Soft silicone rubber in phononic structures: Correct elastic moduli

We report on a combination of experiments to determine the elastic moduli of a soft poly ( dimethylsiloxane) rubber that was utilized in a smart experiment on resonant phononic modes [Liu et al., Science 289, 1734 ( 2000)] and whose reported moduli became widely used as a model system in theoretical calculations of phononic materials. We found that the most peculiar hallmark of these values, an extremely low longitudinal sound velocity, is not supported by our experiments. Anyhow, performing theoretical band structure calculations, we can reproduce the surprising experimental findings of Liu et al. even utilizing the correct mechanical parameters. Thus, the physical conclusions derived in the theoretical works do not require the use of an extremely low longitudinal velocity, but can be reproduced assuming only a low value of the shear modulus, in agreement with our experiments.