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  Acoustic Modem

In the underwater domain, acoustic waves are the best method of transmitting information wirelessly. This communication, however, is prone to errors from changing water conditions, environmental noise, echos, difficult terrain, and a host of other factors.

As the industry moves towards large fleets of underwater vehicles, a significant challenge is how to efficiently communicate within the fleet while remaining robust to communication errors.

The acoustic modem developed here is designed to equip a large network of vehicles for research into robust communication techniques. This device sacrifices speed and long-range performance for cost; it can be built for less than a tenth of the cost of traditional acoustic modems.

The underlying technology utilizes piezoelectric elements which are powered with an electric field of varying frequency. The modulation technique (FSK) uses one frequency to represent a digital high signal and another frequency to represent a digital low. The electric field in the piezo device is transformed into a mechanical deformation, which produces pressure waves which propogate through the water. On reception, the reverse process occurs; the pressure wave causes a deformation in the piezo element, which converts it into an analog electric signal that can be decoded into the corresponding digital signal.

Acoustic Modem Operation
Modem Assembly
Acoustic Communication: Theory of Operation
FSK Modulation
Frequency Shift Keying (FSK) Modulation
Circuit Board Assembly
Modem Assembly
  † Image from Lurton, X., An Introduction to Underwater Acoustics: Principles and Applications. Springer-Praxis, New York, 2002.