  Circular dichroism (CD) is an intriguing chiroptical phenomenon associated
with the interaction of chiral structures with circularly polarized lights.
Although the CD effect has been extensively studied in optics, it has not yet
been demonstrated in acoustic systems. Here, we demonstrate the acoustic CD
effect in a three-dimensional chiral metamaterial supporting circularly
polarized transverse sound. We find that the effect is negligible in the lossy
metamaterial possessing $C_4$ rotational symmetry but can be strongly enhanced
in the $C_2$-symmetric system with inhomogeneous loss. The phenomena can be
understood based on the properties of the metamaterial's complex band structure
and the quality factors of its eigenmodes. We show that the enhanced CD in the
$C_2$-symmetric system is attributed to the polarization bandgaps and the
non-Hermitian exceptional points appearing near the Brillouin-zone center and
boundaries. The results contribute to the understanding of chiral sound-matter
interactions and can find applications in acoustic sensing of chiral structures
and sound manipulations based on its vector properties.
