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.