  Three-dimensional nodal line semimetals (NLSMs) provide remarkable importance
for both enrich topological physics and wave management. However, NLSMs
realized in acoustic systems are twofold bands degenerate, which are called
Weyl NLSMs. Here, we first report on the experimental observation of novel
Dirac NLSMs with fourfold degenerate in sonic crystals. We reveal that the
topological properties of the Dirac NLSMs are entirely different than that of
the conventional Weyl NLSMs. The Berry phase related to the Dirac nodal line
(DNL) is 2{\pi}, which results in the surface responses of the Dirac NLSMs with
two radically different situations: a torus surface state occupying the entire
surface Brillouin zone (SBZ) and without any surface state in the SBZ. We
further reveal that topological surface arcs caused by DNL can change from open
to closed contours. The findings of Dirac NLSMs and their unique surface
response may provoke exciting frontiers for flexible manipulation of acoustic
surface waves.
