Sound deadening

[HarryB]

I know, joke of a quest in an open car, but since I am doing a fair bit of highway commute couple of times per month, I have start wondering what I can do to reduce it a bit. Have done my fair bit of reading but there's a huge amount of people copying what others do (or Dynamating everything) thus unnecessarily increasing weight and cost. So what I will try to do is find what type of noise do I hear the most, and try to reduce that without adding too much weight (my top limit is less than 10kg).

Now what I feel is that a LOT of noise comes through the rear bulkhead/parcel shelf. If you think about it, it is an empty resonance chamber communicating with the cabin through two "tunnels" on both sides of the fuel tank, making it effectively a perfect bass chamber :P. I borrowed a decibel meter from work a while back, and my measurements confirmed that most of the noise indeed comes from back there.

Now the second part; what type/frequency of noise is that? I have done a fair bit of work analysing sound signals to identify cutting tools condition during material removal processes at work, so I wanted to borrow that (calibrated) equipment to play around a bit, but was not practical. Instead I resided to my smartphone and an app called "Sound Analyser" which might be less accurate but will do for my purposes. Did various experiments at various speeds, coasting with and without gear, and through FFT the frequency of 258Hz kept popping up, while most of the noise occurred at frequencies below 500Hz. The following image is from a test coasting at 100kph w/o gear.

http://i374.photobucket.com/albums/oo189/HarryBk88/15591688_10211143158757483_1700670404_n_zps9bbreie l.png (http://s374.photobucket.com/user/HarryBk88/media/15591688_10211143158757483_1700670404_n_zps9bbreie l.png.html)

Putting the car in gear did not affect much the readings so indeed most of the "annoying" noise does not come through the engine, rather from road excitation. Time to fix this...

There are a lot of ways to soundproof any structure; the most common one being mass loading. It is the classical example of having a large flat metal surface hit, producing that tinning sound, and then sticking a chunk of plasticine at the center; it now does not. Very effective, but should be used with care due to the additional mass. What has been found to work well at low frequencies is closed cell foam, which works by having multiple close voids that dissipate soundwave energy. A combination of the two sounds ideal for this occasion; mass loading the center of large panels while foam covering every other surface.

To be continued...

[kung fu jesus]

Lovely! Sub'd for results! I've tried a few things to quiet resonance in my NA. I did have some audible improvement by undercoating the parcel shelf tin cover. I think a less-offensive smell could be accomplished doing the same with layers of plasti-dip paint.

[Agent☣Orange]

I wasn't sure whether to reply or wait for a series. I've done the sound deadening process on a few Miatas including my own and I can say that covering every square inch of surface by mass loading is a waste of time, money and lightness. Noise, heat and vibration are all just various forms of energy but each should be addressed appropriately.

Instead of covering every inch of the interior with Dynamat like on the TV shows, I would mass load broad panels such as the doors and parcel shelf to prevent noise. Then I'd do a layer on the tranny tunnel followed by some Reflectix (feather weight) to block heat.

In the rear, I'd mass load the rear wheel wells a little and then cover with close cell foam pad. Then I'd fill the entire cavity (both sides) with pillow stuffing like a subwoofer enclosure. Yeah, it's not ideal but it weighs next to nothing and sound cannot resonate around that sound chamber.

Finally, blocking the tunnels with foam mat cuts out even more but weighs next to nothing. I know there's a lot of sophisticated and costly stuff on the market but they way I would do it now would prioritize weight savings and targeting correct materials for the job.

[Rogue]

Sub'd as well!

[HarryB]

I wasn't sure whether to reply or wait for a series. I've done the sound deadening process on a few Miatas including my own and I can say that covering every square inch of surface by mass loading is a waste of time, money and lightness. Noise, heat and vibration are all just various forms of energy but each should be addressed appropriately.

Instead of covering every inch of the interior with Dynamat like on the TV shows, I would mass load broad panels such as the doors and parcel shelf to prevent noise. Then I'd do a layer on the tranny tunnel followed by some Reflectix (feather weight) to block heat.

In the rear, I'd mass load the rear wheel wells a little and then cover with close cell foam pad. Then I'd fill the entire cavity (both sides) with pillow stuffing like a subwoofer enclosure. Yeah, it's not ideal but it weighs next to nothing and sound cannot resonate around that sound chamber.

