  The underwater acoustic channel is characterized by a path loss that depends
not only on the transmission distance, but also on the signal frequency. As a
consequence, transmission bandwidth depends on the transmission distance, a
feature that distinguishes an underwater acoustic system from a terrestrial
radio system. The exact relationship between power, transmission band, distance
and capacity for the Gaussian noise scenario is a complicated one. This work
provides a closed-form approximate model for 1) power consumption, 2) band-edge
frequency and 3) bandwidth as functions of distance and capacity required for a
data link. This approximate model is obtained by numerical evaluation of
analytical results which takes into account physical models of acoustic
propagation loss and ambient noise. The closed-form approximations may become
useful tools in the design and analysis of underwater acoustic networks.
