Implementation of a Sound Measurement System Based on an Optical Method for the Primary Free-field Microphone Calibration and the Realisation of the Acoustic Pascal

The current international primary standard of sound-in-air is based on the reciprocity method which provides the sensitivity of laboratory and working standard microphones. The method depends on specific artifacts and in addition does not measure the acoustic pressure directly. For these reasons, a new primary standard is required so that this particular area of acoustical metrology can move from the traditional classical era into a new one, where traceability is derived through fundamental constants and does not depend on artifacts. As a method to replace the current primary standard of sound-in-air, the technique based on optical measurement was proposed and its feasibility has been investigated. In this study, the system to measure the acoustical particle velocity is designed and implemented for free-field sound conditions where it is possible to calculate the sound pressure through particle velocity measurements and the acoustic characteristic impedance. For this reason, a system based on photon correlation is implemented outside an anechoic chamber, targeting a distant measurement point within a propagating sound field more than 1.5 m inside the chamber, where free-field conditions are guaranteed. The design of the optical system has taken into account the necessity to cover the audible frequency range up to 20 kHz, while considering the minimum sound pressure level required at each frequency, in addition to characteristics such as the crossing angle of the laser beams and the size of the required seeding particles. The results achieved by the implemented system are compared to those obtained by the reciprocity calibration method established as primary standard in IEC 61094-3.