Increases in amplitude at resonant frequencies that are a function of the dimensions of a room.
"Just want you to know that I picked my order up from the Post Office today, and I'm very pleased with your work. The sound is excellent! I'm also very pleased that you took the initiative to do the IRU on the one record that needed it. The sound is better now than I remember it being when I used to play it as a kid (over 35 years ago)--I can ..."