of dispersion pattern at the higher frequencies whose wavelengths approach the width of the throat or the width of the diffraction slot. Because the CD horn's high frequencies are more spread out over its coverage pattern, they appear attenuated relative to other horns. The CD horn requires an equalization boost of approximately per octave with a filter knee centered between 2 and (depending on horn design) in order to sound neutral and balanced. Most manufacturers of active electronic audio crossovers responded to this requirement by adding an optional CD EQ boost filter or high frequency shelf filter. For instance, such circuitry was provided via internal jumper links by BSS in their FDS-310 crossover and by Rane in their AC 22S and AC 23B crossovers. Rane allowed for greater front panel control of two bandpasses ("hi-mid" and "high") using CD horn equalization including sweepable frequency range on their AC 24 crossover. Further refinements of the filtering process are available in DSP-based crossovers. Hybrid Constant Directivity (HCD) Firstly published in December 2019 in a Voice Coil article and then at the 148th AES Convention in June 2020, Dario Cinanni presented a new horns family. The HCD algorithm, already used by SpeakerLAB Horn.ell.a software from 2006, transforms any expansion (exponential, hyperbolic sine, hyperbolic cosine, catenoidal, tractrix, spherical, or a new expansion) horn into a constant directivity horn. The HCD permits to maintain the same acoustic load of the original expansion. HCD algorithm reduces reflections if compared to a CD horn, or in general to a multiflare horn, providing low distortion at high sound pressure levels. Similar to the Radial horn HCD offers a constant directivity on one plane, to be specific a progressive constant directivity on the plane along the horn mouth major axis. The progression depends on the selected mouth-ratio. While on the plane along the mouth minor axis we will have an equivalent directivity contour of a circular mouth horn (using the same expansion). Multiple entry horn In 1996, Ralph D. Heinz of Renkus-Heinz received a patent for a multiple entry horn which incorporated multiple drivers for two bandpasses, high and mid, whose sound waves all exited into a single horn but at differing distances depending on the bandpass. It was marketed as the "CoEntrant" horn. The mid- and high-frequency drivers in the Renkus-Heinz ST/STX product line both exited through a "Complex Conic" waveguide. In the late 1990s, Thomas J. "Tom"