Encrypted acoustical communication system using time reversal through a tunable sonic crystal

In this work, we propose a system for encrypted acoustical communication within the audible frequency range. The system relies on a physical key determined by the configuration of a tunable sonic crystal (SC) and utilizes the one-channel time-reversal (TR) technique to spatially encode each byte of the message at one of 256 possible pulse reconstruction sites. The physical encoding-decoding system consists of a rectangular array of 100 asymmetrical columns that can be controlled to rotate around its axis (SC), placed between a sound source and a grid of recording sites. During the message encoding phase, each byte is encoded as the temporally reversed impulse response recorded at a specific grid location, using a particular orientation of the columns of the SC. During the decoding phase, the signal is transmitted through the same system. Only when the configuration of the SC is the same as the one used to encode the message, each reversed impulse response generates a TR focusing peak at the site corresponding to the encoded byte, and the original message can be reconstructed with almost 100% accuracy using transmission rates up to 2.7 kilo bits/s.