  Acoustic metamaterials constructed from conventional base materials can
exhibit exotic phenomena such as negative refractive index, extraordinary
transmission-absorption and sub-wavelength imaging. These are typically
achieved by combining geometrical and resonance effects. Holey-structured
acoustic metamaterials have already shown potential for sub-wavelength acoustic
imaging in air, where it is relatively simple to achieve the required
difference in acoustic impedance needed to create resonances within the holes.
However, the use of polymers for metamaterial operation in water is difficult,
due to the much lower difference in acoustic impedance between water and the
many polymers used, for example, in additive manufacturing. Hence, metals are
commonly used. Here we show that this can be overcome by trapping air layers so
that they surround each water-filled channel, making sub-wavelength imaging in
water possible. The operation of such a Trapped air design is confirmed at
200-300 kHz ultrasonic frequencies via both finite element modelling and
experimental measurements in a water tank. It is also shown quantitatively that
the trapped air design outperforms its metal counterpart. The results indicate
a way forward for exploiting additive-manufacturing for realising acoustic
metamaterials in water at ultrasonic frequencies.
