UNCERTAINTY CONTRIBUTIONS IN THE OPTICAL MEASUREMENT OF FREE-FIELD PROPAGATING SOUND WAVES IN AIR AND WATER

The calibration method for measurement microphones and hydrophones is based on the reciprocity principle that provides the device sensitivity as a function of the acoustic frequency. Though this method is considered an absolute primary calibration standard, it can only be used on specific devices and cannot be modified to accommodate alternative or new emerging technologies. Moreover, it does not measure directly any acoustical parameters and therefore cannot be used for the direct realisation of the acoustic pascal. In recent years, considerable work and effort has focused on the provision of new primary standards based on photon correlation spectroscopy for airborne acoustics and heterodyne interferometry for underwater acoustics. These techniques can measure directly particle velocities due to free-field propagating sound and can thus calculate the acoustic pressure in an absolute manner providing simultaneously the sensitivity of a device under test and a realisation of the acoustic pascal. In order to show the full potential as new calibration methods, it is necessary not only to show good agreement with the existing calibration method, but also to provide detailed uncertainty budgets. This paper discusses uncertainty components and their contributions both for realisation of the acoustic pascal and the calibration of an acoustic sensor.