Creating a Converging Lens for Sound Waves at the Exploratorium in San Francisco

[Pranav Jha]

How can converging lens for sound waves be made?
I got confused because i don't which materials should be used to make such lens. The lens if kept in air cannot be made of a solid material as sound travels faster in solid than in air and thus sound would not be converged. I have read that there is such a lens at San Francisco's
Exploratorium

 

[K^2]

If you use material where sound travels faster than in the air, it will just swap the functions of concave and convex lens that you are used to in optics. In other words, a concave lens will focus sound.

You probably don't want to make the material with too high a speed of sound, though, because this will result in most of the sound being reflected.