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 
could you guys recomend me an Oscilloscope for this use ? i would like a hand held one with a battery that dont need to last that long but 10-20 min, and a DMM in one if possible. what MHz do i need? price is is not an issue.
You don't need much bandwidth since you are looking at 40kHz signals.
The nicest toys come from Fluke and Keysight:
Very nice - but over £2k
Search Results | Farnell element14
This one:
is a lot cheaper at £1053
but I'm sure you could get away with:
about £330 form Amazon
I haven't used any of these - the Fluke and Keysight models will be nicer and will behave like proper scopes - the Hantek will probably be quite good enough to see if systems are working or not and cheap enough for you not care too much if you drop one in the bilges !
MK
thanks for the input, i wasnt expecting sutch a price :Z sorry about that, but do you think this is good enough Hantek DSO1060 Digital Handheld Oscilloscope /Multimeter 60MHz 500MSa/s 2 Channels-in Oscilloscopes from Industry & Busi…
or do i need to go up on the Keysight Technologies U1602BU1602B was thinking about losing it too
i dont think i will go for the fluke, just because im gonna lose money on this deal then, so ether the keysight or the Hantek
or mabye this one 72-9360 - TENMA 72-9360 - Oscilloscope, 2 Analogue, 100 MHz, 500 MSPS, 512 kpts, 3.5 ns | Farnell element14 Norge ?
I would go for the Hantek from Amazon - under EU distance trading laws you can return it (undamaged of course) within a few days of receipt if it isn't any good for you purpose.
I would expect it to be adequate for your purpose.
http://www.eevblog.com/forum/chat/hantek-dso-1060-a-battery-powered-60mhz-dso/
Another two links with a bit more information.
MK