Try using a fix freq. ( the one the sensor specified).
Count the time ( perhaps with a timer) to receive the echo. And knowing the sound propagation velocity in air you calculates the distance.
Have in mind that the propagation velocity is dependant of the temperature ( the square root of T )so this method is not good to sense distance from a hot or cool source, where gradients of temperature are involved. Of course you need sensing temperature to compensate the measurement.

I have done this ( freq. = 40 KHz ) and get a very good result. My sensors were manufactured by MURATA.

Good luck
Daniel