Ultrasonic module Water proof vs non-water proof types
It is possible to measure distance with a single ultrasonic transducer.

First the the transducer is used to transmit an ultrasonic pulse, and then the same transducer is used to receive the echo thrown back from the objects in front of it.

The disadvantage of using a single transducer is, that it is very difficult (impossible) to detect objects that are close to the transducer.

The surface of the transducer is set in motion when transmitting the pulse, and it needs to settle before an echo can be detected. 

The settling time can be significant depending on the transducer type. Typically 1500us, which is approximately 250mm in distance from the transducer.