A low frequency folded horn enclosure intended for use in proximity with at least one planar surface with access to the horn throat from the top of the enclosure. The horn is bifurcated at the throat and folds horizontally around a central trapezoid-shaped columnar back chamber which includes a phase-inverting means. The throat channel expands vertically to the single fold and expands vertically and horizontally to the horn mouth, maximizing back chamber volume within the constraints imposed by footprint size, frequency response, and driver characteristics.