Kennerton's sealed Rögnir headphone is. What happens to its dynamic driver's rear emissions trapped inside? As the inside of their wooden shell shows, they encounter a geometry visually reminiscent of an anechoic chamber's absorptive foam wedges. Obviously wood isn't acoustically absorptive. Here the geometry instead scatters incoming sound waves to ideally exhaust themselves in a game of endless ricochet rather than reflect back out through the thin diaphragm. Such slightly time-delayed leakage is a challenge for box speakers and sealed headphones which eschew effective dampers, absorbers or scatterers. KEF's medieval labyrinth aka Meta material at the upper left is their 'enter to never exit again' absorber said to be over 90% effective at treble frequencies. Vivid's Laurence Dickie famously relies on absorptive straight or spiral tubes of shrinking diameters to trap his drivers' rear waves.
Today's thinking point is that unlike open-baffle dipoles which forego any attempt at trapping their transducers' rear waves, box speakers and sealed headphones intent on time fidelity must worry about not leaking their captive energies back out through their diaphragms. It's notoriously difficult to overcome a sealed headphone's greater warmth over an equivalent open-backed version where 'warmth' is a euphemism for thickness and time blur from trapped air interfering with the direct sound via reflections, resonances, pressurization, cancellation and turbulence. Though such warmth can be perceived as pleasing when it behaves as minor soft focus, it's a function of imprecision. If you'd rather hear more of the music signal and less of your transducers' material makeup, ask what your speaker or headphone designer has done about this basic subject. Case closed? Or should we leave it open instead?