The tip transmits a current to the left speaker/earpiece, the ring transmits to the right speaker/earpiece, and the sleeve grounds the port. The black bands in between are called isolation grommets, which ensure that there is no unwanted mixing of sound between the right and left channels. If you pay attention, some 3.5mm jacks have one isolation grommet, some have two, while some even come with three. One isolation grommet means the connector comes with just a tip and a sleeve without a ring, which leads to a mono sound output. If you had earphones with a single ring, you would essentially have sound coming from only one of the earpieces. One-grommet jacks are primarily used for guitars.
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