Should be able to do up to 130 dB (like cheap $20 noisemeters). Is a 24-bit analog-to-digital converter necessary to capture this level?
Anything that's home-built isn't acceptable for legal purposes. It gets rather complicated to make a calibrated A-weighed SPL meter, and of course it has to be professionally calibrated by a certified lab. Most labs won't calibrate homemade equipment.... You can only make something that's approximately accurate if you can calibrate it to a particular "kind" of noise. The kind of timed & logged measurements you want to do require some expensive equipment. You might be able to rent it... I don't know...Typically there is a range control (sometimes auto-ranging) that adjusts the sensitivity or the gain of the amplifier. You also don't need 130dB of dynamic range to read up-to 130dB SPL because you're not reading down to zero dB SPL.
Find a reputable calibrated noise meter that has serial output and connect it to a datalogger. Find an expertwitness who can witness the use of the system, and the actual readings being observed, and is willing toanswer questions in court about the measurements and their relevance to the legal case - anyone capable ofclaiming 85dB is 55dB is also capable of pulling a few sneaky tricks so be ready. They certainly would trashany claims based on home-brew equipment. I suspect the best course of action is to employ a professional company whose business and reputation iswith noise management - they might see an opportunity to get work from the offender in fixing the problemonce the spectre of a successful prosecution is raised.
I understand what you're saying about the dynamic range. I think that's something I could process from the ADC output.
but what I found at Amazon was $800+. I was looking for < $50.
I live in an area that has a severe noise problem but the citizens cannot fight back against the offender without empirical data.