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.
"Rec'd CD's yesterday and I have to let you know how pleased I am with the results! You've managed to take a rather worn, 24 year old cassette that was recorded on a cheap portable cassette recorder, and turn it into 2 highly listenable CD's. This was the only surviving tape of my father-in-law, and as such, meant the world to my wife and I. ..."