edit: Steve Mould's video "I Made a Lens, But for Sound" demonstrates how balloons filled with gasses of different density than the surrounding air, act as a lens on sound waves. Helium filled balloons will scatter sound because the helium is less dense than air. He shows how a balloon filled with carbon dioxide can focus the sound.
https://www.youtube.com/watch?v=RLBmWF9Xo10
edit: Steve Mould's video "I Made a Lens, But for Sound" demonstrates how balloons filled with gasses of different density than the surrounding air, act as a lens on sound waves. Helium filled balloons will scatter sound because the helium is less dense than air. He shows how a balloon filled with carbon dioxide can focus the sound.