The invention provides a high-frequency acoustic waveguide for use in coaxial loudspeaker systems. The waveguide is made up of a plurality of walls that define a conduit with an input end and an output end. Each of the walls includes a mask layer and a perforation layer. The mask layer has a plurality of holes sized and shaped to make the mask layer acoustically transparent to sound waves below a crossover frequency. The perforation layer has a plurality of micro-perforations sized and shaped to make the perforation layer acoustically opaque to sound waves above the crossover frequency. The waveguide directs sound waves above the crossover frequency, and is acoustically transparent to sound waves below the crossover frequency.