Which ultrasonic sensor for your face?

[hxtasy]

Ok so say i have an application that measures how close your head is to something.


I'm currently looking at ultra sonic sensors, I'm not sure which type to get. I am mostly worried about how accurate they will be. Say I'm measuring your head right now how far away it is from your monitor, i need the signal to bounce off your head, not anything in the background, so I'm guessing it has to be a narrow beam width.

Also, what are typical outputs of these things? I know they operate at frequencies higher than 20KHz, but is there a way they can still give you a headache if one is near your ear for extended periods of time?


any input appreciated.

 

[hxtasy]

Ok so i found some on this site:
http://www.superdroidrobots.com/shop/search.asp

which are way to expensive for what i need. Then i found some on www.mouser.com[/URL]. but one of them is a transmitter/receiver, and in the data sheet it doesn't really say how to hook it up and it appears to only have two wires coming off of it, how could this work?