Researchers at POSTECH built MiPAL, a parametric speaker with one emitter and 2 metamaterial layers. A front acoustic metasurface focuses ultrasound, while elastic metacells suppress vibrations that would spread audio sideways.

MiPAL modulates an ultrasonic wave with an audio signal, using nonlinear behavior in air to send sound along a narrow path. Tests in a mock aircraft cabin covered 500 Hz to 10 kHz, while the adjacent seat stayed quiet.

The modules are 3D-printed, so the design can be adapted to seat spacing, beam angle, or other conditions. Possible uses include personal announcements on planes and audio guides at museum displays.