Sennheisser has figured out how to dampen vibrations in structure of their phones and has been doing it with the HD 800 for some years by using an unnamed damping material in the headband. This type of damping is not to be confused with damping backwaves and the like of sound in the chambers of headphones and speakers, rather it is a totally different problem, how to get rid of the energy going back into the structure of the phones from the driver mounting. Think of Newton's 'for every action there is an equal and opposite reaction. '
Senn is probably using Sorbethane or some similar material; at times they have just called it a 'space age material.' My experiments with Sorbothane show very big improvements in several aspects of sound on Stax phones, including tonal accuracy, dynamics and soundfield. Grado does something similar using what they call a proprietary polycarbonate and I have seen a number of other companies use related damping techniques.
I got onto this issue when I noticed that my Stax SR007 A showed small sonic changes simply by my touching the headband. More by accident than design I got onto using Sorbothane on them ( then on speakers which have the same problem BTW.) I have not seen Senn discuss this issue in their literature on the HE1 as they do on the HD800 series but I can't imagine they are not doing something to tackle this problem here as well since it seems to be a universal problem with audio transducers. I see in their manual for the HE1 that they recommend changing the padding on the headband after a few years so the padding may be part of their damping technique.
I get very good results using small sections (less than 1 inch high in any direction) of high density sorbothane (70 duro,) fairly thick (I use a lot of 1/2 inch) glued using the 3m self-stick that comes on the thinner pieces ( I have to use an expensive industrial glue on the 1/2 inch though since no-one seems to sell the self-stick.) Covering the back of these pieces with several layers of electrical tape enhances their effectiveness, giving what is termed 'constrained damping.'
See
https://www.head-fi.org/threads/dam...s-with-sorbothane-and-other-materials.744839/ for some history. The story changes quite a bit as the thread progresses and more experiments are tried. Also googling 'constrained damping' will pull up engineering articels and some from audio manufacturers.