A waveguide configured to function as a high frequency waveguide for a high frequency sound source mounted to the waveguide, and to function as an acoustical low pass filter for a low frequency sound source mounted behind the waveguide in a multi-way loudspeaker system. The waveguide includes openings that overlay the vibrating surface of the low frequency source. The openings are configured by selection of various geometrical parameters to tailor filtering characteristics as desired.