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Acoustics - what is being measured and why is it important?September 2013

Everyone is affected by noise but noise is often a secondary consideration when designing a building. There are so many building constraints that the best design for acoustics is not often the best design for other factors, such as temperature comfort, and as such noise issues can occur once the building is occupied. By considering noise at the beginning of the design process and educating occupants at the end of the design process noise issues within buildings should be minimised.
(Read about BSRIA's acoustic testing services)

Common noise sources

Any moving or vibrating part in a product can produce noise and this noise will vary with the operation of the product. For example as a fan’s rotational speed changes, the noise produced by the fan changes. Some products contain one noise source and some products contain multiple noise sources. For example, in an air source heat pump the noise sources are a fan, a compressor and a pump.

Noise produced by a product will be transmitted through the air and this is called airborne noise. Sources of airborne noise include fans, compressors, pumps and motors. Vibration is transmitted through structures and may be radiated as noise by connected components; this is called structure-borne noise. Sources of structure-borne noise include casing and ductwork.

Paths of noise transmission

Once the noise sources of a product have been identified, it is necessary to understand how that noise will be transmitted in a real life situation. When a product is installed, noise can be transmitted through various different paths.  Let us consider a simple example of an air conditioning unit located in a plant room of a building that supplies air to the offices above, as shown in Figure 1.

Figure 1: Paths of noise transmission (click image to zoom)
The air conditioning unit produces noise which propagates through the room, as shown by the purple arrows emanating away from the unit. This noise could cause disturbance to people working in the plant room. Airborne noise can also be transmitted through the ceiling, causing disturbance to the occupants of the offices above. The air conditioning unit may be supplying air to the offices and airborne noise will propagate through the ductwork and into the offices. If the duct is lined or an attenuator is installed then the noise will be reduced as it travels along the ductwork.

The air conditioning unit can also cause structure-borne noise, as shown by the red arrows. If the unit is installed with anti-vibration mounts structure-borne noise into the floor of the plant room will be reduced. Depending on the installation of the ductwork, structure-borne noise can also be transmitted through the ductwork.

Standard testing

Most sound tests fall into one of three categories – sound power, sound transmission and sound absorption.

For heat pumps, air conditioners and similar products the test standard is EN 12102, which refers back to EN 14511, and is specified for the UK MCS scheme and the European EHPA Quality Label. This standard specifies how to determine the sound power level of a product, with particular emphasis on carrying out acoustic testing at set conditions, as the noise produced by a product can change with the operation of a product.

Products such as louvres can be tested in accordance with EN 10140 to determine the sound transmission through the product. Finally building materials that are designed to absorb sound are tested in accordance with ISO 354 to determine the sound absorption coefficient of the material.

In some situations it is also appropriate to carry out vibration tests, in accordance with standards such as ISO 10816. This type of testing can be used for operational monitoring, acceptance tests and diagnosis of potential failures. Vibration levels are a strong indication of how a machine will perform over time and tests can be carried out to determine if specific components, such as bearings, are likely to fail.

Importance of test data

Accurate acoustic test data is very important as it will lead to an accurate noise impact assessment of the product for a particular site. This means that the product will be successfully installed and no unforeseen noise issues should occur.

On the other hand, inaccurate acoustic data where the product has not been tested at appropriate operating conditions or a suitable test standard can lead to an inaccurate noise assessment and ineffective mitigation. Once the product is installed it is more likely to receive noise complaints and then additional noise assessments and mitigation may be required. This can be a lengthy and costly process.

It is therefore essential to obtain robust, meaningful and reliable acoustic test data. This will allow installers to make a comprehensive judgment when selecting products for a particular site.

Typical acoustic data

Figure 2: Sound power levels for an air source heat pump (click image to zoom)
Figure 2 shows the sound power levels for a heat pump operating at the standard rating conditions specified in EN 12102. The sound power levels are given for each frequency band and the overall A-weighted sound power level is 61 dB (A). To put that into context 61 dB(A) is slightly louder than a typical household appliance.

Why measure frequency?

Why is it important to measure noise levels at different frequencies and not just provide an overall noise level? Frequency is used to describe the pitch of the sound being produced. A low frequency is a low pitch – such as a bass drum, and a high frequency is a high pitch – such as a triangle. Figure 3 shows the noise produced by three products. The blue line represents a product that produces a broadband noise – equal at all frequencies. The green line represents a product that produces predominately low frequency noise. The purple line represents a product that produces noise with higher noise levels in the mid-frequency bands. Although these three noise sources will sound very different the overall sound power level is identical at 71.7 dB(A).

If you were recommending product developments or mitigation once installed then being provided with just the overall sound power level would suggest you need to just choose mitigation that overall reduces the noise from the product. However, being provided with the noise level at each of the frequency bands means that developments and mitigation can be tailored to the individual product. Some mitigation solutions work well to stop low frequency noises and some work well to stop high frequency noise. Knowing the noise level in each frequency band enables cost-effective mitigation to be implemented.

Figure 3: One-third octave band centre frequency sound power levels (click image to zoom)
How can noise be reduced?

Noise from HVAC systems can be controlled by either minimising the noise at source or minimising the noise transmitted. Minimising the transmission of noise can be achieved by locating silencers in ductwork, utilising acoustic louvres, installing anti-vibration mounts, and examining the building construction.

Minimising the noise at source can be achieved through product development. If an air conditioning unit has a particularly noisy compressor, this could be examined in an attempt to make it quieter. It is important to consider how the noise changes with the operation of a product. If a product is going to be operating at particular conditions (thermal, airflow, etc.) then it is essential that product development takes place under these conditions. Acoustic product development is very beneficial because minimising the noise produced will minimise any noise mitigation required once the product is installed.

Installation and commissioning

Once a product has been installed there may be noise nuisance complaints. This may result in additional mitigation being required to rectify the issue. When dealing with noise nuisance two key questions are “is the product installed correctly and is it making the expected noise?”

Poor product installation is often the reason for noise complaints and something as simple as ensuring casing has been tightly mounted can stop the noise nuisance. Poor installation of building elements can result in higher levels of noise being transmitted, due to gaps or flanking noise paths that the noise can be transmitted through.

The level of noise once installed may be different to when it was tested. This is often due to a change in the operation of the product. If fan speeds, air or water temperatures, and flow rates have changed then the noise produced by or transmitted through a product will change. It is therefore important to carry out initial testing and product development at the operational conditions that the product is being specified for.

Which is better – prevention or reaction? 

Overall, acoustic testing is carried out for one of two simple reasons – prevention or reaction. Prevention by carrying out acoustic tests on a product before it is installed, to qualify for a grant scheme, comply with the regulations or develop a quieter product. Reaction to noise complaints once the product has been installed and then requiring noise mitigation solutions.

When reacting to a noise issue it is often difficult to carry out the solution that will best solve the noise issue within other constraints. Preventing noise issues through initial testing and product development enables an understanding about the product and a chance to effectively deal with any noise issues early on. It also allows the right product to be specified for the right location and
Rebecca Hogg, Acoustic Consultant, BSRIA Ltd
installations to be carried out successfully.

Rebecca Hogg is a Senior Acoustician at BSRIA.  For more information on acoustic testing at BSRIA email test@bsria.co.uk or phone +44 (0)1344 465578.

This article was first published in Climate Control Middle East.

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