If a balloon is filled with C${{O}_{2}}$ gas, then how will if behave as a lens for sound waves ? if it were filled with hydrogen gas, then what happen

Velocity of sound in C${{O}_{2}}$ is less than that in air. Therefore balloon will behave as a convex lens for sound waves. In hydrogen, velocity of sound is greater than that in air. Therefore, balloon filled with hydrogen will behave as a concave lens.