Mechanical equivalent of an electrical transmission line. Baffles, labyrinths, or resonators are placed at the rear of a speaker enclosure to assist in reproduction of very low audio frequencies.
"Thank you for the nice CDs; they will be preserved for years in my granddaughter's cedar chest that I just bought for her. Too bad she will never know her granddad but she WILL hear his voice! (Mine also - when I could sing!) Thank you!"