Realisation, dissemination and application of the unit watt in airborne sound

Domestic appliances and industrial machinery must meet EU Directives that limit the noise they produce in order to protect the hearing of European citizens. A range of regulations require manufacturers to declare sound power levels from domestic appliances such as vacuum cleaners, or power tools in order to ensure public safety.

Assessments of sound power, which is measured in watts - the SI derived unit of power - are routinely made using microphones in a specially designed environment where no sound is reflected. This is known as a free field. In the real world, echoes from walls or objects can amplify noise beyond safety limits, and machinery verified in a lab may fail noise tests in use. Sound power calibrations that consider environmental reverberations to ensure better regulatory compliance require investigation to ensure accuracy before any changes to current practice can be made.

This project investigated calibration methods which reflect environmental effects on sound measurements and proposed that a significant change to existing calibration methods should be adopted.

The project:

• Modelled, designed and constructed new precise sound power sources by creating highly controlled vibrations for use in performing sound power calibrations
• Evaluated these power sources to determine measurement uncertainties and developed corrections, using rooms designed to absorb or reflect sound in a predictable way, and suggested ways to characterise sources using only free-fields
• Characterised commercial sound power standards, based on rotating fan blades, by linking sound power levels to the extremely accurate standards used in NMIs, and developed a calibration procedure for this type of standard
• Established that the test environment and the procedures used introduce systematic differences between measurement methods. This was established using everyday sources of sound including vacuum cleaners, compressors, and angle grinders.

This project has increased our understanding of how sound power levels deviate depending on the environment and improved NMI capabilities to measure sound power. As a result, a complete change in measurement philosophy based on free-field characterisation is being proposed. A formal comparison of sound power measurement methods has been arranged as a first step towards developing equivalence between NMI sound power measurements and the introduction of calibration services. The STAndardization, Innovation and Research (STAIR) Working Group is considering the project’s results with a view to proposing the additional measurement research needed to establish a new EU standard on sound insulation measurements at low frequencies where sound insulation is defined as a sound power ratio. Outputs have also been incorporated into the ISO3744 revision to support the determination of measurement uncertainties and to improve traceability for low frequency sounds.