...quieter after vs before dynamat... errr, yes but expect a few more questions if its not

I've ordered the analog SPL meter from RadioShack. I'll give it a go as you and Tony have suggested.

The noise is mostly engine with backup from the mufflers and rear wheels. The car's fastback shape makes it act like a horn so noise seems amplified. There's also a fair bit of dead space in the rear quarters that I'll need to figure out how to fill. I've replaced the window seals and tires and it helped but not enough. I'll let you know how it goes. Thanks!