Here are two iphone apps that can read decibel levels using the built in microphone on the iphone.
The free app has fewer features and the interface isn’t quite as nice, but it’s FREE.
That said, I have them both and tend to prefer the .99¢ Decibel app.
Since I don't actually own an Android, I can't test any apps. So here is a free decibel reader that I found online
| deciBel (android)