How to display the frequency?

[Lennartbrugger]

I know this project was submitted a long time ago but just 10 minutes ago I found this little application. Exact this is what I'm looking for so many days- thanks for developing.

But - to my question:

I'd like to make a simple application that is able to listen for frequencies around 18000Hz (18kHz). Would it be possible to just filter the input to let it listen for frequencies in the range from 17kHz to 19kHz? And would it be possible to just display these values for the users?

Help would be very appreciated,
Thanks in advance, lennart