I use these microphones with my CCTV system at home and they are very sensitive. I have one in my den hooked up to Vox and it works great. The only down side is it requites 12VDC. It has a built in amplifier. And for $7 you can't beat it.

Here is a link:

http://www.amazon.com/VideoSecu-Sensitive-External-Amplified-Microphone/dp/B0053V5Z9W/ref=sr_1_1?ie=UTF8&qid=1420334069&sr=8-1&keywords=cctv+microphone

Worth it for the warning sticker alone!