Making sense of Noise Surveys
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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Why do I need a noise survey and what is all this jargon in my report?
As an acoustic consultant I see hundreds of noise survey reports every year. Noise Surveys are probably the most common element of work for an acoustician, so what are they, why do we need it and how do we conduct them?
Why do we carry out noise surveys?
For this article, I’m going to focus on noise surveys for new or refurbished buildings. In this area, surveys are conducted for two main reasons.
1. Noise Ingress – In any noise sensitive building (which is most buildings), we have indoor ambient noise level targets to meet within our development. Noise will transmit through the ventilation openings, glazing, doors, walls and roofs in our building façade, therefore we need to know how noisy it is outside and hence the degree of sound insulation we need. This informs the choice of our constructions, particularly important in determining ventilation openings and glazing, typical acoustic weak points. You may hear an acoustician refer to this as a BS 8233 assessment. As a rough example, if I measure an average of 50 dB(A) outside from my survey, our maximum internal target is 40 dB(A), and I know that an open window gives me roughly 13-15 dB(A) of sound reduction, then we can naturally ventilate our building. If we’re on a much noisier site, or our room is highly noise sensitive, we will need to look at higher performance constructions and mechanical ventilation, or preferably, attenuated natural ventilation. We need to measure not only average noise levels, but maximum levels as well. Our building could be beside a train line, it might be quiet on average with one train an hour, but when it does come, it causes a racket!
2. Noise Pollution – Next we have to consider the impact that noise may have on the neighbours of our new development, particularly residential areas and any other buildings considered ‘noise sensitive’. The most common noise pollutant is plant, such as external chillers, AHU’s and fan units from large kitchens. Quite often we’ll have a large school development without any specialist noise mitigation needed, but our small high street takeaway with flats overhead may encounter problems, as it’s not just the size and quantity of plant, but importantly the distance to the neighbours of our building that impacts on noise pollution. To minimise the risk of noise complaints, we should design our plant noise levels to not exceed the existing background noise level, which is why we need a noise survey. Typically, we would take the lowest measured background level from our survey, which could run through the night if the plant is operational at this time, and design to be at least 5 to 10 dB below this. Your acoustician or planning officer may call this a BS 4142 assessment. In some cases, we might have operational noise as well as plant, i.e. a resistant materials workshop, a music rehearsal room or external MUGAs and sports pitches. We may not be working to the strict limits of the lowest background noise level in these cases, but either way, a survey is still likely required to settle up on agreeable noise level target.
When do I need a survey?
Noise surveys are usually carried out at feasibility or early design stages. The earlier we conduct a survey, the more time we have to figure out how to ventilate our building and whether to shield the most noise sensitive rooms on a quieter façade, should noise levels be deemed to be high for simple open windows. We can also work on where plant may need to go (or not go) and any noise level restrictions for the M&E engineer to consider in his or her specification. Building an acoustic fence near our MUGA or sports pitch might be necessary, and operation time restrictions could be needed on music venues, bars and external restaurant areas.
The latter points of the above are normally required as part of the planning submission. Planning officers will then usually stipulate a condition relating to noise, typically regarding noise pollution and operational restrictions. This might need a further survey post completion, measuring the plant in operation to test that it is indeed below the existing background noise level, if demonstration by calculation is deemed insufficient.
The same assessment (a BS 4142 assessment) is required to achieve the one credit under POL 05 of BREEAM (POL 08 in older versions). Credits under HEA 05 will stipulate (for some building types) that indoor ambient noise levels targets must be those specified in the relevant standard (i.e. BB93 for schools or HTM for healthcare), so our survey is important in ensuring our façade does a good enough job of limiting the external noise ingress.
Is a survey always needed?
Not always. In the case of a refurbishment which is not a change of use, there might not be any mandatory acoustic requirements. That is, unless new plant is to be added. But even if you think a survey probably isn’t needed, remember that a survey is usually required for most developments, so you always an acoustician.
How do you conduct a survey?
