Ted Weller and I just published a paper describing novel echolocation behaviors in hoary bats.
Here's a link to the paper
Listen to normal and micro calls in this writeup by Science
Nature wrote a short summary.
Press Release from Wake Forest
Email me if you'd like a PDF.
Corcoran, A. J. and T. J. Weller (2018). Inconspicuous echolocation in hoary bats (Lasiurus cinereus). Proceedings of the Royal Society B. 285: 20180441.