This TECHNAL sponsored CPD explains what designers need to consider when specifying architectural glazing packages that involve increasingly demanding acoustic requirements
Enhanced acoustic requirements are commonly being stipulated in contract documents for many types of developments to help attenuate a progressively noisy external environment. Clients are therefore looking to their project teams to deliver both high-performance and creative solutions when formulating facade designs to meet tough specifications.
This CPD explores what designers need to consider when specifying architectural glazing packages where good acoustic performance is a key requirement. Designers also need to consider other performance and aesthetic criteria as part of the package. This will create better-performing buildings and safeguard investment.
The role of fenestration in noise reduction
Great architectural design is not purely about pleasing the eye but includes creating structures and interior spaces that function well and offer comfortable conditions, whether for habitation, employment, recreation, education, care or other activities.
As a main element to most buildings, fenestration has long played a key role: contributing natural daylight and ventilation, as well as being integral to overall energy performance, in terms of both thermal insulation and solar gain. Arguably, however, it is in respect of acoustic performance that the bar continues to be raised most dramatically in response to the growing impact of noise pollution on our everyday lives.
This worsening situation stems from a variety of sources, including higher traffic volumes, expansion of rail and air travel and a growing tendency towards a 24/7 society in which the population lives, commutes, works, shops and seeks entertainment in ever closer proximity.
UK housing developments are among the most high-density in the world. It is not just the noise of neighbours and passers-by that pose a problem: being beside a trunk road or beneath a flight path can also have a serious impact on places such as schools and hospitals.
Unwanted noise is not merely intrusive and distracting; the impacts can be detrimental to health and general wellbeing. Scientific studies have shown that patients on noisy wards recover more slowly and are more likely to suffer psychological issues, while poor acoustics also lead to lower productivity and slower learning. These factors have the potential to significantly reduce the value of a property or shorten its working life.
Noise poses a commercial risk as well as being a key consideration within developers’ corporate social responsibility, which pushes acoustic performance up clients’ priority lists. Planning authorities too are increasingly focused on sound transmission as well as actual noise abatement.
At present the matter is being addressed on a project-by-project basis, in response to acoustic reports, with specifications tailored accordingly; but a more long-term and holistic approach could have far-reaching benefits.
Acoustic design considerations
To realise these multiple objectives, the project’s site investigation and appraisal should include mapping the geography and geometry from an acoustic perspective. This will provide information that should refine the development’s exact location and orientation as well as design and layout, which will in turn influence capital cost and eventual patterns of occupancy.
For commercial properties the facade, including its glazing, is second only to the building services package in value, normally accounting for 15% to 25% of total construction cost. The different elevations and roof areas must provide protection from the weather while complying with the relevant Building Regulations, notably Approved Document E which covers resistance to sound.
As a result, the architect, structural engineer, fire consultant, specialist facade consultant, acoustician, fire consultant and manufacturers supplying elements for the envelope will need to liaise closely.
The primary objective of co-ordinating and consolidating the input of all these professionals is to arrive at a design for the envelope that delivers on all the key criteria, and therefore enables accurate costings along with certainty of delivery. Expensive design changes and delays during the construction programme can thereby be avoided.
Design guidance for good acoustic performance
Guidance on designing for good acoustic performance can be found in BS 8233: 2014. Environmental assessment methods including LEED and BREEAM award credits for good acoustic performance. These should be read in tandem in order to provide a holistic approach to environmental performance.
It is important to recognise the very complex nature of sound. This includes the frequency or pitch as well as the intensity, which determines how loud the sound is. Intensity is measured in decibels. The specifics of the environment in which the sound is produced also affect how people experience sound. But while the drone of vehicle wheels on a concrete road surface sounds very different from the low rumble of engines ticking over, human voices or the hammering of pneumatic tools, all these sounds can be attenuated and reflected through careful detailing of glass, air gaps, interlayers and frame components.
