ATL stands for Advanced Transmission Line, a term coined and used by the loudspeaker manufacturer PMC Loudspeakers to describe its proprietary cabinet design.

It is a highly sophisticated version of the cabinet design known as transmission line. The idea is to create a long tunnel or labyrinth inside the speaker cabinet, which is carefully lined with acoustic damping materials.

This tunnel (the transmission line) absorbs all frequencies except the lowest ones.

These low frequencies exit through the cabinet opening in phase with the sound from the main speaker, acting as a second "woofer".

The result, according to PMC, is a much deeper, cleaner, more controlled and distortion-free bass response than would be possible to obtain with a conventional cabinet (sealed or ported) of the same size.