A maker built a directional speaker using a 40 kHz ultrasonic array that projects a narrow beam of sound toward a specific target. The device works by exploiting the short wavelength of ultrasonic frequencies and using constructive/destructive interference from parallel emitters. When amplitude modulation is applied to the ultrasonic signal, the air itself demodulates it into audible sound due to nonlinear propagation effects. The circuit uses a 555 timer for the ultrasonic driver, and the beam can be reflected off hard surfaces, though indoor reflections reduce directivity.
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