Ultrasonic Parametric Audio System: Invisible Airborne Speaker
A parametric ultrasonic array that beams audio which becomes audible only at the target location — directional sound with no headphones and no leakage.
Abstract
Conventional loudspeakers radiate sound omnidirectionally, leading to noise pollution and a lack of audio privacy in public environments. This project presents an Invisible Air Speaker based on a parametric ultrasonic array, in which audible sound is transmitted through amplitude modulation of a 40 kHz ultrasonic carrier generated and controlled by an ESP32 microcontroller. The modulated signal is amplified through a gate-driver-based MOSFET power stage and radiated by a 4×4 array of Murata MA40S4S ultrasonic transducers.
Due to the nonlinear acoustic properties of air, the ultrasonic beam self-demodulates into audible sound only at the target location, enabling highly directional audio delivery — visitors hear the audio only at a marked position. According to guidelines from the World Health Organization, Health Canada, and IEEE, airborne ultrasonic exposure at the operating frequency and sound pressure levels used in this project (80–95 dB SPL at 40 kHz, well below the 120 dB safety threshold) is considered safe for humans and animals under controlled conditions.
Where typical commercial directional speakers cost ₹50,000–70,000, the complete prototype comes in under ₹10,000, with a modular array design that scales from a single 4×4 unit to larger N×N configurations. The system offers an innovative, compact, and energy-efficient solution for museums and exhibitions, smart advertising kiosks, silent zones such as libraries, and immersive AR/VR audio — demonstrating the feasibility of safe directional sound technology for academic and practical use.