Look the 555 is the worst way...
I use the 16 bit counter of my AVR....
Look there is an old trick about this.... You can make a robot follow something by this....
Lower frequencies have longer wave length cause the material the pass through remains the same (air).
You can find it by this math: u = f X l (where u is speed, f is frequency and l is wave length)
So if the basic detection range of the 33kHz burst is 30cm let's say, the 38kHz would be 10cm let's say and 29kHz burst would be 60cm!!!!
Let alone you can experiment more easily....
Do whatever you like man!!!
Regards, Lefteris
GReece