New measurements techniques: Optical methods for characterizing sound fields

Acoustic measurements are traditionally based on transducers, and in particular, the most advanced measurement techniques are nowadays based on transducer arrays. This poses a fundamental problem, namely the influence of the transducer itself on the actual properties of sound when the transducer is immersed into the sound field. Typically, this influence is assumed to be negligible when the size of the transducer is small compared to the wavelength of the sound wave, or is rendered negligible by using a transducerbased correction that depends on the frequency. Either solution introduces additional uncertainties to the measurement process. Optical techniques may help overcoming this problem because the sensing element is not a bulky instrument, but a beam of light that does not change the properties of sound. Optical methods are thus non-invasive and can thereby enhance the current state of the art in the measurement of sound.
The present PhD study primarily examines the use of the acousto-optic effect, that is, the interaction between sound and light, as a means to characterize acoustic fields. The acousto-optic measuring principle does not provide a direct measure of the pressure, but the integral of the pressure encountered by the ‘sensing’ light when traveling through the acoustic field. Far from being a limitation, this integral principle is exploited for sound field visualization using tomography. The most innovative contribution of this PhD project is the applicability of the acousto-optic measuring principle to acoustic holography and beamforming. On the one hand, a new method called near- field acousto-optic holography (NAOH) has been proposed and makes it possible to predict properties of sound at planes different from the measuring one. In comparison with conventional near-field acoustic holography (NAH), the suggested holographic method features novel spectral properties in the wavenumber domain. On the other hand, an acousto-optic beamformer has been designed and validated experimentally for the localization of sound sources located in the far field. In this case, a laser beam is interpreted as a line array of microphones with infinite resolution, which makes the proposed acousto-optic beamformer immune to spatial aliasing.
In addition, the present PhD study investigates the applicability of photon correlation spectroscopy as a primary method for microphone calibration under free-field conditions. Various signal processing refinements are proposed to improve the accuracy of this measurement technique.