Investigation on the dissemination of unit watt in airborne sound and applications

Sound power is a widely applied quantity for the characterization of sound sources. Its determination is based on measurements of sound field quantities. Despite the state-of-the art measurement techniques, the sound power determination has some disadvantages. Most significant is the low frequency limitation, with different measurement methods leading to different results, which are expressed in broad frequency bands. A new method is proposed towards the establishment of traceability in airborne sound power. The realization of a primary source enables the free field sound power to be determined. The study investigates the dissemination process by which the sound power of a device under test can be referred to its free field sound power. In addition, the related uncertainty can be explicitly determined. The method of choice is the substitution method, which has been investigated both theoretically and experimentally. Apart from the well-established sound pressure measurements, the implementation of the substitution method also includes sound intensity measurements. The theoretical investigation focuses on the different positioning of the sources used in the substitution method, the substitution of sources of different radiation order and the existence of an impedance boundary. The sound power of aerodynamic reference sound sources has been examined since this type of source has been chosen to be the required transfer standard. For the measurements a specially designed scanning apparatus has been used. Sound power determination in calibration conditions and in situ has been performed. The required correction has been derived and successfully compared to an existing one. Sound pressure and sound intensity measurements at realistic environments have taken place and their sound power has been determined by applying the dissemination process. The sound power determination includes both narrow and broadband analysis along with a transparent uncertainty budget for the spectrum from 20 Hz to 10 kHz