Acoustic impedance is a material property that depends on mass density and acoustic wave speed. An impedance
mismatch between two media leads to the partial reflection of an acoustic wave sent from one medium to another. Active
sonar is one example of a useful application of this phenomenon, where reflected and scattered acoustic waves enable
the detection of objects. If the impedance of an object is matched to that of the surrounding medium, however, the object
may be hidden from observation (at least directly) by sonar. In this study, polyurea composites are developed to facilitate
such impedance matching. Polyurea is used due to its excellent blast-mitigating properties, easy casting, corrosion
protection, abrasion resistance, and various uses in current military technology. Since pure polyurea has impedance
higher than that of water (the current medium of interest), low mass density phenolic microballoon particles are added to
create composite materials with reduced effective impedances. The volume fraction of particles is varied to study the
effect of filler quantity on the acoustic impedance of the resulting composite. The composites are experimentally
characterized via ultrasonic measurements. Computational models based on the method of dilute-randomly-distributed
inclusions are developed and compared with the experimental results. These experiments and models will facilitate the
design of new elastomeric composites with desirable acoustic impedances.
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