As great as open-back headphones can sound, they are simply not practical for all use cases. In the most extreme case, you are on public transportation, or even on an airplane. Not only do you not want to hear your neighbors, but your neighbors almost certainly don't want to hear you - open cans share both ways. At that point it's usually best to use sound-isolating, or noise-canceling headphones. The Mixcder E9s are of the latter type.