Communication Device

Communication devices are a type of wireless technology used by delvers to maintain contact with each other while they are too far away from each other to speak directly, they take the shape of a small speaker that is inserted directly into a delver's ear, similar to real life earplugs.

The device presumably has a microphone capable of picking up a person's voice, and a receiver and transmitter capable of broadcasting signals to other similar devices within range.

Limitations
Communication devices have a limited effective range, the sound quality will diminish the further apart two devices are from each other until it either becomes unintelligible or the devices disconnect entirely. They are also subject to disruption from the forcefield, which is especially troublesome if they are used to communicate between layers, although they can still function if the units are relatively close to one another. It's unknown if they can be chain wired to provide a wider range.