Sound in air
Sound pressure standard is based on artefacts, namely reciprocity calibrated laboratory standard microphones, that have reached the limits of their capability. Moreover the standard has no direct link to SI fundamental units. This is the reason behind the development of a new standard based on the measurement of speed of air particles in a free field (anechoic chamber).
Air particle speed is measured form the autocorrelation function of the signal produced on a single photon detector by photons scattered in a small area in which two laser beams interfere. As the free space acoustical impedance is known from the properties of air (density, speed of sound), sound pressure can be obtained.
The anechoic room is small, designed for this experiment and may also be used for standard microphone calibration by free field reciprocity technique.
A new Sound Power Primary Standard is under development. Started within EMRP SoundPwr project, the realisation and development of sound power primary standard aims at redefining traceability of sound power quantity to SI fundamental units. Improvements are especially important in the reduction of the secondary emission by the mounting flange around the rigid piston generating the sound field, that may be computed on the basis of the piston velocity and is therefore traceable to m and s.