Ultrasim:Ultrasim is an open source toolbox for Matlab. It is an interactive, menu-oriented simulator for finding the sound fields from transducers, primarily for medical ultrasound, but it can also be used for sonar and even loudspeakers. License GPL.
Field II: Field II is a freeware toolbox for Matlab. It contains a set of functions for computing fields for various ultrasonic transducers using linear acoustics. Only binaries are distributed. License AS-IS.