Acoustics A to Z
B = Building Acoustics
The aim of our articles are to break down acoustic terms and concepts as simply as possible, without going too far into the mathematics and every nitty gritty technicality, that acousticians usually love to get stuck into.
So please, if you’re an architect, contractor, developer, planner… or really anyone who occasionally needs to dabble in acoustic design and assessments… then read on…
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What do you mean Building Acoustics?
B gives me another excellent topic which I’m very familiar with, which is… . Now I appreciate that it looks like every topic in this A to Z will be a different word simply followed by the word ‘acoustics’, but I promise… it’s not going down that road, there will be some variety!
Anyway… a common misconception (along with eggboxes on walls) is that Building Acoustics means designing concert halls and recording studios, types of space which have a strong connotation with good acoustics and quality of sound. However, it’s also an important aspect for any building which contains ‘noise-sensitive’ spaces, which is most buildings, i.e. our homes, places of work or education, healthcare facilities and places of recreation and leisure. It can have a huge impact on our living comfort, health and wellbeing, productivity and communication.
Well, the clue’s in the name, so you’ve probably already guessed that it’s to do with the acoustic design of buildings… but still, what does that mean? Let’s briefly look at some of the different aspects of Building Acoustics (and bear in mind, I’ll cover most of these in more detail later in this A to Z series).
The control of sound transmitted through the air, such as speech, television, music or the next-door neighbour’s dog. We usually want a decent level of airborne sound insulation from the room next door so that we don’t hear private conversations or have to listen to their music. We achieve this by specifying wall and floor constructions which have good sound reducing qualities, making sure they all connect together without leaving weak points for what’s known as ‘flanking sound’, and sealing any ‘penetrations’ like ducts and pipes which may run through the separating wall or floor.
Impact sound occurs when an object impacts the structure, typically the floor, such as footsteps and chair scrapes. Carpets and ‘resilient’ underlays help to dampen the impact, as do rubber or spring mounts on raised floors. In most cases, we’re dealing with vertical transmission across intermediate floors from above to below. But in high impact areas, such as sports halls and gym floors, we can also have impact noise transmitting horizontally.
How sound behaves in an enclosed spaced, in particular, how reverberant a room is, which refers to the length of time that sound takes to decay. For example, think of two contrasting rooms, firstly a large sports hall with lots of hard surfaces such as vinyl flooring and concrete block walls, which has a very ‘live’ feel. Secondly, a small control room at a recording studio, with plenty of soft surfaces (which absorb sound) such as carpet, foam wall panels, ceiling tiles and furniture, which has a very ‘dead’ feel. The former has a much longer reverberation time, resulting in poor speech intelligibility and high levels of noise build up. If you think of a classroom, an office or a bedroom, this is somewhere between these two extremes. This is important, after all, can you imagine working in an office as reverberant as a sports hall?
This is the background noise level in each room of a building, when unoccupied but operational, or if you like, when you sit in silence with the television off. In this case, we might hear the sound of passing traffic outside, noise from a nearby school playground, maybe the air conditioning, and perhaps the sound of water rushing through the pipes. So, the aim is firstly to control noise ingress from outside by specifying façade constructions that have sound reducing properties, and also considering the ventilation strategy, i.e. will open windows let too much noise in? Secondly, we need to limit the noise generated by the building services, avoiding noisy mechanical ventilation units in sensitive spaces, using silencers in ductwork, and perhaps lagging pipework or encasing it with plasterboard. We need low enough indoor ambient noise levels to avoid sleep disturbance in our bedrooms, and to work or learn without distraction from noise (although in open plan offices, some background noise can actually be beneficial in masking the sound from our colleagues, and providing comfort to ourselves, after all, no-one wants to feel like the whole office is listening to their phone conversation, especially if you can otherwise hear the proverbial pin drop).
As you can probably guess, this is the control of noise generated by rain impacting on the roof, particularly if it’s a lightweight roof, and especially if it has rooflights. If you’ve sat in a conservatory when it’s raining heavily, you’ll know it can get very noisy.
This relates to vibrations through the structure which then radiates sound into a room. Indeed, impact sound is a form of structure-borne sound. But it might not be a sudden or intermittent impact, structure-borne sound could be excited by a continuous source of vibration. Examples of this might be mechanical plant with moving parts (such as a fan unit), which if not installed well and isolated from the structure, can cause continuous vibration to be transmitted through the floor, walls and fittings, which then radiates noise into rooms nearby. Or, the vibration could arise from the foundations, particularly if the building is close to a railway line.
This perhaps fits under the area of Environmental Acoustics rather than Building Acoustics, but as I’ve referenced noise and structure-borne sound generated by building services, it’s worth mentioning. This mainly refers to controlling the external noise emissions generated by the mechanical plant of a proposed building, to ensure it does not adversely impact on those that live and work in the surrounding area. But we also must consider how ‘activity’ noise might impact on others, particularly if a building has high noise level spaces (i.e. music rooms, dance studios, workshops, etc).
Well, ideally yes… but the necessary level of detail in the design and the performance will vary for different types of buildings, rooms, and adjacencies. If we live in an apartment, we’d want a good level of sound insulation from the flat next door, but we probably wouldn’t be so bothered about it between the rooms within our dwelling. If we live above a nightclub, then we’d definitely want a VERY good level of sound insulation! Low indoor ambient noise levels in our bedroom are important for sleep, but not so important on the dancefloor of the nightclub downstairs. Many guidelines exist (in the UK) which govern the acoustic design of different buildings, such as Approved Document E (Part E) of the Building Regulations (residential), BB93 (education), HTM 08-01 (healthcare), BCO guidelines (offices) and BS 8233 (multiple building types).
With the exception of assessing external noise ingress through the building envelope, where a site with high external noise levels might not be suitable for a sensitive building unless designed and mitigated appropriately, and perhaps some high-level internal design as part of a feasibility study, the bulk of Building Acoustics design is usually (though not always) done after gaining planning permission, depending on the project. So, this is on new construction or refurbished developments from RIBA Stage 3 onwards. That is, detailed design, technical design, and construction stages. We may also have to ‘test’ the building at handover and completion, to demonstrate that the acoustic design criteria have been achieved. Occasionally some input from an acoustic consultant may be needed for developments that have been in operation for years, in response to a complaint from the occupant, usually as a result of a refurbishment or a change in the surrounding environment.
I hope you enjoyed this short article, and keep an eye out for more articles on the common questions that I get asked by clients in my job as an acoustic consultant. Feel free to connect and message me through LinkedIn, send me an email at email@example.com, or through our Contact Us page.
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An easy and obvious topic to start the A to Z… Acoustics! What is it and how does it affect our day to day lives?