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NOVA Acoustics

Air-Conditioning Case Study

A frequently encountered type of work which clients often ask for further information about is a Noise Impact Assessment.

In general, Noise Impact Assessments are required when there has been the introduction (or planned introduction) or a new ventilation or air-conditioning system. This is usually requested by a local council when it is thought that there may be a chance that the new installation may have an impact on the amenity of a residential property (known in the acoustic consultant trade as Noise Sensitive Receptors or NSRs).

Many owners and operators of commercial residences are surprised that they need to have a Noise Impact Assessment carried out in order to simply install something so commonplace. This is doubly vexing to many owners when, in their viewpoint, all they are attempting to do is improve the way in which they deliver their business or service.


However, without such assessments, it is difficult to know whether or not the local residents will be affected. In fact, the purpose of the assessment is to understand whether or not the new installation will be likely to draw complaints. This likeliness of drawing complaints‚ is exactly what is assessed by BS4142:1997. This British Standard is referenced by acoustic consultants when they carry out a Noise Impact Assessment; it helps to provide the client with an idea of what they can expect for their new installation. Although not all Noise Impact Assessments are performed to this standard (perhaps you are installing a wind turbine?), this is certainly the most common and relevant to many client‚ needs.

The Proposal:

NOVA Acoustics were commissioned by a client to assess the noise impact caused by two air-conditioning units which the client had recently had installed at their property. This would entail measuring the typical background noise at the site and using these measurements to make an assessment of the likelihood that they would impact on the amenity of the nearest noise sensitive receptors (NSRs). These background noise measurements would be analysed in relation to the difference in noise measured when the units were operational. The work would be carried out in accordance with BS4142:1997:‚ Method for rating industrial noise affecting mixed residential and industrial areas. It would therefore provide the noise Impact Assessment of the air-conditioning unit with the following objectives:

Determine the existing noise climate at the client property
Assess the likely noise impact of the new air-conditioning units
Make recommendations (if necessary) regarding how to achieve suitable noise levels with the new air-conditioning units
The Site:

The proposed Noise Impact Assessment of the client’s air-conditioning units was to be undertaken at a busy, residential area in London. The units themselves were installed on the first floor of a rooftop terrace, with a residential property adjoining this area; this meant that the neighbour’s windows were very close to the location of the air-conditioning units. This can be seen in the picture below.

The Methodology:

In the first instance, a background noise measurement, which would be representative of the typical background noise in the area during operational hours of the units, would need to be made. Following this, the two air-conditioning units were measured at their maximum output (ensuring a worst case scenario is captured) to provide the contrasting measurement which the BS4142 assessment would be based upon. A picture of NOVA Acoustics‚ noise monitoring equipment being set up at the site can be seen below.

The Result:

After obtaining the necessary measurements, performing the required calculations and comparing the results with the recommendations provided in BS4142, it was found that the likelihood of complaints would be high – this would have been particularly true at night time (when the world tends to get a little quieter).

The Recommendations:

Following our arrival at this result, NOVA Acoustics were able to offer free remedial advice about the air-conditioning units, in order to ensure that they would fall within the recommended noise limits that would be unlikely to prompt complaints. Alterations to the air-conditioning units were the only option in this particular instance as the client had already installed the units; in circumstances where it is at all practical to do so, we encourage clients to contact us prior to installation, so we can advise on the most appropriate unit (sometimes a cheaper air-conditioning unit can be a false economy if you have to spend more money to ensure that it complies with recommendations!). In this instance, NOVA Acoustics were able to provide our client with a simple, cost-effective solution which meant their new air-conditioning unit purchase had not been in vain.

If you have recently installed a new air-conditioning unit or ventilation system, or even if you are considering doing so, contact NOVA Acoustics today for a comprehensive and cost-effective noise survey.

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