DEI NA/NB insulation kit

Miatas are great, but they're not exactly quiet cars. This kit won't turn your Miata into a quiet Rolls, but it's a good step in the right direction. This kit, from DEI, uses their Under Carpet Lite insulation, which is a 1/2" thick multi-layer composite material specifically designed to absorb sound and insulate against heat. They take that fancy material and CNC-cut it to custom-fit the NA and NB Miatas. Small variations in the years (e.g., ECU location) are accounted for by scored lines that are easy to cut if you need to remove material but strong enough to hold everything together if you don't. 

This kit is intended for LHD (left hand drive) cars, although it should work on RHD cars if you're willing to trim the insulation as needed to make it fit. Please note that it's not pre-scored to address this. 

Covers the floors, transmission tunnel (except under the dash), rear bulkhead, and rear package shelf. Easy install - remove the seats, pull up the carpet, slip this into place, then put it all back - no dash removal required. 7.25 lbs total, so a trivial weight increase. Be sure to add the Boom Mat sound damping kit for the best in sound deadening. 

How much quieter is it? That's a surprisingly tough thing to quantify. While the decibels do decrease, it's not by much - the biggest difference is that the harshness and tone of the noise is much decreased / improved. That's a tough thing to quantify, but it does make a big difference to how pleasant the car is.