Communication by Ultrasound Using Radio Modulation Techniques

There are many methods to send electronic signals, including by wire, radio, light, infra-red light, and audible sound waves. Ultrasound consists of sound waves that are of frequencies above the human audible range. Certain animal species, such as bats and dolphins, use ultrasound for navigation and localization by echolocation. In recent years, ultrasound has become prevalent in medicine and industry. However, little prior work has been done on communication via ultrasound. Ultrasound offers many benefits. For example, it does not use electromagnetic radio waves and therefore avoids issues with radio interference, crowding, and privacy. Yet, it shares the same ability as radio to communicate around obstacles, which infrared light cannot. Sources of ultrasound noise are less common than those of audio noise in the natural environment and the signal-to-noise ratio can potentially be very good.

This project involves the design, creation and testing of a communication system that can send and receive an information-bearing ultrasound signal, using modulation-demodulation techniques commonly used in radio. The information is converted back to the audible range for interpretation. The circuits utilize standard transistor design with common-emitter voltage amplifiers, common-collector current amplifiers, filtering, a few integrated circuits, audio amplifiers, ultrasound-sensitive microphones, and a tweeter that works in the ultrasound range. This is a study of the feasibility of using ultrasound to send information, including the distance and signal fidelity that can be achieved. Using amplitude modulation with a carrier frequency of 25 kHz, with 100% modulation using voice information, 2 watts peak envelope power and 99 dB SPL peak sound level, the system is able to send a signal a distance of 61 meters outdoors (where sound reflections are minimized). The device is able to send a variety of modulation signals, including tone, voice, music and digital. A digital signal can be sent at 1000 baud rate a distance of 11 meters, and a 1.7 MB photo can be sent digitally in 2 minutes a distance of 6.5 meters across a room. The demodulated signal is surprisingly clear; the sound fidelity is comparable to that of a handheld AM radio.