Due to the self-demodulation property of finite amplitude ultrasonic waves propagating in air, a highly focused sound beam can be reproduced. This phenomenon is known as the parametric array in air. However, because of the nonlinearity effect of ultrasonic waves, the reproduced sound wave suffers from high distortion. Several amplitude modulation techniques have been previously proposed to mitigate this problem. Currently, no comprehensive study has been carried out to evaluate the performance of these amplitude modulation techniques. This paper attempts to provide theoretical and experimental studies of the effectiveness in mitigating the distortion from different amplitude modulation techniques. Objective measurements, such as sound pressure level and total harmonic distortion are used to compare the performance of these modulation techniques.
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