The term 'SOUND CANYON' is a combination of two words, 'SOUND' and 'CANYON'. The word 'ECHO' is a four-letter word that is related to the term 'SOUND CANYON'. An ECHO is a SOUND wave that is reflected off of a surface and returns back to the original source. This phenomenon is often seen in canyons, where the walls of the CANYON reflect the SOUND waves and create an echoing effect. Therefore, the term 'SOUND CANYON' is used to describe a CANYON that has an echoing effect due to the SOUND waves reflecting off of its walls.