*** Edited** Just saw someone already point this out ***
This "acoustic Bass Lens" information is from the Sony MDR-F1 headphone.
I think he's referring to "Bass Lens". Fostex used the idea in their original T50 or at least in their OEM version for NAD, in which they have a ring/donut of felt on the ear side of the driver covering only the outside portion with a hole in the middle. Here's a nice picture that's been posted a few times in the ortho thread. I find that it can also work to remove peaks in certain areas of the upper midrange and treble without reducing perceived detail (I find covering the whole front of the driver can cause a reduction of perceived fine detail)
Edited by Twinster - 11/6/11 at 5:19pm