welcome to the community.
What you want to do is a distance measurement to get something happening.
First you want to measure distance. May this chapter helps http://playground.arduino.cc//Main/InterfacingWithHardware#DistanceSensing
The you may want switch an existing unit that growls with a transistor. May this helps http://arduino.sundh.com/2011/10/transistor-controls-other-power-source/
You can use any Arduino for that job pending on how small the unit shall be. May the best is to buy a arduino uno with a prototype shield...that's always a good starter.