  In recent years high-resolution 3D printing has enabled a diverse range of
new, low-cost, methods for ultrasonic wave-front shaping. Acoustic holograms,
particularly, allow for the generation of arbitrary, diffraction limited,
acoustic fields at MHz frequencies from single element transducers. These are
phase plates that function as direct acoustic analogues to thin optical
holograms. In this work it is shown that, by using multiple polymer 3D
printing, acoustic analogues to 'thick' or volume optical holograms can also be
generated. First, an analytic approach for designing a volume hologram that
diffracts a set of input fields onto a desired set of output fields is briefly
summarised. Next, a greedy optimisation approach based on random downhill
binary search able to account for the constraints imposed by the chosen
fabrication method is introduced. Finally, an experimental test-case designed
to diffract the field generated by a 2.54 cm, planar, PZT transducer onto 8
distinct patterns dependent on the direction of the incident field is used to
validate the approach and the design method. Field scans of the 8 target fields
demonstrate that acoustic analogues of optical volume holograms can be
generated using multi-polymer printing and that these allow the multiplexing of
distinct fields onto different incident field directions.
