Actually I think it’s plastic. I haven’t looked real closely for a while. Anyway, I’m talking about the disc that covers the driver that has holes and slots in it. I’m pretty sure the factory foam pads cover everything, but what I have on my 35s now expose that middle section. On my original pair of 75s I removed the disc and drilled more holes in it per some mod I read on headfi or someplace. This was a few years ago. I can’t say that it necessarily improved anything except that the theory was that it would make it more open an detailed sounding. On $15 headphones I have no problem trying crazy stuff to see if it works. A couple years ago I took a pair of closed back Sennheisers HD-201 that my kids had blown and put Koss 75 drivers in them after modifying the mounting baffle of the Sennheisers. They sounded like ass. But it was a cheap experiment in hopes I could have a cheap pair of closed backs for work.