Sound is part of any residential, commercial, or mixed-use city building experience— from the people interacting in nearby apartments to music drifting from shops and restaurants. However, when you get inside your home, hotel room, or office, it is vital that this commotion is left outside of your space. Disruptive noise levels can damage sleep and interrupt concentration and productivity. Nuisance noise can also lead to long-term health issues.
Acoustic separation in curtain-walled buildings is a particular challenge for façade designers. Often constructed using lightweight and hollow aluminium profiles and glazed panels, they can offer multiple potential paths for sound to travel from room to room. Therefore, it is important that designers consider both the structural and sound absorption properties of building elements and partitions, and understand how to treat the voids that will occur between them early on in their process to enable the creation of acoustically comfortable and functional spaces for occupants.
MEASURING SOUND
Before we explore direct sound transmission paths and how to reduce them, it is important to clarify some of the key measurements used when talking about building acoustics. People often refer to decibels (dB). Whilst a decibel can tell you the sound pressure level by itself, it doesn’t indicate the content of the noise being measured. This is why acoustic metrics are employed to set this metric in a context:
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Decibel (dB) RW
An Rw is the weighted sound reduction index. It is a laboratory-measured value for identifying the airborne sound insulation performance of a ‘building element’. It is used for internal or external walls, ceilings/floors, windows, doors, or any separating element. The higher the Rw value, the better that element performs in reducing sound transmission. Because it is a laboratory-tested level the Rw does not take into account any flanking noise paths which would occur on site. This means it is highly unlikely a Rw 50dB wall will reduce noise by 50dB onsite.
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Decibel (dB) Dnf,w
A laboratory-tested performance value of a ‘system’ such as a curtain wall façade, raised access floor, or suspended ceiling. This is a flanking sound transmission, tested in a similar way as described above with the system installed between two rooms.
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Decibel (dB) DnT,w
There are variations on a level difference, including the DnTw. A DnTw normalises the Dw to account for the reverberation time – or echo – in the receiving room. This allows us to compare measured sound insulation results between different rooms and at different times, irrespective of the amount of echo in the receiving room. In theory, the DnTw sound insulation performance should remain the same when testing between a pair of rooms when they are unfurnished compared to tests when the rooms have been furnished and carpeted.
POTENTIAL SOUND TRANSMISSION PATHS
Whilst there are many ‘flanking’ sound transmission paths in glazed façades, the following five paths can typically be considered the most significant with the total combined performance of each of these paths giving the Dnf,w value.
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Glazing Element
Sound waves hit the vision glass causing vibrations within it. These vibrations travel through to adjacent rooms via the structure of the glass itself and re-radiate as noise. Spans of glass and spandrel panels, the number of separating transoms and their stiffness all play a part in how sound travels along this path.
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Transoms
Sound energy strikes the transom and becomes structure-borne. The vibrations travel through the transom profile into the spandrel panel and then radiate into the adjacent room. Spandrel panel height, construction, transom design and the number of separating transoms affect how sound travels along this path.
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Mullions
Sound energy hits the mullion structural element and becomes structure-borne. The vibrations then travel through the frame and radiate into the adjacent room. The height of the spandrel zone and the design and frequency of mullions have an impact on how the sound energy travels along this path. Library test data suggests that this path can cause the most issues in the 500Hz — 1000Hz range of frequencies – a range that the human ear is highly sensitive to. Furthermore, results from testing have shown that continuous mullions spanning multiple floors are the main performance limiting factor (PLF) in facade design.
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 The Hollows of the Mullions
Sound energy hits the mullion and passes through into the hollow of the profile, which is a highly reverberant space, then travels up or down the building before breaking back out again into the adjacent rooms.
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Movement Joint/Firestop Zone
The area around firestop zones, where different building elements meet, can present a path for flanking sound transmission. Carefully designing and treating these zones with high mass, and isolating materials with fire-resistant elements is crucial for achieving good sound insulation as well as for meeting the required fire resistance performance.
It’s important to note that the overall performance of a curtain wall system is only as good as its weakest link. Addressing the weakest sound transmission path is crucial for achieving substantial improvements in noise control.
KEY STRATEGIES FOR EFFECTIVE NOISE CONTROL
There are several strategies designers can take to reduce the noise transmission through these paths, enhancing the acoustic performance of curtain-walled buildings, while maintaining their aesthetic appeal:
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Splitting Mullions at Floor Levels
A significant increase in potential overall performance can be achieved by fully decoupling the mullions at the floor level. This is achieved by splitting and then rejoining mullions with adjoining spigots at the floor line.
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Twin Transom Design
Single transom designs will often limit the possible overall floor-to-floor performance. Using twin transoms instead can help reduce noise transmission through these elements.
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High-Performance Glass Units
Specifying glass with higher acoustic properties can help to mitigate noise transmission from outside to inside and through the glazing element internally.
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High Mass Treatments
Incorporating high-mass materials, such as acoustic matting or panels, can significantly improve noise control, particularly in the floor to façade junction.
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Decoupled Framing
Decoupling the building’s framing from surrounding structures can effectively reduce the transmission of noise through structural elements.
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Acoustic Insulation
Incorporating layers within the structure such as elastomeric interlayers can significantly reduce the transmission paths. This is effective when addressing flanking transmission.
Beyond these design strategies, there are also several acoustic enhancement products that have been specifically engineered for use in façades. They are designed to be incorporated within the curtain wall system to reduce vertical and horizontal noise transmission.
ACOUSTIC ENHANCEMENTS OF FACADE SYSTEM ELEMENTS
As highlighted above, the ability of an individual product or system to limit noise transmission is quantified by its Sound Reduction Index measured in dB RW. Whilst the exact performance requirements and product selection will depend on the project, choosing solutions that have been designed, developed, and tested specifically for noise control in curtain wall applications can help to ensure compatibility.
Different options are available which have been tailored to meet the needs of common conditions encountered in these façade types, ensuring both effective noise control and ease of installation. High-mass materials incorporated between internal partitions and curtain wall mullions can also be useful when considering the noise transmission from the framing structure.
As mentioned above, another key area to consider is the junction between the floor slab and the curtain wall system— in what is known as the firestop zone, movement gap, or perimeter joint. Considering the acoustic performance of the firestop itself is key— as well of course as ensuring it meets the required fire integrity and insulation (E & I) performance. From both a fire safety and an acoustic insulation perspective, it is also crucial that the firestop can expand and compress with the façade’s dynamic movement. Failure to do so can result in gaps forming, providing a ready path for noise, fire and smoke to travel internally through the structure. Where the acoustic performance of the firestop itself doesn’t meet the required performance, acoustic overlays can be applied to improve sound performance by adding mass and absorbent materials to treat sound transmission.
In terms of external treatments, specialized mullion covers have been developed to be installed over the top of curtain walling mullions where partitions abut. This substantially improves their acoustic performance and can typically be finished in the designers’ choice of colours to integrate with the overall aesthetic. These products are ideal for projects where internal acoustic upgrades to the mullion have not been used or are not practical. This includes situations where the internal layout is unknown during construction or when fit-out or operation has already begun. Its simple installation process allows it to be installed quickly with minimal disruption to building occupants without the need to empty spaces of furniture or equipment.
CUT THROUGH THE NOISE
Noise control in lightweight aluminium curtain walling systems is a challenging but fundamental part of ensuring a successful project and functional building. By incorporating the 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.
It’s important to note that the overall performance of a curtain wall system is only as good as its weakest link. Addressing the weakest sound transmission path is crucial for achieving substantial improvements in noise control.