MX5 doors aren't big relative to most car doors, but they're still big expanses of mostly flat sheet metal, which makes them very prone to resonance and rattling - knock on the outside metal and take note of how loud and rattly it is. This kit, like our sound damping kit, uses pre-cut pieces of DEI's Boom Mat sound damping material to absorb those vibrations and quiet down your interior. They also make a big difference to how good your music sounds (since your speakers won't rattle the doors anymore), and give a solid "thunk" when you close the door, as opposed to the normal rattle. Easy install, just remove the door panel and the plastic sheet behind it (watch that black goo, it gets on everything), firmly press the pieces into place, re-assemble, and enjoy your quieter car. Includes enough pieces for both doors (one kit = one car).
This kit is designed for the NA and NB, but will work for the NC and ND as well with a little trimming (it's not too bad to cut with a razor blade or good scissors).
Boom Mat is a premium quality sound damping material that keeps interior panels from vibrating and generating noise by trapping that noise between an asphalt-free butyl lower layer and a 40 mil (.004" / .1mm) thick aluminum top skin (.080" / 2mm thick in total). Since Boom Mat doesn't need to cover the entire area to do its job, you spend less money and add less weight to get the job done.
While this kit doesn't make a huge difference to the decibel measurement, it's in the right range, and keeps the metal panels from acting as transducers. Which makes the car much more livable an improves the sound of the stereo.