| Περίληψη: | This work presents novel findings on sound reproduction from laser-generated plasma acoustic sources. It addresses fundamental aspects of the physics behind laser-based optoacoustic transduction as well as engineering aspects for the utilization of the laser-plasma transduction mechanism in the development of audio reproduction systems. It presents a joint experimental, theoretical and signal processing approach for the study and modeling of sound radiation from spherical, omnidirectional but also cylindrical, directional laser-plasma sound sources. Based on experimental findings, a novel phenomenological model is developed that allows for the estimation of the frequency spectra of the plasma-generated sound waves in terms of the thermal light emitted by the heated volume, clearly demonstrating the correlation between secondary light and sound radiation following laser-induced breakdown of gases. The model’s predictions are validated by comparison with acoustic measurements, showing good agreement. Also, an acoustic model based on the classical acoustic line source model is developed that calculates the acoustic radiation of plasma filaments in 3D space in terms of the spatial distribution of the deposited optical energy. It is shown that the plasma geometry and axial pressure distribution can be determined by the spatial profile of the thread’s luminescence. The model’s predictions are validated by comparison with acoustic measurements, showing very god agreement. Moreover, the concepts of a novel laser-sound technology are presented and a laser-based optoacoustic transducer prototype is described and experimentally evaluated. The prototype is capable of generating 1-bit or multibit ΣΔ optical pulse streams, that are transduced into acoustic pulse streams directly in the ambient air or on metal targets. The presented prototype has an effective bandwidth of 1kHz, which is sufficient for the proof-of-concept demonstration of the technology, while it is shown that generalization of the results towards a wider frequency range, such as the audible range, is a matter of laser specifications. Experimental results from the reproduction of bandlimited test signals, such as single or multiple sine waves and sine sweeps, as well as real audio signals, particularly speech and music, are presented. Moreover, a computational model is developed that allows for the simulation of any ΣΔ-base laser-driven sound reproduction system, in terms of the laser and modulation parameters. The model takes as input the original sound signal and transforms it into the desired modulation signal via convolution of its ΣΔ representation with the pressure profile of a single laser-generated acoustic pulse. The model’s predictions are validated by comparison with the experimental measurements, showing good agreement. Finally, ongoing work on a complete physical model of the energy deposition through optical breakdown of gases is outlined and a new method for the prediction of the frequency spectra of uniform-sampling Pulse Width Modulation signals originating from random modulating signals is demonstrated.
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