A hallmark of a well-designed building is that noise from its hvac system does not cause problems for the occupants. When there is excessive noise from outside or from hvac systems it can cause discomfort and unease amongst those working in the building. It may also contribute to difficult communications, especially in open spaces. This can result in reduced productivity, since those who are not comfortable in their surroundings are not fully effective.
The services engineer is expected to take responsibility for control of noise, whether from the mechanical plant or external noise transmitted through the system. Where noise is related to the building design, such as natural ventilation openings, the engineer needs to liaise with the architect and agree suitable locations for air inlets.
For natural ventilation systems air inlets are normally placed where they are remote from sources of air pollution and traffic noise, but they may still be exposed to other sources of noise. In some cases the services engineer may be given responsibility for all services noise in a building, including pumps, lifts and escalators.
Helping hand
Building services designers can now turn to the latest addition of the revised set of CIBSE guides for advice on this issue. The new CIBSE Guide B5 – Sound and vibration control replaces the old B12 and provides comprehensive guidance on the identification and prediction of sources of noise and vibration in ventilation plant. It also details the tools and techniques that are available to control them.
After an introduction to the issues of noise control, the Guide follows the path of a typical mechanical ventilation system, starting with the initial source of noise, usually the fan, and ending in the occupied space. This is illustrated schematically in figure 1. As well as fans, other potential sources of noise are considered, along with measures that can be taken to minimise their impact. There is also a specific section addressing issues of noise control in plant rooms, which considers both noise to atmosphere and health and safety within the plant room.
Source of interest
The Guide contains detailed discussion on the sources of noise performance data for equipment, taking the form of relevant octave band noise data. It explains how this should be obtained under British, European, international or equivalent standards procedures, and how this relates to performance of the specific item in situ, rather than in a lab. This section is essential reading for designers who have to address why specific items are not performing as expected, as it identifies clearly what it is, and is not, reasonable to expect based on quoted data.
The hvac installation should be treated as a complete system, the separate parts of which influence each other as the air passes through. The Guide therefore moves on through the system as the air passes through ducts, bends, fittings and attenuators, which can add to or attenuate the noise.
The influence of these various components and their contribution to the overall room sound levels is discussed in chapters five and six. These consider the noise of airflow in ducts, and the ability of the ducts to transmit noise down the ventilation system. They also address the problem of noise escaping from the duct system, known as breakout noise, and highlights options to control it.
Chapter eight considers specific problems associated with the transfer of noise into and out of the building. There are two angles to consider here: the noise emissions of plant within the building, which may well be subject to regulations or consents, and the introduction of noise into buildings through the natural ventilation elements of the fabric.
An important element of hvac design is controlling noise to meet the specified noise characteristics in the occupied spaces, while having minimum effect on the cost and aerodynamic performance of the fan duct installation. Chapters nine and ten provide a summary of the noise criteria that may be specified. There is also a more extended look at these in the appendix, and a discussion of noise prediction, complete with worked examples.
Energy and noise
A theme running through the Guide is the link between energy and noise, since energy inefficient and turbulent installations tend to be unnecessarily noisy. Generally, the higher the air velocities of the system, the greater the likely noise. Energy conservation measures are therefore likely to deliver quieter systems as a by-product. This is largely beneficial, although there needs to be consideration of whether there is adequate masking noise throughout the spectrum. An analysis of this is given within the discussion of room noise and criteria.
Finally, chapter 11 deals with the specialist requirements for vibration control. The Guide concludes with a summary of the main issues and advice. There are several appendices covering numerous topics, including: acoustic terminology, prediction equations, interpreting manufacturers' data, basic techniques for prediction of room noise, vibration instrumentation, and noise instrumentation.
The whole package provides a thorough treatment of this specialist area and will be extremely useful to hvac engineers.
To complement the publication of the new guidance, CIBSE is holding a one day seminar on 29 January. In addition to topics addressed in the Guide, the seminar will also feature case studies showing the principles of the Guide applied in practice. A copy of the Guide is included in the price for all seminar attendees.
Hywel Davies is CIBSE research manager. The production of the new Guide B5 was supported by the DTI Partners in Innovation Project. The Guide is published with the consent of the DTI, but the views expressed are not necessarily accepted or endorsed by the DTI.
CIBSE Guide B5: Noise and vibration control is available from CIBSE Publication Sales, 222 Balham High Road, London SW12 9BS. Tel: 020 8675 5211, fax: 020 8675 3302, or e-mail pubsales@cibse.org. The price £29 for CIBSE members and £58 for non-members. For information on the CIBSE seminar visit the website at www.cibse.org.uk.
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