Hi,

as your Tx and Rx will be on the same side, both facing the object to be detected ...

sound will have a travel equal to ... Twice the distance you look for.

As simple as that !

2 x 10 cm / 340 m/s = ... 0.588 ms = ~ 600 µs

10 cm mini ??? haha ... I let you discover why ... and why to cancel detections under these values ...

Alain