height is typically the smallest dimension, and floor and ceiling reflections (including higher order reflections such as sound bouncing first off the floor and then off the ceiling) are responsible for the vast majority of reverberant sound degradation
While I agree that floor and ceiling reflections are very important (particularly their tonality), usually creating the first wave of reflections with very little chance for proper diffusing (unlike side walls), I would not say that this creates the vast majority of sound degradation linked to reverb.
Build a speaker with a very narrow vertical beamwidth (less than 60 degrees: aka +/-30) and you find clarity much, much farther into the room than with a conventional speaker
I fully agree in theory. Practically, you would need to do that for basically the complete frequency range being localizable to the human brain, on an equal level of attenuation outside the listening window beam width. That is roughly from 500 Hz to 7,000Hz. +-30deg vertical is not only too wide in my understanding to properly attenuate floor/ceiling reflections, it is also very difficult to achieve constantly over such a broad range of wavelengths.
Which speakers do achieve such even vertical pattern, except from CBT or (virtually) curved line sources or maybe very very big horns (which are difficult to meet the ´point source´ condition at the same time)?
I am highlighting this because a lot of people seemingly mix up the minimum vertical listening window angle which appears narrow on paper (like +-20deg with some d´Apollito concepts) and the average or constant angle over the aforementioned band which can be much much wider. And in practice, it regularly is, and the resulting tonality of early floor/ceiling reflections with some bands dominating, will not solve the problem you have brought up. Rather the opposite.
Me thinks, the goal of attenuating floor and ceiling reflections will be easier to achieve with some sort of line source, limited and frequency-dependent in length, so you can really create a more or less constant vertical directivity pattern. Sounds easier than it is.
it makes some sense to prioritize the horizontal reflections, except that you have to include the virtual vertical sources to properly understand the distribution of your virtual sources even in the horizontal plane. Those vertical reflections reinforce the sound and don't widen the soundstange at all.
Absolutely true, and maybe one of the reasons why coaxial speakers are more likely to create a stable and coherent virtual imaging, with both the phantom sources and the perceived reverb surrounding them making sense to our brain and blending nicely.
Humans can sense vertical position. Not very well, mind you, but we can. Research on this shows that vertical localization is inconsistent when using multiple speakers with the signal manipulated through stereophonic intensity and timing differences
Vertical localization is mostly HRTF-related, so very much dependent on the our ability to perceive tonality and compare tonality pattern of different sound events (such as direct sound, early reflections, multiple reflections and diffuse reverb). It is a well-known phenomenon among recording engineers that a frontal phantom localization might cause the impression of the source being elevated, as the HRTF-related change in tonality between +-30deg and 0deg brings our brain to such a conclusion.
One of the effects in such setups or in concert halls with hanging reflectors is vertical broadening.
In concert halls and theaters, such are mainly used to reduce the initial time delay of the first wave of vertical reflections in order to improve clarity and intelligibility. Vertical broadening can be a positive side-effect, surely.
I disagree with the benefits of narrow vertical directivity speakers being worthwhile. I cannot stand hearing spectral shifts as I move my head.
Why should there be spectral shifts with constantly narrow vertical directivity?
Moving the head I would more associate with horizontal movement, or turning one´s head. Vertical movement is not covering a vast angle, unless you permanently sit down and stand up while listening in a nearfield environment.