Vibrant times for Phononic Metamaterials

The boost experienced by acoustic and mechanical (phononic) metamaterial research during the past years has been driven by the ability to sculpture the flow of sound waves at will. Motivated by the desire to engineer artificial structures in the form of metamaterials and the quest to map quantum mechanical phenomena onto classical waves such as sound has led to vast possibilities in material designs for control of wave motion and the potential for engineering applications. Some striking material properties have shown in the past to be able to leave an objective acoustically concealed, hence they are cloaks of “unhearability”. Also, fascinating materials have been designed to be able to bend a ray of sound the “wrong” way when, for example, a loudspeaker turned on irradiates such artificial structure with a negative refraction index. In this talk, I like to review some of the key achievements made in this field and wish to address some of the unanswered questions that might lead to a breakthrough in the future.