  In recent years, metamaterials have gained considerable attention as a
promising material technology due to their unique properties and customizable
design, distinguishing them from traditional materials. This article delves
into the value of acoustic metamaterials in room acoustics, particularly in
small room acoustics that poses specific challenges due to their significant
cavity resonant nature. Small rooms usually exhibit an inhomogeneous frequency
response spectrum, requiring higher wall absorption with specific spectrum to
achieve a uniform acoustic environment, i.e., a constant reverberation time
over a wide audible frequency band. To tackle this issue, we developed a design
that simultaneously incorporates numerous subwavelength acoustic resonators at
different frequencies to achieve customized broadband absorption for the walls
of a specific example room. The on-site experimental measurements agree well
with the numerical predictions, attesting to the robustness of the design and
method. The proposed method of reverse-engineering metamaterials by targeting
specific acoustic requirements has broad applicability and unique advantages in
small confined spaces with high acoustic requirements, such as recording
studios, listening rooms, and car cabins.
