You choose how much of the world to let in or keep out, or have the headphones do it for you automatically based on your surroundings and motion.

Libratone—the Danish manufacturer best known for building specialty wireless speakers like the Zipp, the One Click, and the Too—is plunging deeper into the headphone market. Its new Track+ wireless in-ear-monitor headphones are geared towards those with an active lifestyle, sporting an IPX4 rating for sweat and splash resistance, plus some new noise-cancelling features.
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https://www.techhive.com/article/3280804/libratone-track-wireless-headphone-review-automated-adaptive-noise-cancelling-done-right.html