In theater, a shell is a curved, hard surface designed to reflect sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 towards an audience.

Often shells are designed to be removable, either rolling away on wheels or flying into a flyspace. Shells are most commonly used for orchestras, bands and choirs, although they can also be used in any application that requires passive sound amplification. Shells are generally made of hard materials because they are designed to absorb as little sound as possible.