Subsonic Repulsors create an ultra sonic harmonic field that "confuse" hostile mobs into not targeting players. Perhaps make mobs attack other mobs. Repulsors are mid-game progression because they require a fairly high amount of RF to operate (500-1000rf/t), thus making simple coal generators insufficient to power them. Material cost to craft them is trivial, but interesting materials (such as magnets) could be required to encourage use of other machines and/or mechanics.

Subsonic Repulsors with Subsonic Amplifiers (range boosters) can be used as wireless RF charging stations with Energy Transducers (upgrade? separate block?). They simply wirelessly charge RF accepting items, they're not a wireless energy network.

I'd imagine they slightly hum when standing next to them. Perhaps crackle a bit, maybe just use the vanilla fire burning sound? Paul, you could engineer the perfect sound.

For now, going on the backburner as EIO adds something like this