  The recently-emerged concept of "invisible gateway" with the extraordinary
capability to block the waves but allow the passage of other entities has
attracted great attentions due to the general interests in illusion devices.
However, the possibility to realize such a fascinating phenomenon for acoustic
waves has not yet been explored, which should be of paramount significance for
acoustical applications but would necessarily involve experimental difficulty.
Here we design and experimentally demonstrate an acoustic invisible gateway
(AIG) capable of concealing a channel under the detection of sound. Instead of
"restoring" a whole block of background medium by using transformation
acoustics that inevitably requires complementary or restoring media with
extreme parameters, we propose an inherently distinct methodology that only
aims at engineering the surface impedance at the "gate" to mimic a rigid "wall"
and can be conveniently implemented by decorating meta-structures behind the
channel. Such a simple yet effective design of AIG also enables analyzing the
physics behind this extraordinary phenomenon in an analytical manner, which
agrees quite well with the numerical and experimental results. Furthermore, our
scheme offers possibility to manipulate the symmetry of AIGs freely, which is
proven both theoretically and experimentally by demonstrating two distinctive
examples of one-way and two-way AIGs. We anticipate the realization of AIGs
will open new avenues to illusion acoustics with potential applications in
acoustic measurement and acoustic device, and provide inspirations for similar
researches in other fields.
