  Efficient sound transmission across the water-air interface has always been
expected in the field of ocean exploration. However, the existing researches
are mainly concentrated on the narrow-band transmission based on resonance,
which greatly limits the transmission capacity and efficiency. Here, we
combined the air-based and water-based metafluids to realize an exponential
gradient impedance matching layer for broadband water-air sound transmission.
By cooperatively adjusting the sound velocity and thickness in the matching
layers, we modulated the required acoustic parameters of each layer into a
reasonable range, which can be conveniently achieved by the proposed
metafluids. A matching layer sample was constructed and validated in a water
tank. Experimental results show that the proposed matching layer can achieve an
average sound energy transmission enhancement above 16.7dB from 880Hz to 1760Hz
across the water-air interface. Moreover, we use the proposed matching layer to
demonstrate a multicolor picture transmission from air to water, which shows
extremely high communication capacity and accuracy. Our work is promising for
more applications based on water-air transmission and opens a new avenue to the
design and implementation of the extreme impedance matching case.
