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When an intensity-modulated laser beam illuminates a column of water, it produces, via thermal expansion, a highly directive sound beam. The pressure field radiated by such a thermoacoustic source is analyzed, first by determining the impulse response of the system. The thermoacoustic pressure is then expressed as a convolution between the impulse response and the time derivative of the laser intensity. The analysis is valid in both the nearfield and the farfield of the source and, in principle, for any spatial or temporal distribution of the laser intensity. This approach provides valuable insight into the nearfield of this type of transducer. Resulting theoretical predictions of pressure waveforms and directivity patterns compare well to experimental results, and agree in the farfield limit with existing farfield analyses of thermoacoustic radiation.
Berthelot et al. (Sun,) studied this question.