The value of an acoustic report
In compiling the acoustic report, the acoustician conducts a detailed survey, measuring the noise levels incident on the projected footprint, at multiple points and at different times. Each level of the building is assessed to determine the degree to which the facade system and its glazing elements must be specified, taking account of the minimum attenuation required to deal with the noisiest periods. This may be in order that bedrooms permit people to sleep undisturbed, or for offices to provide a comfortable and productive working environment throughout set operating hours.
However, the rising levels of sound in our environment are rapidly leading construction clients, local authorities and other stakeholders to push the boundaries on what is economically achievable – demanding ever smarter specifications and technically optimised solutions.
There are numerous forms of contractual arrangement for big construction projects, including those where the architect is novated to a main or management contractor. Ultimately, the client and leading consultants must weigh up the benefits of potentially suitable products that meet the design criteria – with factors to consider including the physical weight of the glass units, which might potentially compromise facade features such as cantilevered fenestration.
Avoiding acoustic bridges
The next stage in achieving good acoustic conditions within a building is to understand how the noise attenuation offered by different materials and envelope elements can be compromised by the interconnections and surrounding structure.
The phenomenon known as flanking sound or flanking noise refers to sound waves that pass over or around a barrier rather than directly penetrating it. This means that although an insulating glass unit or architectural glazing system may offer good acoustic insulation, noise can still penetrate to the building interior, or pass between spaces.
The Dnfw value measures the total flanking sound transmission of the facade system. It does, however, comprise a number of individual sound transmission paths, with five main ones to consider. These include the glazing element, the transoms, the structural element of the mullion, the hollow of the mullion and the movement zone.
The system provider and facade consultant must therefore seek to address rigid connections, especially metal-to-metal interfaces, in order to prevent sound waves being transmitted through pressure plates, frame profiles, joints, brackets or anchor points.
When considering the potential for flanking sound passage via hollow frame elements within curtain walls, two main modes of transmission can apply:
- Where the hollow section actually forms a separating element between two adjoining areas, for example at a mullion junction to an internal partition or at a transom abutment to a raised floor or suspended ceiling. In this case the sound insulation index associated with transmission “through” the section effectively controls achievable performance levels.
- Where the hollow section passes from one area to another without interruption, such as mullions crossing an internal floor line. This offers the potential for sound to break into the hollow section in one area and then exit in the adjoining area. In this instance sound passage occurs “along” the length of the framing component.
Remember that sound passage via hollow frame components represents just one of a number of potential mechanisms and factors that collectively form total flanking sound transmission.
Achievable acoustic performance improvement levels will vary significantly based on a number of specific factors that include:
- Transmission mode
- Section material
- Section wall thickness
- Internal dimensions.
Flanking noise can therefore be reduced greatly by careful detailing, such as acoustic inserts to mullion and transom profiles, and the creation of additional acoustic barriers between the frame and floor slabs.
While the inserts may be employed for the purpose of improving the sound transmission characteristics of the building envelope (that is, outside-to-inside performance), more commonly they are used to assist in reducing flanking transmission between adjacent internal areas. This includes room-to-room and floor-to-floor flanking sound transmission.
Below are some examples of sound transmission treatments using specialist acoustic insulation designed to reduce vertical and horizontal sound transmission in buildings that feature curtain walling.
‘Through’ frame treatments
The “through” frame treatments involve the introduction of a continuous insert that fully fills the section’s internal void. The inserts can be purely absorptive or a high-mass/absorptive core composite combination.
Absorptive inserts, available in two main material types – resin-bonded rockfibre and impregnated acoustic foam – offer modest but useful improvement levels, mainly by reducing reverberation within the internal void. Mass composite versions directly improve the sound reduction index (SRI) of the frame by effectively increasing the section’s wall weight. The elastomeric nature of the high-mass outer membrane offers additional beneficial dampening characteristics.
‘Along’ frame treatments
The “along” frame treatments generally consist of the introduction of a localised closure or baffle at the crossing point of adjacent internal areas within the building.
