Acoustic testing reduces the trials of noise
Noise from build-services equipment can be a constant source of annoyance and can even result in a failed planning application — hence the importance of acoustic testing, as Rebecca Hogg explains.
Noise is an important issue to consider when manufacturing or installing a product. Noise measurements, either in a laboratory or on site, allow a manufacturer to accurately declare the noise level produced by a product.
When installing a product in a real-life situation these declared noise levels can be utilised to carry out a noise-impact assessment to determine if the new product is likely to cause a negative noise impact on people living or working in the proposed vicinity of the product.
So what does ‘noise level’ actually mean, and what is being measured.
In our experience one of the most common misunderstandings when declaring or interpreting noise levels produced by a product is the difference between sound-pressure level and sound-power level.
A basic analogy for explaining sound is heat. An electric heater emits a certain amount of heat, similar to the sound power emitted by a sound source. To quantify the effect of the heater in the room, temperature measurements may be taken around the room. The measured temperatures vary around the room and depend on the thermal characteristics of the room. Similarly, sound-pressure levels depend on the acoustic characteristics of a room.
A sound source, such as an operational heat pump, in a room emits sound power, expressed in watts. The sound power emitted travels away from the source, causing small fluctuations in the air pressure throughout the room, much like ripples across a pond.
Sound power cannot be measured directly, so the pressure fluctuations are measured. The sound-pressure levels will vary around the room, due to the specific acoustic characteristics of the room, even if the sound power level emitted by the heat pump remains constant.
The sound-power levels and sound-pressure levels are stated using the decibel (dB) unit. The decibel can be confusing as it is often assumed to be an absolute unit, such as metres or grams. The decibel is actually a logarithm of a ratio of two different values. Because the absolute values for sound power and pressure cover a huge range, the decibel unit provides a more convenient range of values.
Why is acoustic testing important?
With more awareness of noise issues and many developments occurring in already built-up areas it is essential to provide an accurate specification of the noise produced by a product.
Heat pumps in residential areas, pumps in plant rooms, air-conditioning units in office buildings and kitchen extract systems in commercial premises are just a few examples of situations where noise issues can occur. This can result in failed planning applications or noise nuisance complaints once the products and systems have been installed.
Acoustic testing is an important first stage of assessing noise impact. It is important to manufacturers as it enables them to provide their clients with useful information about the sound-power levels of their products.
This information enables specifiers and consultants to carry out a noise-impact assessment for real-life situations before installing a new product or system. For example noise-impact assessment in accordance with the BS 4142 standard compares noise levels from a product against existing background noise levels. This is particularly important in mixed residential and industrial areas.
Every council has its own planning policies relating to noise, but the overall aim is the same. Councils want to protect the existing or potential inhabitants of an area and will require justification that a new product will not produce noise levels that could cause a negative noise impact. Without an accurate specification of the noise produced by a product, errors can occur, resulting in unrealistic noise assessments, under- or over- specifying noise mitigation, wasted time and greater costs.
Rebecca Hogg is acoustic consultant with BSRIA.
