Measuring sound frequencies using Raspberry Pi or Arduino
Since you've provided a few more details, I'll tell you how to go about it.

Install Raspbian (Debian Linux for the Raspberry Pi.)
Find and purchase a USB sound card and a separate microphone, or an integrated &quot;USB microphone.&quot;  Plug it in and make sure your Pi recognizes it.
Install GNU Radio on your Raspberry Pi.  Here are instructions for installing GNU Radio on a Pi.
Follow any of the widely available tutorials on using GNU Radio with a sound card.  Most will show you how to use the sound card to capture audio, do a Fourier transformation, and then display the spectrum.

If you do all of that, then you will learn a fair bit about how software and hardware interact, and how the Fourier transformation is used.

GNU Radio allows you to write your own modules in Python.
With NumPy, SciPy, and GNU Radio, you can do just about everything it is possible to do with sounds.