I recorded this passage last night. I took a look at the sonogram for a visual representation of the sound and found an interesting surprise.

http://www.voicesinthevalley.com/media/batcall/index.html

Included in it are a Bull Frog, Green Frogs and a Grey Tree Frog plus an inaudible bat that found a meal! The Bat’s call starts around 25,000Hz and is well above the average persons hearing level. Some younger persons can hear high frequencies like those from dog whistles etc…

I play the passage twice and then slow it down approximately 10x. This results in the high frequencies pitch being slowed down, so they are within the normal hearing range of humans.

If you listen closely, and haven’t damaged your hearing from to much Metallica, you can hear the chirp of the bat echo-locating bugs, then it explodes to a very fast trill of 20+ chirps where it is centering on it’s prey, then resumes the hunt. The trill is very soft, but if you listen close with headphones, you can hear it.