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Why Not Asphalt?

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#1 Rudeboy



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Posted 29 September 2010 - 09:45 AM


Close to half the vibration damping/noise mitigation threads on other forums are debates about using asphalt based materials as a vibration damper. I’ve spent way too much time during the last 5 years typing and re-typing responses to these questions. Let’s keep it in one place to reduce litter and redundancy.

This topic was my introduction to aftermarket “sound deadening”. I had a noisy car, wanted to quiet it down and started to do research. This was 2005. Several respected “authorities” were vocal advocates for asphalt. Part of this came from the belief that products like Dynamat Xtreme were part of a conspiracy to fleece consumers. Part of it was the often repeated “fact” that roofing materials and vibration dampers sold specifically for aftermarket automotive use were exactly the same thing. One cost pennies per ft² and the other dollars so it was worth investigating.

Some of you may remember that Sound Deadener Showdown used to be a testing and review site. Since I wasn’t able to find any conclusive answers on the forums, I decided to buy small quantities of every product I could. Two things were immediately apparent:
  • Nobody had ever had their hands on all of these products at the same time or they would have seen the obvious differences.
  • The people claiming that Dynamat Xtreme and roofing materials were exactly the same thing didn’t know what they were talking about.

I discovered a few other things very quickly. Many sellers were making outrageous claims. Some were wildly overstating obvious physical characteristics like thickness and mass/area. Others were either claiming outright that their asphalt products were butyl or were using intentionally deceptive descriptions like “rubberized compound” to describe their adhesive layer. Nobody seemed to be drawing the right conclusion from sellers’ attempts to hide the fact that they were selling asphalt.

For several years the argument was about durability. Asphalt had an unhappy tendency to melt or fall off. At the time, there were no reports of butyl adhesive failure – that had to wait a few years until one seller decided to re-purpose some low quality butyl roofing material. I spent a lot of time trying to figure out why this was happening. All of these materials are asphalt with some sort of rubber added to increase heat tolerance. The generally accepted melting point for these compounds is 180°F. That seemed high enough for use in a vehicle, so what was going on?

Our perception of the heat these products are subjected to was wrong. We thought in terms of air temperature inside the vehicle, maybe 140°F-150°F? It turns out things are very different at the sheet metal. Park a car in the sun during the summer, south of the Mason-Dixon line and you can easily get to 180°F+. That explains immediate failures in hot climates.

An insidious part of this problem is that many failures were reported during the second or third summer, many in fairly mild regions. The explanation was pretty simple. The rubber added to asphalt deteriorates over time when exposed to temperatures much lower than those required to melt the fresh material. As the rubber deteriorates, the melting temperature drops. Suddenly it doesn’t take much heat anymore.

The durability question has been answered to my satisfaction. Many high profile asphalt devotees, including some who went to almost insane extremes to install the material “properly” have had their installations fail over the years. A few of the brave ones have come forward and made their results public.

The pennies vs. dollars argument still nags. What if you plan to junk your vehicle after a few years (worse, plan to trade it in and don’t care what problem you are passing on)? Is asphalt a reasonable choice when durability isn’t a factor? Nope.

There are two main reasons people believe they can substitute asphalt roofing materials for CLDs:
  • They look alike – shiny on one side, black and gooey on the other.
  • They don’t understand how a constrained layer vibration damper works.
See: Caddy Shack, swimming pool scene for all you need to know about point 1. Point 2 needs more consideration. I used to believe that vibration dampers work by adding mass to a panel and lowering its resonant frequency below the audible range. This is completely wrong. They work through a fairly complex sequence of events that occur in the adhesive layer and between the adhesive layer and the constraining layer and substrate. I won’t go into the details here, please see:

Vibration Damping
By Ahid D. Nashif, David I. G. Jones, John Phillips Henderson

The important point is that everything depends on the material property viscoelasticity. Basically this describes something that can be deformed and will then return to its original shape more slowly than it was deformed. The strains created during these events account for the conversion of vibration to heat that we’ve all heard about. Butyl adhesives formulated for vibration damping are viscoelastic. Asphalt isn’t – it doesn’t need to be to seal a roof.

