The basis of Ultrasonic is high frequency sound echo which reflected by objects in their path. By calculating the time interval between the transmitted sound and the received echo, we can determined the distance of that object in the path.
This is how bats flies in the darkness of the night seeking for food.
High frequency tranducers (Ultrasonic transducers) are the common 'Transmitter' module, they are usually sold as a pair with its matching 'Receiver' module.
These modules are usually fixed 40kHz frequency range and their effective range is only a couple of meters. If larger range is required it is better to use IR modules instead.
Many circuits available on the net where Ultrasonic modules are used.
Here a circuit I found and built for car-reverse parking indicator, a bit old but it's still working!
Thanks to a.riccibiti