NAD drivers emit sound in just one direction--towards ears. So no acoustic reactions and bouncing frequencies to take care of as in the case of other drivers. Looking at Tyll's pictures of a stripped down HP50, the NAD drivers are like mini speakers the size exactly of the back metal plate with magnets circling the metal plate.
??? I don't think it's what TMRaven meant. In the video that has Jude and Paul Barton in it and also the one at innerfidelity with Tyll, Paul Barton talks of how he (or they) worked hard on making a good structure inside the drivers so that the sound does not get messed up the way that TMRaven said can happen.
Also, no space is still something to think of when making closed-backs I believe compared to a open-back design.
Heck, even open backs' metal mesh or whatever is put behind makes a difference (much like loudspeaker damping I guess (room treatment?)).
Edited by Jeff Y - 3/25/15 at 1:20am