Acoustic testing - What is actually measured?

Rebecca Hogg, Acoustic Consultant at BSRIA, explains the mystery behind the measurement.

In our experience one of the most common misunderstandings when declaring or interpreting sound levels is the difference between sound pressure level and sound power level.

Very simply, sound pressure levels are dependent on the specific acoustic environment, and sound power levels are independent of the acoustic environment. The aim of this article is to explain what is actually being measured during acoustic testing.

What is sound?

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. In order to quantify the heat emitted by the heater temperature measurements may be taken around a room. The measured temperatures vary around the room and are dependent on the thermal characteristics of the room, much like sound pressure levels being dependent on the acoustic characteristics of a room.

What is being measured?

A sound source, such as a heat pump, located in a room emits sound power, expressed in Watts. The sound power emitted radiates 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 and therefore the pressure fluctuations are measured using a sound level meter. The sound pressure levels will vary around the room, due to the specific acoustic characteristics of the room. The sound pressure level measured at different points will differ, even if the sound power level emitted by the heat pump remains constant.

Sound pressure levels v. sound power levels

A sound pressure level of 0 dB is the threshold of human hearing and a sound pressure level of 120 dB is the threshold of pain. The human ear is frequency dependant, being more or less sensitive at certain frequencies, so a weighting system is used where the A-weighted sound pressure level is an approximation as to how the human ear perceives sound, expressed in dB(A).

Table 1 gives typical sound pressures and sound pressure levels for a range of sound sources.

The measured sound pressure level and the acoustic characteristics of the room where the measurements were carried out are used to determine the sound power level.

Table 2 gives typical sound powers and sound power levels for a range of sound sources.

Acoustic testing at BSRIA

In recent years there has been greater awareness on the importance of noise impact assessments and this is reflected in various product certification schemes currently in operation.

The recently completed reverberation chamber at BSRIA provides an acoustic testing facility capable of determining sound power levels for a range of products, including air conditioning units and heat pumps. The testing facility has the capacity to provide a specific thermal environment enabling acoustic testing to be carried out over a series of parameters.

Sound power levels are provided to manufacturers/consultants, enabling a noise impact assessment to be carried out for a real-life situation. Sound power levels are therefore a useful and practical quantity and as such robust acoustic testing is very important.

Find out more

BSRIA provides a range of acoustic testing and consultancy services.

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