Compared to other animals, the human ear is not very sensitive. We can hear sounds with a frequency of up to 20 kilohertz (kHz) — a normal speaking voice is about 1 kHz. Bats, dogs and some insects can hear sounds has are pitched much higher, which we cannot hear at all. Children can usually hear a bat squeaking, although this sound can sometimes be as high as 120 kHz, but in adults the ability to hear these high-pitched sounds is usually lost. ‘Supersonic’ dog whistles are used to call dogs, and their sound is so high-pitched that humans cannot hear it at all.
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