karlok:
Ok, thank you very much.
And yes, it is a (almost) right angle where the signals should be reflected and so get its echo.
I will note that it really does need a right angle and something sound will bounce off of. It doesn't need to be a perfect 90% angle, and it doesn't need to be a big flat board. But, hitting a wall at enough of an angle or far away hitting something like a pillow will show no ping echo. Most objects are 3 dimensional so there's at least some part of the object that's fairly close to a right angle. But, if you wanted to trick the sensor, it would be fairly easy to make it fail. I don't think any of this is an issue at all to your project. You should be able to get very consistent readings with your setup.
Tim