Sennheiser’s HD 800 series is ever-expanding. It began with the original HD 800 almost a decade ago. That model is still on sale, but a few years ago, the company launched an HD 800 S model with a more subtle and softer top-end, in part seen as addressing concerns of many prospective HD 800 owners. But then came the HD 820.
The HD 820 is the closed-back variant of the HD 800 Series. High-end, closed-back circumaural dynamic headphones are relatively rare, primarily because they are very difficult to manufacture without sacrificing performance in the process. Sennheiser decided to it was worth the effort, however, and the HD 820 was born.
Closed-back dynamic headphones are extremely popular in the professional world. Studios, field recordists, video production, and electronic news gathering teams use them exclusively to monitor sound, because any kind of open-backed design would leak sound into the recording itself. Nevertheless, this creates its own price ceiling (Sennheiser’s own HD 25 series are a popular choice among the field recorder and video production set, because they are small, light, rugged... and cost about one-eighth the price of a pair of HD 820’s).
Sennheiser set itself a bold challenge; to make a headphone as good as the HD 800/800 S, in a closed back design. That meant using the ring-radiator drive unit developed for those models, and that posed a real problem for the company; it’s one of the most uncoloured transducers in audio because it is so open. If you look to the ear-cup of an HD 800 or HD 800 S, it’s effectively an open basket to hold the drive unit. That means the HD 800 and HD 800 S leak sound like a very leaky thing that just got used for target practice by six guys with shotguns.
Sennheiser’s answer: glass. The HD 820 has a concave glass cover on either earcup encasing the main part of the diaphragm, so no back radiation can spill out into the real world. Perhaps surprisingly, glass is the main way of stopping the leak of the HD 800 and HD 800 S. Equally surprisingly... it works!
Of course, changing the system from open to closed back does require some extensive redesign and testing, if for no other reason than the addition of a piece of glass to seal the case completely changes the pressure loading of the diaphragm (the older models expect to see a free space rear excursion, and by simply covering that diaphragm with your hand, you can hear how the pressure changes from you cupping the driver alters the sound. That means quite extensive changes to the internal layout, although the exact nature of the changes is not disclosed by Sennheiser.