FOURTH, how do bats detect sound?

Bats are a fascinating group of animals. They are one of the 
few mammals that can use sound to navigate-- a trick called echolocation. Of the some 900 species of bats, more than half rely on echolocation to detect obstacles in flight, find their way into roosts and forage for food. Interestingly enough, bats not  only make use of echolocation, they use ultrasound to conduct echolocation. 
(REMEMBER! :) Ultrasound is sound beyond the normal human hearing range)

A young person can barely detect a sound with a frequency of 20,000 vibrations per second. A bat furnished with this specially designed "sonar system" sends these sounds in all directions 20 or 30 times each second, and then listens carefully to the echoes that return to it. The bat's brain processes the returning information the same way we process our shouting sound using a stopwatch and calculator.

The echo of the sound is so powerful that the bat not only understands the existence of objects in its path, but also detects the location of its swift-flying prey.

The interesting ear anatomy of bats enables them to detect the high-frequency ultrasound that they produce. The ears and brain cells in bats are especially tuned to the frequencies of the sounds they emit and the echoes that result. 

With regards to bat ear biology, a concentration of receptor cells in their inner ear makes bats extremely sensitive to frequency changes: Some Horseshoe bats can detect differences as slight as 0.000l Khz!

Of course, some sounds that bats make can also be heard by humans! The squeaks and squawks that bats make in their roosts or which occur between females and their pups can be detected by human ears, but these noises are not involved in echolocation.