Car sound deadening (for Mazda MX-5 / Miata)

These cars aren’t known for their quietness. There was a definite rumbling / booming noise that from what I can understand is only found on the Hardtop (PRHT) NC MX-5 / Miata as it causes an echo chamber. With the roof down, the boom isn’t there. Infact the car is extremely quiet. So I set off trying to find what others have done to reduce the noise.

Carpet underlay
Sticky spray foam

A lot of people point out that there are giant gaps – almost channels – that allow the noise to filter from the boot straight to the cabin. They fill these gaps with all sorts of things from carpet underlay style foam, to old pillows, even blankets.

There are even people who have used spray foam to fill these channels. I don’t advise doing this.

What I found is that due to most MX-5 / Miata fans rejecting the NC and especially the hardtop model, all of the noise solutions focus around road noise, not the booming that I am experiencing.

Resonance

Finally I discovered the resonance answer. This involves adding density to a metal panel so that it doesn’t vibrate as much. It absorbs it before it has a chance. The way they do it is with a rubber substance called Butyl mat. It’s got a layer of foil on top to help mould it to shape, and it is sticky but not terrible like tar-backed variants.

All the different layers to help with noise reduction.

This Butyl mat varies widely in price, thickness, quality, etc. The official branded version is usually called Dynamat. However there are cheaper and sometimes thinner versions easily found on Ebay.

Brand-name version.
Cheaper version.

I ended up finding a supplier in Australia that made fairly thick matting, was adhesive, and looked just like the real deal.

Disassembly

It was fairly easy to get to the metal that I needed. It’s all stuck together using plastic push clips that are common in vehicles. This had 6 total sections – 4 plastic covers around the edge, the felt floor cover, and a wooden panel at the base of the pan.

Carpet and wood removed
All coverings removed including the fuel line panel.

Once down to the bare surface, I made sure to mark each important screwhole, plastic push pin hole, vent holes, and any other important areas that shouldn’t be covered. For the remaining area I gave it a good clean so that this had the best chance of staying stuck.

Cutting and rollering

Cutting is easy with a stanley knife and a guide. I would highly recommend purchasing a roller for the application.

Where it doesn’t seem to have stopped the flex enough, add a second layer.

For me, the application took a couple of hours.

Fuel line cover also covered. I’m unsure on how much this actually achieves but others had done it, and it is fairly thin, so I figured why not.
I ended up doing two layers in the lower pan as it seemed to be where the majority of the noise came from. Also gave a much cleaner look.
Wheel wells were left mostly uncovered, I just stuck a small amount in there if I had some offcuts.

Reassembled

For those curious, this is the finished shot. There is nothing to show that this material is there.

All put back together

Verdict

What this has done is it has definitely removed the booming. It has made the sound inside much more muffled like I would have expected it to be initially. A conversation can be held while driving now!

There is still road noise but maybe this has made it more obvious to me by not having the booming there. For that I have read the best answer is new or better tyres, even in a size larger than standard. That’s next on my agenda.

Highly recommended!