  Topological features embedded in ancient braiding and knotting arts endow
significant impacts on our daily life and even cutting-edge science. Recently,
fast growing efforts are invested to the braiding topology of complex Bloch
bands in non-Hermitian systems. This new classification of band topology goes
far beyond those established in Hermitian counterparts. Here, we present the
first acoustic realization of the topological non-Hermitian Bloch braids, based
on a two-band model easily accessible for realizing any desired knot structure.
The non-Hermitian bands are synthesized by a simple binary cavity-tube system,
where the long-range, complex-valued, and momentum-resolved couplings are
accomplished by a well-controlled unidirectional coupler. In addition to
directly visualizing various two-band braiding patterns, we unambiguously
observe the highly-elusive topological phase transitions between them. Not only
do our results provide a direct demonstration for the non-Hermitian band
topology, but also the experimental techniques open new avenues for designing
unconventional acoustic metamaterials.
