how does ultrasonic sensor transmit signal

How does an ultrasonic sensor transmit signal?

Here is a really good link which will get you some very detailed answers:

http://lmgtfy.com/?q=ultrasonic+sensor+transmit+signal

If you are referring to a piezoelectric transducer, and if by 'transmit' you mean the generation of ultrasonic energy, then it uses the piezoelectric effect to convert an alternating voltage into mechanical energy (or vibration) that shoves air molecules back and forth causing regions of alternating high pressure and low pressure that radiate through the air (using the air as a medium) away from the transducer like waves in water radiating away from an agitator.

Otherwise, you'll have to elaborate.