  Sound absorption or dissipation principally involves joint interactions
between sound waves, material morphology and the air medium. How these elements
work most efficiently for sound absorption remains elusive to date. In this
paper, we suggest a fundamental relation concisely cross-linking the three
elements, which reveals that optimal sound absorption efficiency occurs when
the pore size of the material is twice the thickness of the viscous boundary
layer of the acoustic air medium. The study is validated by microlattice
materials comprising of well-controlled regular structures that absorb sound in
a tunable manner. Optimized material morphology in terms of pore size and
porosity is determined to provide a robust guidance for optimizing sound
absorbing materials.
