is it possible to use an raspberry pi to detect ultrasound of 40000 Hz ? if so what parts and what software. and i want to have a small screen that readsout the Freq. would help me a lot! Thanks for the input 
okay tested what you guys said, and it worked like a charm, thanks for all the help
okay tested what you guys said, and it worked like a charm, thanks for all the help
Glad it worked OK - thanks for letting us know.
MK