..
Some old analog crossovers had a button\jumper "CD EQ", where CD means "constant directivity"
Also new products apparently:
"High-Precision Stereo 2-Way/3-Way/Mono 4-Way Crossover with Limiters, Adjustable Time Delays and CD Horn Correction"
"CD Horn Compensation
When a driver radiates into open space via a horn, its efficiency increases. Over the past few years, constant-directivity horns have gained widespread popularity, since they are designed to produce consistent dispersion over their entire frequency range – however, the higher the frequency, the lower the efficiency. To compensate for this, the CX3400 includes a switchable pre-EQ for CD horns that ensures a fl at frequency response, even before equalization is applied. This pre-EQ raises the signal gain by 3 dB at 3.5 kHz, which then increases by 6 dB/oct. up to 22.5 kHz."
www.behringer.com
Nothing I intend to buy. I just saw that it was there. That functionality.
Edit:
Although new and new, ..it's Behringer so it's probably some old construction that they (illegal?) copied in true Behringer spirit, I can imagine.