The term "waveguide" is used to describe horns with low acoustic loading, such as conic, quadratic, oblate spheroidal or elliptic cylindrical horns.
Oblate spheroid waveguide (OSWG) horn designs attempt to improve directivity pattern control, provide a lower frequency of directivity (the lower the better to match woofers and mid-woofers' directivity), and--as claimed by the inventor--they mitigate higher order modes (HOM), a form of linear phase/amplitude-distortion.