There was a similar discusion recently at another place https://www.diyaudio.com/forums/multi-way/375458-constant-directivity-home-post6749276.html and in this context constant directivity is synonymous with uniform directivity, not as in public adress systems an actual constant directivity throughout the speaker's bandwidth. Therefore, you will also find that Don Keele spoke about a constant-energy response monitor, the goal beeing no directivity errors. https://www.pearl-hifi.com/06_Lit_A...her_Publications/Improvements_Monitor_LSs.pdfWhat does constant directivity mean to you? To me it means the directivity index sees no change through the frequency range. This speaker sees a narrowing as frequency increases, like most speakers do.