Noise control

There are three distinct stages to the noise control process:

- Source - Transmission - Reception. The acoustic engineer needs to evaluate the noise reduction measures which must be implemented to provide a quiet comfortable working environment within these three stages.

Basically there are two weapons to combat noise reduction – mass and absorption – usually applied in a combination. The term mass, for example, meaning the plantroom build of heavyweight materials and absorption being the splitter attenuators strategically located to acoustically seal the ducts where they penetrate the mass barrier.

Terminal equipment and ducting

Almost all noise problems that come to the attention of the public seems to be created a long way downstream of the main plant and are located within the terminal devices, e.g. fan coils, diffuser, or possibly ductwork and balancing dampers.

There could be a number of reasons why this is the case and these could be sub-divided into the following key areas:

• Acoustic theory and design calculation

• Acoustic engineers’ specification

• Manufacturer’s performance data

Dealing first with the theory. We should all realise that the whole acoustic/attenuation subject is broadly derived from empirical data. This is not to say that it is incorrect but rather it does not completely follow any classical acoustics equation that if disputed could be proved mathematically. There are therefore accepted methods of calculating acoustic performance of air conditioning systems which have in the past proved reliable. Within these there are a range of recommendations and assumptions that have to be made to achieve the resultant calculated noise level within the occupied space.

These calculations are normally made by the specialist acoustic engineer or independent consultant the latter being the more popular choice.

This brings the issue on to the second point, namely the acoustic specification. The specialist engineer is dealing with a complex subject, largely using empirical methods. He has a duty of care to his employer and his practise and does not want to create problems in his design which are expensive to overcome. So what does he do? He obviously errs on the safe side.

Just as an aside, if you gave the same project or air conditioning terminal system to a number of engineers from the same practice their acoustic conclusions based on calculations and judgement would be different.

For example the actual insertion losses in the octave bands that would be used to select an attenuator would not be identical and could result in different attenuators being selected.

This is mentioned because there is always a judgemental factor – and whilst the end result can vary from individual to individual there can be a vast gulf between different acoustic consultants.

Taking another example called Room Correction. Some USA manufacturers use a figure of up to 20dB and some UK consultants would suggest 0dB. At Trox we believe a conservative figure is around 4 to 6dB depending on room conditions rather than the 8dB that is often quoted in the UK, but the range of judgements is obviously vast.

With the lower figure the air conditioning equipment has to be that much quieter and which generally speaking incurs a cost penalty. So if a specification on acoustics mentions only a room noise level, for example NR35, then the manufacturer has to make an assumption on room correction. This could lead to some manufacturers making optimistic assumptions which would lower their price and reduce any safety factor that is required in the system design.

Refinement of these room corrections and their associated calculation procedures to produce an agreed industry standard would be a major step forward in this area.

The third point is manufacturer’s performance data. The first questions is – have all the components within the system been tested to the up-to-date acoustic standards? What documentation exists to prove the figures at test are the same as the figures in the catalogue? Have the products been tested at all?

It is amazing that during a recent survey conducted by Trox that over 50% of the consultants interviewed said they had first hand experience of noise problems within fan coil systems.

In the January 2006 edition of BSEE, Phil Stonard of BSRIA said: “One of the main challenges that the manufacturers are facing in this competitive market is guaranteeing performance of their products. From its extensive experience in fan coil testing, BSRIA have found that some of those products had the performance which is 25-30% down on catalogue data.

“Reliable performance data demonstrates that equipment is fit for use and is key to minimising risks on project.

“Independently tested and certified products provide the guarantee that the performance has been accurately published and verified, and help manufacturers to compete on product quality with confidence. Also, the testing provides reassurance to project teams that the systems will satisfy particular requirements.”

At Trox we have tried to eradicate these acoustic nightmares by taking on total fan coil system responsibility. In this way we can test each individual component but more importantly the complete system. For example if the consultant’s specification states an arbitrary figure for downstream pressure loss of 50Pa but in reality it is only 25Pa this could create a noise problem.

There are many more aspects which can affect the system acoustic performance and these can be eradicated by this system approach and detailed mock up testing.

By offering a guarantee on room noise levels it gives the design team the confidence that fan coils will not be noisy.

Main plant

Here we are discussing fans, chillers and air handling plant – generally physically large and capable of producing high noise levels. The acoustic engineer must take a number of important issues into consideration:

What noise criteria needs to be achieved in the occupied areas?

Is the criteria thus established likely to be achievable without the application of noise control techniques?

If noise control is required, ductwork design must leave sufficient space, preferably in straight duct runs. Plantrooms and plant should be located away from noise sensitive areas. Are plantroom walls substantial enough? Is sufficient pressure development available on the fans for attenuators?

Are neighbours likely to be affected by noise? What, if any, atmospheric noise control is required?

If plant air handling units are to be located close to noise sensitive areas or in false ceiling spaces what additional care should be taken in the initial selection of noisy items?

Is the client aware that space restrictions imposed by him could result in noise problems; has the client, in applying for planning permission, had any requirements related to noise imposed upon him?

All the above are concerned with the main plant acoustics and generally speaking are dealt with quite adequately during the design phase. Failure to do so can create significant problems and horrendous cost implications if the main plant is too noisy.

There are therefore adequate factors of safety applied by designers and testing of main plant equipment is quite vigorous, fully documented and generally independently verified so the risk of acoustic problems is very low – not so with some terminal fan coil systems!

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