Principles of Noise Mitigation

A noise problem starts with a noise source such as a stream of traffic on a highway. The noise is transmitted through a path and then arrives at the receiver. The noise will be perceived as a problem when the noise is so high as to be a nuisance to the receiver.

The severity of the problem depends on the strength of the noise source (such as heavy or light traffic) or the length of the path, that is, how large is the separation between the noise source and the receiver.

To reduce environmental noise, one can consider the following methods:

1. Control at noise sources
   It is often a primary consideration to reduce noise at its source.
   
   Whenever possible, quieter working methods or technologies should be used.
   
   In Hong Kong, noise from motor vehicles including motorcycles is under control and should meet internationally recognized noise emission standards (For details, please click here) for first registration in Hong Kong. Moreover, road surfaces can be laid with noise absorptive materials to reduce road/tyre interaction noise.
   
   Operation of some noisy products is also under controls. Hand-held percussive breakers and air compressors are two examples. They have to comply with internationally recognized noise standards (For details, please click here) and be attached with "green labels" during operation.
   
   Certain new and quieter technologies have enabled some construction works to be done much quieter as compared with conventional noisy equipment. For instance, some building demolition projects have adopted the more environmentally-friendly hydraulic crusher instead of the conventional mounted breaker. In some projects involving installation of underground utilities, pipe jacking is used instead of the conventional open-cut method.

Enclosing the noise source can also be used to serve both acoustic and other purposes. A noise enclosure for reducing machine noise is commonly made of an exterior metal skin, an interior perforated sheet, with some absorptive materials such as fiberglass filled in between.

2. Noise reduction at the transmission path
   An obvious way of reducing noise is to separate the sources of noise from noise sensitive uses. This is however often not practical in a compact and high-rise city to rely only on distance attenuation to cut down the noise such as in the case of tackling road traffic noise. Additional attenuation, which can be provided through screening by natural landscape (such as earth bunds), structures of noise tolerant uses (such as carpark, commercial blocks or acoustic-insulated office buildings), purposely built podium decking, noise barriers or enclosures are often employed. Proper land use planning to avoid busy highways cutting across residential developments or coming too close to sensitive uses; locating noise tolerant uses to screen noise sensitive developments, and a combination of the different noise attenuation means can often pre-empt noise problems at the design stage. Options to avoid or minimize noise, say, through adopting alternative transport such as railway, pedestrian link, cycling path, underground roads can also be considered at the early planning stage.
   
   

3. Protection at the receiver end
   By arranging noise sensitive uses such as bedrooms facing away from the noise sources, the impact of noise on the receiver can be reduced.
   
   While acoustic insulation by good glazing can cut down noise, its application for residential buildings practically deprives the receiver of an "open-window" life style and requires the provision of air-conditioning due to the warm and humid climate in Hong Kong. As such, it is often used as last resort only.