Where can I find an affordable instrument to measure sound volume?

The neighbor kid (teen or young adult) has converted his entire trunk area into a sound system (complete with lights), which he uses to play ear-splittingly loud “wump-wump-wump” music at all hours of the day and night, parking the car in his driveway. (Windows and doors closed, A/C on, white noise machine or television on – it’s still clearly audible. Ear plugs don’t work because you can feel the vibrations in your chest. Pictures on the wall move.) The city has a noise ordinance after 10 pm and I’ve called the police numerous times (they probably consider me a pest by now). This goes on at 2pm, 11:30pm, 3am – any time. (I had family staying with me around Christmas and he woke the babies up at 2am with a rousing rendition of “Fuck you muthafucka”.) He’s smart enough now that he sees them coming around the corner and turns it down, and the police won’t do anything unless they catch him at it.

His mother can’t control him and has given up trying, plus she only seems to be home once or twice a week. I can’t reason with him (he stuffed a newspaper in my car tailpipe the last time I tried), and he’s been arrested for pointing a gun at someone while cruising the neighborhood.

Out of desperation, I’m wondering if I can document his behavior, videotaping his presence and a decibel reading, and then taking it to the police. Are there affordable sound level readers available? Any other ideas? (I have an ongoing Malatov cocktail fantasy.)