Any Idea for a DIY Sensor?

[audioguru]

Simply place transducers close together, inline and pointing in the same direction.
If you are inline then they point directly at you so make a strong beam. To one side, one is closer and another is further away so their phases are different and they cancel.

 

[Dick Cappels]

Which implies that the spacing of the transducers with respect to wavelength is important (among other things)