The AXYS® Asymmetric Boundary Flare (ABF) has been specially developed for use in road traffic tunnels to form part of the voice alarm and announcement systems. The technology involved has been patented. The ABF geometry (essentially, half of a horn) is based on the principle of acoustical mirroring, in effect the mirroring creates the other half of the horn. The ABF is mounted on the ceiling which acts as a waveguide, reducing lateral reflections and obtaining a coherent wave front.