Automatic sound field sampling mechanisms to disseminate the unit watt in airborne sound

For the dissemination of the unit watt, the sound pressures emitted by primary and secondary sound power standards are to be measured. Disseminating the unit in reverberant fields requires the measurement of a volume average whereas in hemianechoic fields, measurements on an enveloping surface are appropriate. For the latter situation, automatic sound field scanning mechanisms were developed by LNE, SP and PTB. At PTP, an arc with 24 microphones is used which can be tilted to cover a hemispherical measurement surface. Different radii can be realised by manual adjustment of the microphones. At SP, a single microphone is moved on a semicircular arc. Different latitudes can be realised by manual arrangements. LNE developed a scanning mechanism consisting of a hemicircular arc. A microphone can be positioned on arbitrary altitude positions on the arc and the arc can move on a circular path. This way, arbitrary positions on a hemispherical measurement surface can be realised. The contribution introduces the different mechanisms and lists advantages and disadvantages of the different technical solutions.