The input frequency is translated to something you can hear.
The PSR-1 VIetnam era seismic intrusion detector has input frequency that's well below human hearing and shifts that up. When you hear the shifted sound you know what you are hearing, i.e. a man or woman or child walking, a cat, a dog etc.
PSR-1 Seismic Intrusion Detector
If you want to listen to the pings from bats which are ultrasonic you use a converter that translates that down to human hearing.
Listening to Bats
This unit has two mikes and so you get stereo which helps a lot in knowing where to look.