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Fluorescent materials convert some visible or invisible electromagnetic radiations from a certain range of frequencies to another one, usually from higher to lower frequencies, rarely, the other way around ("anti-Stokes"). Is there any equivalent mechanism in acoustics? Are there any structures that are able to vibrate at a lower frequency when stimulated by sound, like a mountain that would make an echo that sounds lower than your own voice?

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There are nonlinear systems that present a sub-harmonic response. That is, a harmonic with a frequency lower than the excitation frequency. For example, a spring with a cubic nonlinearity can present this type of behavior.

There are also systems that present multi-stability, where you have different stable configurations with different values of energy (see reference 1). That is, we have an energy landscape with several local minima. So, I think that you combine both of these things to get a phenomenon similar to fluorescence if you have a structure with some local minima with higher values than others.

References

  1. Shan, S., Kang, S. H., Raney, J. R., Wang, P., Fang, L., Candido, F., ... & Bertoldi, K. (2015). Multistable architected materials for trapping elastic strain energy. Advanced Materials, 27(29), 4296-4301.
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