The airborne watt: New measurements of sound power
Sound power is the rate at which sound energy from a source or object is emitted per unit time, and is measured in watts (W), the SI unit of power. Until now, the sound power was considered to be independent of the environment and of the distance from the source, and could therefore be used to describe the acoustic output of both domestic and industrial technical products. But in practice, sound power can only be determined from other measurements such as sound pressure at a certain position, which is measured in pascals (Pa). There are many different ways to determine the sound power of a particular source, but results can vary widely. Even worse, uncertainties can only be compared if the measurement procedures were the same, and not for an individual measurement result. This makes it difficult to compare the measured sound power levels for regulatory compliance with noise protection legislation. Within this project, primary sound power sources and measurement techniques were developed to reduce measurement uncertainties. Measurements for low frequency sounds were also carried out, for which there is little guidance at present. It was found that the assumption that the sound power is independent of surroundings is not the case for low frequencies, which is a completely new concept for airborne sound. Improvements to sound power measurements will be beneficial in both industrial and domestic environments.