Directional sensor?

ak, I'm just going to suck it and see. It's just a toy so there is no call for accuracy. If it will "look" towards whatever happens to be nearest it (which might be a person) that is all it needs to do. I want to create a "is that thing watching me?" effect, rather than obvious and precise tracking. I might give it eyes with a bit of mechanical linkage so they lead the head a little. So I guess it would be indoors, and I think "ambient effects" might just make it more interesting!
But out of curiosity, how do you create a "lens" for ultrasound? Some bits of tube perhaps?
I'm quite happy to go with the "left a bit, right a bit" system - it even occurred to me I might (with a bit of tuning) be able to gate the received pulses directly to get the pwm for the servo. Haven't had time to think about that though.

A load of plates angled used to be popular on hf speakers as an acoustic director or lens.
https://www.google.co.uk/search?q=acoustic+lens+tweeter&biw=1280&bih=889&tbm=isch&imgil=YEgSijIEUFkRnM%3A%3B8n8QAKSotvLxJM%3Bhttp%253A%252F%252Fforums.audioholics.com%252Fforums%252Fthreads%252Fvintage-jbl-acoustic-lenses-do-these-things-work.84676%252F&source=iu&pf=m&fir=YEgSijIEUFkRnM%3A%2C8n8QAKSotvLxJM%2C_&usg=__2MGogg3qEnEr9Gh25jZJ_QTFG70=&ved=0ahUKEwjUtbve3PDRAhXIHpoKHapoBBgQyjcIJw&ei=i8mSWJTEBsi96ASq0ZHAAQ#imgrc=YEgSijIEUFkRnM:
If you look too much you'll get bogged down with a whole pile of maths and audiophools.