What is an acoustic metamaterial?

The topic of acoustic metamaterials has been included as a regular special session at the ASA for over 10 years, with hundreds of papers presented. In many of these presentations, the author begins with a definition of ‘acoustic metamaterial.’ While many of these definitions are similar, subtle differences exist in the defining characteristics. So what is a metamaterial? What are the defining features as accepted by the research community? This presentation will review some of the common themes in the definitions and include opportunity for light-hearted interactive audience participation to evaluate those defining themes.