Finally, blocking the tunnels with foam mat cuts out even more but weighs next to nothing. I know there's a lot of sophisticated and costly stuff on the market but they way I would do it now would prioritize weight savings and targeting correct materials for the job.

My thinking is quite similar to yours. Most large panels only require minimal mass loading at strategically placed points to stop them vibrating as crazy. Next step in my quest is removing carpets and use a small mallet to excite said panels, then record their response to excitation and find the respective frequency. After that, place some mass and try again till I am happy. Plan to do this during the x-mas vacation, hope life does not decide otherwise!

[Martin]

I'm cheap and lazy

http://assets.academy.com/mgen/52/10515552.jpg

[Bubbax]

There is a heatsheild on an NB over the muffler that can generate a fair point of noise I put some silicon on it then another layer of sheet metal Probably could have made it lighter than I did but it was effective The foam in the two tunnels to the left and right of the fuel tanks was effective I did the majority of the tub with 1# mass loaded vinyl not that effective There is a website called Sound deadener showdown that has a lot of good info Can someone tell me how to locate the two other tunnels that were mentioned? (Not the ones to the left and right of the fuel tank)
Bob

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[Bubbax]

I also used 1/8 open cell foam to

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[Agent☣Orange]

Can someone tell me how to locate the two other tunnels that were mentioned? (Not the ones to the left and right of the fuel tank),Bob

Bob, there aren't any other "tunnels" besides the ones on either side of the fuel tank. However, there are cavities in the rear, wheel-well areas. Once you remove the trunk and B-pillar paneling, you can see those spaces that act like echo chambers.

[Bubbax]

Bob, there aren't any other "tunnels" besides the ones on either side of the fuel tank. However, there are cavities in the rear, wheel-well areas. Once you remove the trunk and B-pillar paneling, you can see those spaces that act like echo chambers.
Remove the B Pillar paneling from the inside of the cabin that is to the left and right of the rear wall between the seats and the trunk? And the carpet and cardboard cover over the spare tire?

[Agent☣Orange]

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.

[wannafbody]

I shoved car wash sponges in the rear tunnels of my Miata. Seems to have helped slightly. Maybe placebo but it's cheap.

[Agent☣Orange]

Tunnel mitigation helps but the real noise comes from the rear wheel wells.

[kung fu jesus]

I used a 2 pack of patio pillows from a big box store. They are light and materials resist mold and mildew. I stuffed one in each tunnel. They are easy to remove.

[vote4pedro]

I put dynamat on the trans tunnel, rear parcel shelf and along the inner lower 1/4 of the door panels and it made a nice difference, especially with the hard top on. I think doing the B pillars only would add to it (IIRC, mazda put jute on the inside of the NB B pillars).

[kung fu jesus]

I put some mat around the speakers and the scraps on larger portions of the door and I experienced the same as you. It helped a lot. I was surprised doing the same helped the noise coming from dash, too. Just small pieces placed on larger, flat surfaces helped remove some of the echo and buzzing/rattling from there. I put a larger piece on the under side of the instrument cluster hood and that yielded a nice result, too.

[Agent☣Orange]

Good point here about the factory jute. Get rid of that crap as soon as possible. It is basically a moisture sponge and I have witnessed inexplicable and unnatural mushroom-like growth beneath them. If it were anything like magic mushrooms, I'd recommend it but I wasn't brave enough to try.

Regardless, scientific decibel readings really is overkill for a Miata.

[Hammerhead]

Regardless, scientific decibel readings really is overkill for a Miata.

BOLLOCKS!! I want to achieve the same level of quietness in my late-eighties designed, convertible monocoque roadster, as you can find in a modern Maybach sedan.

[NCGreasemonkey]

BOLLOCKS!! I want to achieve the same level of quietness in my late-eighties designed, convertible monocoque roadster, as you can find in a modern Maybach sedan.

I was thinking Rolls.... but tit for tat. All with 70d engine mounts. HT, cage, Borlla, I.L. diff. bushings and weight reduction.

[Hammerhead]

I was thinking Rolls.... but tit for tat. All with 70d engine mounts. HT, cage, Borlla, I.L. diff. bushings and weight reduction.

I think you meant to say, to-MAY-to, to-MAH-to...

[NCGreasemonkey]

I think you meant to say, to-MAY-to, to-MAH-to...

I was going to say Panthers 2015 vs. Panthers 2016. But that was too depressing. And Randy would have had a heyday!