The term 'sound barrier' refers to a physical or psychological barrier that prevents sound from passing through. An earplug is a device worn in the ear to reduce the amount of sound that enters it. A REEF is an underwater formation of rocks, coral, or sand that can act as a barrier to sound waves. Finally, a COAST is a strip of land along the edge of an ocean, sea, or lake, which can also act as a sound barrier. All of these solutions are related to the term 'sound barrier' in that they all provide some form of protection from sound.