The term 'hollow sound' can be described by two four-letter words: 'BONG' and 'PONG'. A BONG is a type of percussion instrument that produces a deep, resonant sound when struck. It is often used in religious ceremonies or rituals. PONG is a game that involves bouncing a ball off a wall or other surface. The sound it produces is similar to that of a BONG. Both of these terms describe a hollow sound, which is a sound that is empty and reverberates.