A survey always requires attending site to measure during the proposed operational hours of the building, in positions representative of the building façade and the nearest noise sensitive receptor, i.e. the nearest house. Some consultants will use a longer term ‘fixed’ measurement position, with several short ‘spot’ measurements. The ‘fixed’ position measures for several hours to capture the variance in noise level over the day, and if necessary the night. For the latter, I need a secure location to store the equipment unattended, so don’t be surprised to find my microphone on a roof or suspended out on an open window. ‘Spot measurements’ might be anywhere from 1 minute to 30 minutes long, at various positions. The purpose is to establish how noise levels vary across the site, and by measuring at synchronised time periods to our fixed meters, we can make a direct comparison.
The more complex a site is in terms of the noise levels, the locations of noise sources, the density of neighbouring buildings and how big or irregular a shape our building is, the more positions we should measure in. Sometimes we will measure in several fixed positions, and measure over an extended period, depending on the noise source. The motorway nearby may consistent day to day, but the recycling centre next door might work with glass on a Tuesday, so it’s always important to survey for a long enough period, and on the correct days.
For any new build we will of course be measuring long before the building is in place, so won’t this be unrepresentative? Well yes, the building could change level significantly, through the screening of noise behind the building, and the noise reflections off the building itself, and nearby properties. New landscaping and fencing may also effect noise levels.
So in some cases we will consider building a noise map, calibrated to our measurements from site, with the new building then modelled. If we go back to the earlier example of an external level of 50 dB(A), I’m not too concerned, our new building is unlikely to raise these levels significantly, if at all, so we can naturally ventilate without a noise map model. If it’s 60 dB(A), we’re now looking at the increased costs of mechanical ventilation or an attenuated natural vent strategy. Without a noise map, one might apply this strategy across every façade. With a noise map, we might be able to show that the facades at the back of the building, without line of sight to the busy road outside, may be quieter and therefore can be ventilated through open windows. Of course, if there was another road facing this rear façade, a train line, plant equipment or maybe we’re under a flight path, then this may not be the case, but this is why a noise map model is important here.
If you’re ever faced with a design that has mechanical ventilation or high performance glazing across all facades, and a noise map hasn’t been produced, you should check with your acoustician.
I can’t make sense of this report, what do all these terms mean?
In the acoustics world we deal with a lot of different parameters. The main one’s you’re likely to find in a noise survey, are:
· LAeq – can be thought of as an average noise level, this is the equivalent continuous sound level which would contain the same sound energy as the time varying sound. Indoor ambient noise level targets are specified as an LAeq, hence we use this parameter to assess noise break-in.
· LAmax – this is the maximum noise level, used to assess occasional loud noises which might not affect our average level that much, but would still cause a disturbance. Like the LAeq, we should use this in assessing our façade against noise ingress.
· LA90 – used as a measure of the ‘background’ noise level, this is the level which is exceeded 90% of the time. For a noise pollution assessment in line with BS 4142, usually the lowest, modal or mean LA90 is chosen as the target noise level that plant noise should not exceed.
Sometimes you might see these written without the ‘A’, i.e. dB(A) Leq, rather than dB LAeq (these mean the same thing), or with the time period the measurement is conducted over, such as LAeq,16hr or LAeq,30min. The odd ‘S’ or ‘F’ might slip into notation, i.e. LASmax, Slow and Fast, on a slow setting noise levels are recorded every 1000 milliseconds, compared to every 125 on fast. We usually use fast, but slow can give a smoother level fluctuation which is easier to read in an environment where the average noise level is constantly changing.
Other terms you might encounter include the LA10 (level exceeded 10% of the time, often used to assess traffic noise), Lday, Lden and Ldn variations of the Leq over different day, evening and night periods, and LCpeak (the peak sound pressure level). The ‘C’ in this last term is a C weighting. In acoustics we use a weighting to give a single figure dB, which encapsulates all the frequencies in the noise. Usually an ‘A’ weighting is used, reflecting the response of human hearing in normal levels of noise, for example noise at very low frequencies and very high frequencies are heavily weighted against, because we cannot perceive them as well as frequencies around 500 Hz to 6 kHz. At excessively high noise levels the frequency response of our hearing changes again, which we use the ‘C’ weighting for. Any ‘Z’ terms that pop up define when no weighting has been applied, these are the raw sound pressure levels, commonly used if we’re not considering the effect that sound has on humans.
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 firstname.lastname@example.org, or through our Contact Us page.
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