Evolution of Engineering Acoustic Waves with Metamaterials

The control of acoustic waves have critical applications across many fields, including ocean geology, [1] marine biology, [2] and biomedical engineering. [3] Controlling the direction and dissipation of acoustics waves plays a critical role in noise cancellation [4,5], ultrasonic imaging, [6] and many other applications. Acoustic metamaterials play a significant role in the development of technology used to control acoustic waves. Acoustic metamaterials have interesting properties that interact with incoming pressure waves in unique ways. In particular, these metamaterials are able to exhibit a negative bulk modulus. [7] Contradictory to our natural intuition (nature materials with positive bulk modulus), a negative bulk modulus refers to an increase in volume from an external compressive force [8]. This negative bulk modulus phenomenon occurs at some particular frequency range near monopolar resonance. When acoustic metamaterials are subjected to an oscillating signal near the resonance frequency too fast for the medium to respond in time, a phase shift can be observed. This phase shift impliesπ that the response signal is the negative of the incoming signal. Due to this phase delay, the volume of the metamaterial unit cells responds in a negative manner in respect to the oscillating pressure field, resulting in negative bulk modulus [8]. Similar effects can be observed for mass density subjected to dipolar resonance in which the delay in response causes the center of mass of each unit cell to decelerate when pushed by the oscillating pressure field. In the frequency range where the monopolar and dipolar frequency ranges overlap, the metamaterial exhibits both negative bulk modulus and negative density. The effects of these two negative material properties result in a negative phase velocity with a negative refractive index. Foundational Research