The new research, recently reported in PLOS ONE, builds on previous Oxford University research showing that elephants call ‘bee-ware’ and run away from the sound of angry bees. Whilst the ‘bee’ and ‘human’ rumbling alarm calls might sound similar to our ears there are important differences at low (infrasonic) frequencies that elephants can hear but humans can’t.~ from, http://phys.org/news/2014-03-elephants-human.html
Seems pretty logical to me that elephants would have different sounds for alerting to different threats. The big question, for me, is how much of a threat do they perceive humans to be; Do they actually understand how dangerous humans are/can-be?
And someone emailed me to point out, that yes, elephants can distinguish a lot about humans; from, Elephants recognise human voices.