Speed of sound in beryllium and boron is very similar.
But boron could be grown around a rod and made into an extremely thin, incredibly stiff, and light pipe.
All beryllium cantilevers that I’m aware of are rods.
I’ve heard both and prefer pipes.
Yes, by the late 80’s into the very early 90’s... it had been decided in the general sense, by all involved, by manufacturers and buyers of said cartridges, that hollow boron tubes for cartridge cantilevers..was the ’ne plus ultra’ of materials for making the best phono cartridges.
Everything else was not as good a combination of mass, damping, resonance, and frequency of resonance. By the very existence of that particular attribute in the best cartridges, it was, by fiat and overwhelming example...declared ’king’.
We’ve slowly been going backward ever since, regarding mainstream use. Or course, quite a few more cartridges were made back then, and costs were lower and so on. So now, the hollow boron tubes are pretty well extinct and we’ve got inferior materials for the mid to high priced cartridges and then aluminum for the bottom, as per normal. Only in the extreme high end, nearly moving into 6 figures for a price, do we finally get back into better or newer with extreme aims in quality. Things like diamond are attempting to happen. Of course, I’m no expert in this but the lay of the ground does tend to look a lot like this particular recipe/mix.