Some important things to note:
- Its range is 64 blocks
- It can hear through solid blocks
- It will output a signal even if there is a solid block below it
One possible use is to greatly extend the length of a Redstone signal without using wiring or Wireless Redstone. A note block at one end can be triggered with a signal, causing an Arcane Ear at the other to emit that same signal. Have that one power another Note Block, and so on. The different blocks and ears should be at different frequencies, however.
If a friend has a doorbell you can use it to tell when someone enters his or her house.
It can be used to trigger a trap - have a harmless looking note block with a lever on it, no wiring leading elsewhere. Once the victim plays the note, however, the ear sends a signal to whatever trap (Sand Block, Dispenser, TNT, etc.).
The Arcane Ear is a device that is capable of detecting the sound emitted from nearby note blocks. If it hears a note it recognizes it will emit a short redstone pulse.
The Arcane Ear needs to be tuned in a matter identical to note blocks, the tone and note it listens for is depends on the material it is placed upon.