How, please explain?
The Genelecs 8351B uses the MTM cancellations to effectively increase its vertical directivity.
Please explain how you compare this speaker’s off-axis and band limited dip to Genelec One series?
I didn't mean to do a direct comparison. just that when designed properly it is quite effective. the post right above mine shows that there is a fairly decent amount of control in the vertical radiation. Narrow, but controlled.
There’s no such thing as insensitivity to vertical directivity. Any off-axis FR irregularity will affect the FR in the room. Just look at any speaker tested on ASR. Maybe you can show us another speaker that has a 40dB lope, which is “fine”? If you think in the near field that’s not the case, look at the studio picture I posted above: How will the reflection FR from the desk surface will be when there’s a 40dB lope in the FR?
It is very very common knowledge that we're much less sensitive to vertical directivity, it is why every 2-way design out there is usually placed with the tweeter facing up or down but never with the speaker on its sides.
Even all of Harman's preference research heavily favors horizontal directivity. For example, Listening window only includes +/- 10 degrees vertically, while horizontally it includes +/- 10, 20 and 30 degrees. For Early Reflections it gets even more interesting, we get +/- 90 degrees horizontally, and 40 degrees below the tweeter and 60 degrees above it. No idea why there is a mismatch there, you're the expert here so maybe you have an idea?
As for ''Speakers with 40 dB nulls'' being fine. Here is the Genelec S360 vertical directivity showing a very sharp -20 db null. I could probably find a worse example with something like the JBL M2 with an even larger mismatch (big horn + big woofer) but I will leave that for you incase you're curious. Honestly for me -20dB is just as bad acoustically as -40 dB.