This helmet has large semi-spheres that cup over your ears. These metal earmuffs are lined with wool and quite cozy.
Even while not attuned to the helmet, while you are wearing it you have disadvantage to any checks that involve hearing. However, you are immune to being Deafened and you gain advantage to saving throws against taking Thunder damage.
When you are attuned, as an action you can focus your hearing in another location. When you do this you create an invisible sensor within 1 mile in a location familiar to you (a place you have visited or seen before), or in an obvious location that is unfamiliar to you (such as behind a door, around a corner, or in a grove of trees). The sensor remains in place until you dismiss it and can’t be attacked or otherwise interacted with.
When you do this you can hear without the hearing disadvantage imposed by the helmet, but you can only hear as if you are in the sensor’s space.
A creature that can see the sensor (such as a creature benefiting from See Invisibility or truesight) sees a bulbous fleshy ear about the size of your palm.