Question

Why do we not hear the voice of bats ????

Answer 1

Unlike birds, they do not fly into window glass because it is opaque in their sound world. Basically they emit a 'chirp', and listen to the echoes. ... Bats emit sounds at typically 2 or 3 times higher pitch than we can hear. Sounds above human hearing are usually called ultrasonic.

Answer 2

beacuase they produce ultrasounds which have a frequency of more than 20000 hertz and out of audible range of humans