  The recently discovered non-Hermitian skin effect (NHSE) manifests the
breakdown of current classification of topological phases in
energy-nonconservative systems, and necessitates the introduction of
non-Hermitian band topology. So far, all NHSE observations are based on one
type of non-Hermitian band topology, in which the complex energy spectrum winds
along a closed loop. As recently characterized along a synthetic dimension on a
photonic platform, non-Hermitian band topology can exhibit almost arbitrary
windings in momentum space, but their actual phenomena in real physical systems
remain unclear. Here, we report the experimental realization of NHSE in a
one-dimensional (1D) non-reciprocal acoustic crystal. With direct acoustic
measurement, we demonstrate that a twisted winding, whose topology consists of
two oppositely oriented loops in contact rather than a single loop, will
dramatically change the NHSE, following previous predictions of unique features
such as the bipolar localization and the Bloch point for a Bloch-wave-like
extended state. This work reveals previously unnoticed features of NHSE, and
provides the observation of physical phenomena originating from complex
non-Hermitian winding topology.
