire protection. These include:
MI Mullion/Transom Acoustic Insert
Inserting acoustically absorptive or mass-based inserts within hollow mullions and transoms can significantly reduce horizontal and vertical flanking sound transmission between adjacent internal spaces via the hollow channels in both stick-built and unitised curtain wall systems. They improve the performance of glazed areas and are ideal for enabling future change of occupancy and office refits, as they maintain the interior aesthetic. Various solutions are available to suit different curtain wall configurations and noise control applications.
MC Mullion Cover
This is an external treatment that can be used to overclad curtain walling mullions where partitions abut. It can substantially improve acoustic performance through the mullion. MC is also available as a fire-rated variant that can provide up to two hours EI fire resistance.
As a decorative treatment, MC can be finished in the designers’ choice of colours to integrate with the overall aesthetic. It is ideal for projects where acoustic infill upgrades to the mullion are not practical, such as retrofits and situations where the internal layout is unknown during construction, or when fit-out or operation has already started.
FIP Acoustic Panel
An acoustic panel developed to reduce noise transmission at the junction between internal partitions and curtain walls or window mullions. It provides a slim solution where high dB Rw performance is needed.
Acoustic Barriers (CW-AB, CVB/C10)
Installed around CW-FS Curtain Wall Firestops functioning as a perimeter fire seal at the perimeter joint, these barriers can effectively reduce floor-to-floor sound transmission. Unlike traditional mass-barrier materials such as steel or plasterboard, Siderise barriers can accommodate building and façade movement.
Closing the curtains on noise
Noise is an inherent aspect of residential, commercial, and mixed-use urban environments, from the sounds of conversations in nearby apartments to music drifting up from shops and restaurants. When inside your home, hotel room or office, however, this commotion should not infiltrate your space.
By incorporating appropriate design strategies and investing in acoustic product enhancements from the outset, architects and contractors can deliver buildings that provide huge returns in acoustic comfort and privacy.
View the full range of curtain wall acoustic treatments on NBS Source or learn more about Siderise by visiting it’s website.
