You may find some answers from the scientific community, especially those folks working wildlife. Just this morning I watched a program about elephants and the different sounds ways they use to communicate with each other. Scientists have discovered that the elephants use subsonic frequencies to send signals over long distances, over covering more than 110 sq. miles. These sounds are not audible by the human ear, they can only be captured with recording equipmentmiles::

http://www.elephanttag.org/Professional/professional_elephant%20vocalizations%20and%20behavior.html

On a similar note

Similarly, researchers have discovered that trees can "sing"; using sing". Using specialty microphones (similar to hydrophones, I think) and recording at insanely high sample rates, they can hear the trees "vocalizing" as the weather changes:

http://emusician.com/daw/emusic_going_wild/

You may find some answers from the scientific community, especially those folks working wildlife. Just this morning I watched a program about elephants and the different sounds they use to communicate with each other. Scientists have discovered that the elephants use subsonic frequencies to send signals over long distances, over 110 sq. miles. These sounds are not audible by the human ear, they can only be captured with recording equipment:

http://www.elephanttag.org/Professional/professional_elephant%20vocalizations%20and%20behavior.html

On a similar note, researchers have discovered that trees can "sing"; using specialty microphones (similar to hydrophones, I think) and recording at insanely high sample rates, they can hear the trees "vocalizing" as the weather changes:

http://emusician.com/daw/emusic_going_wild/