Here's an app for your iPhone that can give you an idea of how loud anything is using the iPhone's internal microphone as a sound pressure level meter. The Ultimate Ears app isn't a true sound pressure level meter, but it does give you a sense of how much environmental sound invades or is invited into your home.
Ultimate Ears app is basically a marketing tool for their custom made earphones, but the free app is a fun way to see how loud your stereo can get, how loud that garbage truck in the morning annoyingly is, or how loud your dog can howl. Just point and the visual meter will give you an approximation of the decibel levels (the app is clearly noted for "recreational" use). Download the app at the iTunes store.