Slater
Headphoneus Supremus
Beyerdynamic dt 911
From when Beyerdynamic started using neodymium magnets, lower impedance to work well with the new digital sources (CD players and similar), and computer simulation to design the enclosure.
Driver an enclosures is very similar to the later DT931
The interesting things, uniques to the 911 and 931 (and also the closed counterparts 901 and 831) are the side vents on the magnet, and a more open driver (the plastic vents on the back) that is also common with other models such as the dt531 dt150 and dt250 (although those have a different magnet).
That is really cool. I wonder if those side vents are intended to cool the voice coil?