Noise Alert is a security app for the Android platform that lets you turn your Android phone into a noise detection device. This app is capable of detecting the noises around the phone and then calling a set phone number once the noise exceeds the sound threshold you've set.
Rigging up this app is very easy. All you need to do is go into settings, set the alert phone number you want it to call, set the sound threshold level you want it to call the number at, and also set a sleep interval incase you want to save battery. After that, you can simply start the app up and let it work.
If you think about it, this app is useless on its own. But, say you're in a situation with two phones near you. Perhaps a family setting. If you're worried that your child is sneaking out at night, set the phone up plugged in near their room or the front door and have it call either your home phone or your spouses cell phone. Once the threshold is triggered, the phone automatically calls the line and stays on so you can hear everything.
This is a really handy app that can be used for anything from security to pranks. It works very well, but be careful with setting it on 5. At 5, it takes quite a lot of noise to set it off. More than anyone would make walking near your phone unless they drop a pan while doing so. Definitely check out Noise Alert.