Today’s jargon buster is about sound insulation. Very broadly, the level of sound insulation of a partition indicates how well it stops sound from passing through. It’s used, for example, in testing party walls and floors to see if they are compliant with Approved Document E. It also comes into play when we consider speech privacy in offices, hospitals and schools. When people refer to “soundproofing”, sound insulation is generally what they mean.
Airborne sound, as you might expect, is sound transferred through the air – for example, someone’s voice, or music from a loudspeaker, sound from a television. This is defined as a variant of either Rw or Dw (see below for more information), and is usually specified as a minimum value which must be achieved.
Impact sound occurs when energy passes into a partition from an impact, and re-radiates as noise on the other side. The example of this you will likely be most familiar with is footsteps upstairs heard in the room below. Impact is a form of structureborne noise, which many of us have also experienced when neighbours decided to get the drill out and do some DIY too early on a Sunday morning! This is specified as a variant of Lw and, in contrast to airborne sound insulation, is specified as an upper limit which must not be exceeded.
So remember – with airborne sound insulation, the higher the figure the better the performance. But with impact sound insulation, the lower the figure, the better.
Sound insulation is referred to in many different ways, depending on exactly what is being described. For example, DnTw,Tmf(max) is used in testing schools to refer to the on-site sound insulation measured, corrected for the mid frequency reverberation time (Tmf(max)) measured in the receiving room!
It’s generally the acoustic consultant’s job to worry about all of the flavours and variations of sound insulation, according to your particular project. But it is useful as a client to understand the difference between the “Rw” and “Dw” types of specification.
If you are specifying partitions using manufacturer’s data, for example selecting partitions from The White Book, the airborne sound insulation performance will be presented in terms of a laboratory tested Rw figure. The actual performance you get in your site-specific application will be affected by all kinds of things which your friendly acoustic consultant can help to assess, including the volume of the rooms, the area of the partition, reverberation time, background noise, and sound flanking.
So what is sound flanking? Well, that’s all the sound that transfers from Room A to Room B by a route other than directly through the partition. With a wall, sound might transfer through the junctions with the floor or ceiling slab or with adjacent walls, if not well detailed and sealed. It’s common on site to find a beautiful partition where someone has then drilled a large hole through for cables, which then hasn’t been sealed properly. Some flanking is inevitable – especially in a refurbishment – but it can be dealt with, with consideration.
Expanding foam is the bane of every acoustic consultant’s life – sorry, but it won’t stop the noise passing through, whatever it says on the can from a Well Known High Street DIY Store. Mass is the key to sound insulation (as well as the air gap between sides of a partition, for higher performance). Small openings need to be properly stopped up with mineral wool and mastic – larger openings may need a cover plate to make them good. Ductwork passing from room to room is also a common issue – crosstalk attenuators can be used to stop ducting acting like an old fashioned “speaking tube” and allowing speech to transfer directly from room to room.
Very rarely, impact sound might be a concern for walls (e.g. balls hitting a sports hall wall next to a classroom). Sensible design should mean that such adjacencies don’t occur. But impact occurs all the time in floors. It’s less of an issue where there is a soft floor covering such as carpet, but in domestic situations often leads to complaints when carpet is replaced with wooden or laminate floors.
Resilient underlays are widely available commercially to help combat this issue. It’s also worth considering the overall construction of a floor and the ceiling below – a mass-air-mass construction, possibly with the ceiling resiliently hung, will typically perform better than a single layer floor (unless that single layer floor is a LOT of concrete).
The Planning (Agent of Change) Bill gets its second reading in Parliament on October 26th. It’s going to be a while before we see it enshrined in law but we are already seeing local authorities being more considerate of the effect that new residential development will have on existing noise sources such as clubs, pubs and industry.
(UPDATE – the second reading will now be on July 6th. There was no time for the bill to be heard on June 15th due to Parliament running out of time while an MP tried to block a bill on upskirting. This was not a good day for democracy!)
The bill is long overdue – and doesn’t introduce anything which shouldn’t be a part of good planning practice anyway. We have long advocated that it should be the responsibility of developers to protect their tenants from existing noise sources as part of their design – but unless the local authorities insist upon this and enshrine it within planning conditions, non-constant noise sources such as from events are often conveniently ignored.
The ‘problem’ with noise from venues often isn’t the overall noise level, it’s the low frequency noise which tends to disturb residents. Mitigating this can be expensive, as low frequency is harder to control than higher frequencies – it can mean thicker glazing units, and thicker partitions. The cost of providing mechanical ventilation systems where noise levels with windows open for natural ventilation are too high is also a huge issue for developers. Understandably, from the Developer’s point of view, they need to comply with planning but they also need to keep control of costs if their development is to remain viable. How can we establish an appropriate balance?
The existing standard which is typically used when considering noise for new residential developments is BS 8233:2014. Unfortunately this sets noise limits as a single figure only, and doesn’t delve into the frequency content of the noise source.
Some local authorities such as Manchester have published comprehensive guidance on planning and noise which considers low frequency noise when a new residential premises is structurally connected to an entertainment venue (or vice versa) but doesn’t set criteria for the frequency content of noise transferred to residents via windows and ventilation. We’re also coming across other authorities now which are trying to implement the principles of Agent of Change but there’s no consensus on what appropriate design standards should be implemented.
Even ignoring, for a moment, the low frequency noise issues, there’s another thorny issue rearing its head.
If facades, glazing and ventilation are designed appropriately to control noise intrusion, what about ‘purge ventilation’, i.e. opening the window when you burn the toast? There’s a strong argument that people should have the freedom to open their windows at any time, as long as they have the option of sufficient ventilation and noise insulation with the windows closed. Arguably, if you then choose to open the windows, you are also choosing to expose your self to higher noise levels. But some planning officers are becoming nervous about this, wary that they will then still receive noise complaints about existing venues and find themselves in a very difficult position.
So. We have existing venues who want to stay in business, developers who are cost conscious, planning officers who don’t need extra work, and future residents who probably won’t understand why the noise they can hear when they open their windows isn’t actually a nuisance. What’s the solution?
Developers – I’m sorry. The bill probably does mean that you’ll be paying more on ventilation and glazing. But see this as an opportunity to extract more value from your acoustic consultant!
The noise climate around a site near a venue can be complex, and it’ll change further post-development when the building itself provides some natural screening. Where there’s a venue nearby, or for any large development, a simple noise survey isn’t enough. You should expect your consultant to produce a noise model of the site, calibrated against the survey and then with the development added to see the effect.
Every site is different, but in many cases you will find that although the acoustic requirements for glazing and ventilation are indeed increased on the ‘noisy’ facades of the development, there will be opportunities to save money on the ‘quieter’ facades. If you get your consultant involved early enough (RIBA Stage 1 or Stage 2) they can even guide you on building form and internal layout to provide natural screening and reduce the number of noise sensitive rooms exposed to the highest external levels. Of course, it’s important to do this while your architect is still at the concept stage!
We’re already seeing good venues being closed due to the mere prospect of future development. As responsible developers, designers and planners, we need to be proactive in ensuring that both future residents, and existing venues, can live harmoniously with each other.
At dBx Acoustics, we’ll be watching the progress of the Bill with interest. In the meantime, we’ll continue to advocate for pragmatic acoustic design which balances the needs of all interested parties.
If you’d like us to keep you updated about Agent of Change and how the emerging proposals are likely to affect acoustic design, just click here to leave us your details.