An increase in sound intensity as reflected waves and direct waves combine in phase. May also be due to the natural vibration of air columns or solid bodies at a particular audio frequency.
"I just wanted you to know the CD arrived safe and sound yesterday. This recording was made at a local radio station in Greenwood, SC, in the early 1950's. My father sang with a gospel quartet that had its own radio show at the time. This is the only known recording we have of my dad singing and I am thrill to have been able to preserve it for ..."