Similarly to treatments for “through” frame transmission, both absorptive and mass barrier solutions are available. The most commonly employed “along” frame inserts are absorptive baffle inserts, typically supplied in 300mm to 600mm lengths and installed centred on the midpoint of the compartment line.
Ensure airtightness while balancing ventilation
Similarly, airtightness – which has been the focus of tightening limits aimed at reducing energy loss – is also very important for acoustic insulation. Any gaps in the building fabric will allow air leakage, which also represents an acoustic breach. Here, the structural engineer’s input will be vital in quantifying the extent to which sway or deflection will affect the envelope’s integrity.
It is therefore essential to ensure that the window, door or curtain walling system chosen features effective acoustic sealing in tandem with effective sealing for airtightness. These acoustic gaskets perform best in combination with concealed accessories, as they can be applied continuously, internal to the frame without the need for cut-outs.
To balance a more airtight envelope, controllable background ventilation is required under Part F of the Building Regulations. In the past trickle ventilators, either clipped to the glass infill or slotted through the frame, provided this. Acoustic trickle ventilators are available, but only provide a certain level of sound attenuation and can be bulky in appearance. Generally, a less expensive and superior alternative is acoustic through-the-wall ventilators.
The selection of glass type and configuration
The choice of glass type and dimensions, including that of the cavity, impacts dramatically on the level of acoustic performance. Industry standard double glazing will deliver a 30dB to 35dB reduction in sound transmission against normal passage through air.
This can be improved upon significantly by specifying thicker panes and special acoustic-grade glass and by widening the cavity or introducing secondary glazing – which can generate a 40dB or greater reduction. Because the decibel scale is logarithmic rather than linear, every 3dB reduction represents a doubling in performance. And just as combining varying densities of material is effective at combating sound transmission, the best results for glazing units are obtained when the inner and outer panes are of different thicknesses.
It should also be assessed whether to include a laminate interlayer for enhanced acoustic performance – potentially also improving thermal efficiency and safety – or perhaps to go from double to triple glazing. The downside of triple glazing is an increase in overall unit thickness and cost, by some 50%. Additionally, the increased weight may be beyond the capacity of the framing system originally considered.
Installation may also be adversely affected if the size and weight of glazing units mean they require specialist lifting equipment, even taking up valuable crane time on site. As well as the difficult handling characteristics, large triple-glazed windows and doors can be too heavy to operate easily, or require alternative hardware, such as stronger hinges. Extended lead times on delivery can be another penalty.
By contrast, specifying an acoustic interlayer might add 60% to the total materials cost but improve project feasibility through slimness and lower weight. It is also an option to create two smaller opening lights instead of one, although this has an obvious impact on sightlines.
In order to ensure that the specification for a facade achieves good acoustic performance, here is a short checklist of the information that should be provided to the architectural glazing manufacturer:
- Schedule of the elevations Shows the types of windows and curtain walling being proposed, along with the overall aesthetic.
- Acoustic report Contains the requirements for sound attenuation from outside to inside and flanking from both room to room and floor to floor, ensuring the specification can be correctly customised.
- Thermal model Details the U-value figures, air leakage rates and other energy targets that must be met to satisfy the Building Regulations or standards such as Passivhaus.
- Fire strategy Highlights the need for fire-rated or fire-tested curtain walling, windows and doors, enabling selection of the right system. Any visual inconsistencies occurring as a result of smoke ventilation or intermediate fire-stopping requirements can be flagged up early on.
- Movement and tolerance report Will ensure the selection of the correct product group in circumstances such as where a window wall or a 50mm/60mm curtain wall is selected. For buildings that need more tolerance due to racking, for example, a 60mm system may be better able to cope with slab edge deflection due to live load.
Possessing the right information and taking a collaborative, cohesive approach from the beginning, recommendations from the systems manufacturers can contribute to tighter cost control, fewer on-site issues, improved programme certainty and continuing client confidence.