Posted: 27th April 2021
One of the most regular questions we get asked by clients, is why their new air conditioning condenser needs an acoustic enclosure when the neighbour’s units (or their own existing units) don’t have any noise control treatments. If this sounds like you, then here’s a brief guide to try and explain what can be a very frustrating situation.
When you want to install a new air conditioning condenser or some air source heat pumps, you will need to apply to your Local Authority for planning permission. The council will require a noise report as part of that planning application, demonstrating that noise from the new condenser complies with their acoustic requirements.
Each Local Authority has their own assessment method and noise criteria. Some councils use BS 4142:2014+A1:2019 to determine the acoustic impact while others have their own unique policies and criteria.
BS 4142 uses the anticipated noise level from the condenser to outside nearby residential windows and various correction factors for characteristics which could make the noise more noticeable or disturbing. This calculated ‘rating level’ is compared against the existing background sound level. If the rating level exceeds the background sound level outside residential windows then the higher that excess, the more likely it will be disturbing. But, importantly, when assessed in accordance with the Standard, BS 4142 doesn’t just use these numerical results.
The context of the proposal needs to be considered to determine the final assessment outcome. A new condenser being installed next to other existing units is unlikely to cause adverse impacts, even if the numerical calculation shows a small excess above the background noise.
Unfortunately, many Local Authorities choose not to implement BS 4142 in full. Instead, they remove the context, choosing to set a strict numerical limit relative to the background noise.
It might be helpful here to quickly explain what we mean by the background noise level. The background noise uses an index called the LA90. In simple terms, this is the sound level that is exceeded for 90% of the time over the measurement period. It is the underlying sound level during the quietest 10% of the time and excludes short-term individual noise events.
This is important to remember where there are existing air conditioning condensers. Because the condensers switch on and off maintaining a constant internal temperature, it will be these times with the condensers off which determine the background sound level.
We usually measure sound levels in 15-minute samples. BS 4142 is clear that the background sound level isn’t the lowest level but is a level which is representative for the area over the period. For example, imagine a noise survey showing consistent LA90 sound levels over an evening with only one or two occasional lower periods. BS 4142 takes the consistent levels as being the representative background sound level. BS 4142 also advises that particularly low background levels in the middle of the night are unlikely to be as important as levels measured at the start and end of the night.
However, some councils specifically require the lowest measured 15-minute period is used as the background sound level. In these boroughs a low 15-minute measurement in the middle of the night can become the defining level when setting the noise criteria.
We mentioned above how some councils choose not to implement BS 4142 fully, instead using a simplified method or removing context from the assessment. These Local Authorities usually set a numerical criterion relative to the background sound level. It varies between councils, from some setting the rating level to not be above the existing background sound level to others needing the new condenser to be up to 10 decibels (dB) below the existing background noise.
In other words, if you are in one of those council areas then the noise from your new condenser to outside your neighbours’ windows could need to be 10dB quieter than the periods when the existing condensers are not running – in the middle of the night!
Because we have worked on projects for new condensers, air source heat pumps, and all manner of new mechanical services equipment all over the country, we know what your council’s criteria is and, importantly, what information they will expect to see in the acoustic report (that varies between councils as well).
Once we have calculated noise levels from your condensers and compared that against your council’s criterion, we will be able to confirm if you need any noise control treatments. This is where our wealth of experience and innovative design approach kicks in. Rather than recommending an acoustic enclosure and be done with it, we actively investigate all other possible ways to mitigate the noise. For example, does moving the unit to another position or using natural screening from the existing building mean we don’t need that expensive enclosure?
Of course, there are times when we simply can’t avoid recommending noise control treatments, but if that’s the case you can be sure we’ve exhausted all other possibilities first!
Our experienced acoustic consultants will be happy to discuss your project and how including our acoustic report with your application can help you obtain your planning permission. Just get in touch with us to find out more.