The causes of noise are many and varied: kitchen extract systems in commercial premises, heat pumps located in residential areas, pumps in plant rooms, and air conditioning units in office buildings. In fact, 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 may produce a noise with a distinct component, such as a high pitched whine or periodic hum. Noise with a distinct tone has been found to be more annoying to the listener (a penalty is given to such noise sources when noise impact assessments are carried out).
Some systems contain a single source of noise, while others contain multiple noise sources. For example, in an air-source heat pump the noise sources are a fan, a compressor and a pump.
Noise can be measured in different ways. Product manufacturers can carry out a sound characteristic test on their products and declare the noise level of the product in order to satisfy the client's criteria.
There are many international, European, and British Standards that outline how product noise can be measured. BS EN 12102:2008, for example, covers HVAC systems. It outlines the methodology for determining the sound power level of products used for space heating and cooling. This Standard highlights the importance of measuring noise level at controlled conditions including air temperatures, water temperatures, flow rates, and relative humidity.
In recent years there has been greater awareness of the importance of noise. This is reflected in various product certification schemes. One such scheme is the Microgeneration Certification Scheme, which states that heat pumps must be tested for thermal performance and noise. The scheme details the thermal conditions under which the sound characteristic tests must be performed.
The relationship between thermal conditions and noise is very important. A product like a heat pump will operate differently at different temperatures, and consequently the noise produced by the heat pump will also change.
Noise produced by a product will be transmitted through the air. This is termed airborne noise. Vibration transmitted through structures, and which may be radiated as noise by connected components, is called structure-borne noise.
Figure 1 shows a building with a plant room in the basement and offices above. The red box shows an air conditioning unit in the plant room. This produces noise which propagates through the room, as shown by the blue arrows emanating away from the air conditioning unit. This noise could cause disturbance to people working in the plant room.
Airborne noise can be transmitted through a ceiling, causing disturbance to the occupants of the offices above. The air-conditioning unit may be supplying air to the offices and therefore airborne noise could propagate through the ductwork and into the offices.
Noise transmitted through duct systems is a common reason for crosstalk between meeting rooms, where the conversation from one room can be heard in the other. The air-conditioning unit can also cause structure-borne noise, as shown by the green arrows.
How can noise be controlled?
Noise from HVAC can be controlled by either minimising the noise at source or minimising the noise transmitted. Reducing noise transmission can be achieved by placing silencers in ductwork, by using acoustic louvres and anti-vibration mounts, and by examining the building construction to identify relationships between sources of noise and occupied zones.
Minimising noise at source can be achieved through product development. For example, 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 characteristics of noise can change with the operation of a product. If a product is going to be operating at a particular environmental conditions (such as thermal or airflow conditions), then it is essential that the product is developed under these conditions.
Acoustic product development is very beneficial, as minimising the noise produced will reduce the amount of noise mitigation that might be needed after the product is installed.
Rebecca Hogg MEng is an acoustics consultant at BSRIA. For more information contact email@example.com. BSRIA provides Acoustic Testing of Products and Sound Insulation Testing for Part E of the Building Regulations.