At first I thought they meant Bang & Olufsen, a high-end brand that prefixes all their products with Beo [0]. But I guess the industry is making Beryllium drivers now [1].

[0] https://www.bang-olufsen.com/en/headphones/filter/over-ear

[1] https://blog.masterdynamic.com/article/know-your-sound-tool-...