measuring an acoustic wave flight time dependency on the temperature

Hi,

Firstly I'm a newbie on comsol, so my problem has been maybe already solved in this forum, but I actually sought and I really didn't found how to make it

My problem is : I want to model an ultrasonic thermometer.

The concept is really simple, we use transducer transceiver to generate an acoustic wave in a open closed waveguide which is full of a defines gaz ( air ). And we have to measure the flight time which is dependent on the temperature.

So I just want to know, how can I generate an acoustic wave and measure the return of this wave in the same point.

I hope that my problem is clear and sorry about my english :)