Remove everything from the trunk. Then on the inside, you'll have to remove the door sills then detach the B-pillar seatbelt anchor bolts paying close attention to the arrangement of spacers that allow for swiveling. Then remove the B-pillar panels and the entire cavity will be exposed.
That's when I apply a double layer of Dynamat-type mass on the actual rear wheel wells and then stuff the entire cavity full of poly-fill. The wheel wells are narrow so you might be lucky to mass load with 5"x14" strips of Dynamat-type stuff or just use some scraps and keep sticking them on the wheel well. If you have some adhesive foam pad available, it would be good to use that here, otherwise, it's just another expense.
Remember, my purpose is to mitigate noise with the lightest available option and poly-fill (a/k/a pillow stuffing or subwoofer fill, available at WalMart is key). The denser you can pack in the poly-fill, the better, just keep it away from the seatbelt-retraction spool.
It's never going to be as quiet as a Cadillac but there is a lot you can do with lightweight materials and low cost. Remember, we're just talking rear noise here. Every broad metal panel such as doors, hood floor and front fenders also resonate but if you keep mass loading, you're defeating the lightweight purpose of the Miata.
Reply With Quote