Better Acoustic Coupling - Weird Way

I bought packet of those boards: But, not sure whether bigger boards can give me even better results in acoustic coupling. Some examples are here (it will take a while until it arrive): All in all, found that poor acoustic coupling of the ultrasound transducers due to low 'resistance' of the air can be increased. By means of 'resistance' or impedance, I mean - the hardest the material, the better 'conductivity' of the sound in this material. While in electronic, the more resistance, the less current will flow - it resist motion of the electrons. In acoustic, higher resistance helping higher velocity of propagation, but also better mechanical transfer of the motion. This transducer is intended for big container, where water or other liquid in it has better coupling factor, higher velocity of propagation. My goal is to make it so that it can work in air without fear of damaging due to too high amplitude of the piezo-electric ceramic, and too high SWR (Standing Wave Ratio). In other words - I want to make better transfer of ultrasound from transducer to air.