Converting obsolete buildings to new uses preserves the carbon in their structures, and minimizes the upfront emissions from new construction. But how to retrofit them to modern standards without adding so much embodied carbon that it risks defeating the point?
That’s the tricky problem facing acoustics engineering specialists like WSP’s George van Hout. In New Zealand, where he is based, structures are typically made from lightweight materials such as timber: safer than masonry in the event of an earthquake, but less good at reducing noise transfer. This can pose a real challenge on adaptive reuse projects where a single-function building is converted for multiple uses – for example, an underoccupied office repurposed to accommodate a mix of residential, commercial and leisure. If there are disused spaces or dead ends where there’s no natural surveillance, or a lot of graffiti, these things make people feel they’re at higher risk of being a victim. This particularly affects women, who are often reluctant to use public spaces for fear of harassment or attack. For example, research by my WSP colleagues in London found that 94% of women felt threatened when using public transport, and that 76% avoided doing so at night.
“Fundamentally, the way sound transfers between two spaces depends on the mass between them,” he explains. “There are innovations coming out all the time, but it always comes back to the same principles. We can provide cavities and insulation, as well as including vibration isolation to separate spaces more efficiently – but to reduce noise, you need mass (because of what we call the ‘mass law’) and that won’t change.”
How is sound transmitted through buildings?
George and his acoustic engineering team consider three key types of sound transfer: “The first is sound transmission between two spaces: how much sound is reduced from one side of a wall or floor to the other, particularly for speech privacy. The second is reverberation time, or how much the sound builds up in the space. If you’re in a restaurant that gets noisier and noisier until you can’t hear the person across the table from you, that’s because the reverberation time is too long. The third is impact noise. In lightweight structures, you may be able to hear people walking on hard surfaces above you. Or if there’s a gym, you might hear the dropping of weights or people jumping up and down.”
In a brand new building, everything can be controlled: the mass of the structure, the wall construction, the surface finishes. With an adaptive reuse, there are more constraints, says George. “A lot of old buildings have quite low floor-to-ceiling heights, which reduces the scope to add drop ceilings, or to build up the floor to control noise transfer and vibrations.
