How far away can elephants hear?

The sound structure of elephants is very advanced. See, they can hear infrasound (i think it's called that) which is basically low rumbles that they can somehow send to each other. They can send them over long distances too, kind of like telephone calls. We can't hear them, by the way. Apparently African elephants can also detect the calls and foot stomps of other elephants through underground vibrations.
Elephants hear through their ears. Elephants ears have infrasound capabilities for long range communication.