Asphalt adds mass to a panel. It may stiffen the panel, but since stiffening raises resonant frequency, the two mechanisms are offsetting and reduce effectiveness even further. Those who claim to have used it and had good results aren’t giving you the full picture. These claims can be better stated as:

I used asphalt. It was better than nothing. I haven’t used a proper purpose designed vibration damper so I have no basis for comparison. It hasn’t failed yet.

Sometimes the endorsement is explicitly stated:

I used asphalt and it hasn’t fallen off.

Not falling off is a pathetically low standard of performance. Testing I’ve done has convinced me that it takes between 6 and 10 times as much asphalt to achieve something approaching the same result you will get with a real vibration damper. This puts the pennies/dollars question on its head. Add the durability concerns and the huge amount of extra work required and the answer is pretty obvious.

I’m sure some will want to argue the points I’ve made here. If that’s you, please stay away from “I used it and it worked for me”. That’s how we got into trouble in the first place and doesn’t make any more sense than concluding that cigarettes are good for you because your grandfather smoked a pack a day and lived to be 90. If you want to go there, please show us some evidence.
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#2 Julian


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Posted 29 September 2010 - 09:52 AM

Great post Don.
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#3 98GMC


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Posted 29 September 2010 - 09:53 AM


I vote Don to be on the SSA Tech Team dedicated to the Sound Deadening Forum.
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#4 Aaron Clinton

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Posted 29 September 2010 - 09:57 AM


I vote Don to be on the SSA Tech Team dedicated to the Sound Deadening Forum.

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#5 Duran


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Posted 29 September 2010 - 10:11 AM

Or moderator of this section! Excellent post Don, we're all lucky to have such a knowledgeable asset to the forum.
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#6 Alton


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Posted 29 September 2010 - 10:11 AM

Don, that is a fantastic write up. I enjoyed reading it.
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#7 Rudeboy



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Posted 29 September 2010 - 10:39 AM

Thanks guys.
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#8 Sir-Lancelot


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Posted 29 September 2010 - 10:51 AM

Nicely done Sir.
Thank you.
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#9 Teampapi79



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Posted 29 September 2010 - 04:49 PM

Great read! And another point to mention, Asphalt based "dampeners" smell like a freshly paved road. Not something I wanna smell every time I am in the car. Real hot days and the smell is even worse! I don't care what they claim. When I had some the smell never went away!

Don is the man and I use his products! Wont use anything else as long as he is around! :drink40:
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#10 jay-cee


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Posted 29 September 2010 - 05:20 PM

Great information man and the last paragraph/analogy was killer, keep up the good work man.
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#11 pandoras box

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Posted 30 September 2010 - 01:57 PM

this was really helped and made me watch caddy shack to figure out what you were talking about. anywhay i wam thinking of using something for my car to help with the vibrations and sound. we havent started it yet, but i want to know as much as i can before i start.
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#12 Aaron Clinton

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Posted 30 September 2010 - 02:18 PM

Everyone better +1 for this topic. :)
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#13 SteveMead


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Posted 30 September 2010 - 02:28 PM

its funny how those of us that grew up in the dynomat era dont have a clue as to what a real deadener/barrier really is. atleast myself. :suicide-santa: back then everything was the same. now we have some great products to chose from. Cheers to Don at Sound Deadener Showdown!
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#14 watts09


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Posted 30 September 2010 - 02:34 PM

Good read :) makes me want to learn more about this stuff. :bigok:
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#15 Nick_W



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Posted 30 September 2010 - 05:13 PM

Good read, very informative without being boring. +1 to you, sir.

I do have a question but I'm not sure how well it would fit into this.
Weather stripping?
I'm not sure if it's asphalt based or what but I've heard of a few people using this with supposedly great results. But it seems like it would only add mass since it's not meant for this line of business and like you said, raise the resonant frequency.

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