Possible to build sound impact sensor instead of buying one?

Hey guys,

I'm looking to build a project that I saw on YouTube for controlling LEDs with the use of a sound impact sensor. A Parallax brand to be specific. Download - Parallax

The circuitry seems pretty simple but I was confused when it came to the potentiometer? Is it after or before the mic or is it used to set the op-amp gain or after the op-amp output? Or do I have it all wrong?

I'd buy the sensor if I didn't have to wait for shipping but I think I might be able to make it out of parts at the Shack.

Thanks!
Rob

Google "electret microphone arduino" and you will gets lots.