Collier's Encyclopedia . We as humans can hear many things. From the giant truck driving by to the writing of a pen. The few sounds we cannot hear are bat sounds. Their frequencies of producing sound is much higher than ours and so we can't hear it.
Frequency is how fast an object vibrates and sound vibrates in waves.
The vibrations are measured in Hertz. Humans can hear from 20 Hz to 20 kHz. Anything higher is called ultrasound or ultrasonic sound. The reason that bats use ultrasound is because it has such a high frequency and it has a low diffraction or it bends less. They use this sound to do a couple of things like to catch their prey and also just to get around.