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.
Acoustical shells were developed to reflect sound outward, focused in one direction as opposed to the ″sound in the round″ diffused from all sides of the open gazebo bandstand.
The Hollywood Bowl shell, based on a 1928 prototype by Lloyd Wright, has been rebuilt several times with the present structure being the fifth on that site.
Rectangular pavilions with enclosed shell and stage, similar to one in Boise, Idaho (pictured), were being built after 1900.