A parametric array loudspeaker (PAL) can achieve sharp directivity by using ultrasonic waves. When an intense amplitude-modulated wave that has a different frequency corresponding to the target signal is emitted from a PAL, the target signal is demodulated on the propagation axis of the amplitude-modulated wave due to the nonlinearity of the air. However, the non-target listeners on this axis can hear the demodulated target signal. To address this issue, we propose an audible area control method using an intermodulation-based audible masker generated by two PALs. One PAL emits an amplitude-modulated wave, while the other emits a band-limited white-noise-based carrier wave. While the target signal is demodulated on the propagation axis of the amplitude modulated wave, an intermodulation-based audible masker is generated by the interaction of the amplitude-modulated wave and the band-limited white-noise-based carrier wave where the two ultrasonic beams overlap. This allows the target signal to be masked for non-target listeners on that axis. The effectiveness of the proposed method is confirmed through objective experiments on the sound pressure level of the masker and subjective experiments on speech intelligibility.
Moriyama et al. (2025) studied this question.