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Old 06-26-2018, 12:06 PM   #1
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Installing Sound Deadening Material - Engine Compartment Noise

Not sure if this is the best place to post this, but since we did this for our 78 Argosy MH, I am posting here, a write-up my husband put together on the recent installation of sound-proofing in and around our engine compartment doghouse. Admin - if you think it is best moved elsewhere, please do so.

Due to noise considerations in the cab, we decided to install Sound Deadening Material (SDM) and Sound Absorbing Material (SAM) in and around the doghouse. There were several options available but all sounded basically the same:
SDM: Is said to be “viscoelastic damping compound with aluminum foil backing (“viscoelastic” appears to be a petroleum-based rubber-like material). All suppliers have this in about an 80-mil thickness. It generally comes int 12" x 24" sheets and in 18" wide rolls.
SAM is an acoustical foam with a foil backing and an adhesive face. There are a few varieties but be sure to use the "under-hood' rated product.

For timing reasons regarding installing the material and to see if one product may work better than another, we ordered both SDM and SAM from Rattletrap and Summit Racing. On the SDM, there did not appear to be a difference. For the SAM, Boom Mat was slightly thicker than the Rattletrap SAM (.750" vs. .810"). The BoomMat was also slightly smaller in total square footage. Having to commit, I used the RattleTrap SDM and the BoomMat SAM. The two SAM products are: FatMat Aluminum Hood-Liner (RattleTrap) and Boom Mat Underhood Thermal Acoustic Lining (Summit Racing). There really is no way to tell if one product is far superior to another.

Regarding the RattleTrap SDM, I ordered the 50 sq. ft. roll. I figured that this would be easier to cut pieces from a roll that would fit better in the various places under the dog house. This was good strategy but I ended up with a lot of extra material due to very little wastage.


SDM Installation: When we started, the only remining original SDM for the engine compartment was a piece of 1/8” rubber glued to the top side of the stationary doghouse cover. Everything else was gone from age / deterioration / removal.

Installing new SDM is fairly straight forward. I washed the engine compartment with degreaser and rinsed with water. As our engine compartment is kept pretty clean this was easy enough to do. If you have a dirty engine compartment, clean it until there is no oil / greasy residue.

Almost all of the SDM pieces that were installed were rectangular with 90° corners. As the main intent of SDM is to prevent ‘oil-canning’ and structure-borne sound transmission, I left about 1/4" of exposed metal all the way around each piece for easier installation. I found that cutting with a good pair of scissors was easier than a utility knife. With either tool, mark with a Sharpie and cut. Once cut, dry fit the piece to make sure it is a good fit.

Now comes the part that is a little hard to describe. Do not peel the entire backing paper off when you are ready to install. SDM is very sticky. The warmer it is, the better and more aggressively it sticks. You will not be able to remove SDM once 2 to 3 square inches are in contact with anything. I found that installing as follows worked the best: Peel back the paper in one corner about an inch or so; place the SDM where it belongs with the backing paper between the SDM and the metal. Reach in between the SDM and the metal and gently pull the paper off while smoothing the SDM to the metal as you go. This is somewhat difficult in tight fit areas (e.g., above and behind the radiator, sides of the stationary doghouse). This may take two sets of hands but it is the only way to get it installed in tight fit areas. Installing SDM under 70°F is more manageable.


Installing SAM:
Cut the SAM pieces to be reasonably tight fit on the edges as SAM is much more of a sound insulating material than SDM. At corners individual pieces of SAM should touch. SAM cuts much better with good scissors than with a utility knife. For tight-fit areas, use the same procedure as with SDM (peel back a small corner; slide into place; then peel off the paper smoothing as you go.).

One thing I did not find mentioned anywhere was the treatment of edges and joints of SAM. I even asked one vendor and was told that the foam edges can remain exposed. This seemed to me to be a poor option from both an appearance and functional point. I sealed the joints and edges with aluminum foil ducting tape. Good choices are Nashua or 3M tapes that are rated UL 181 A-P or UL 181 B-FX. Do not use cloth-inserted or vinyl duct tapes as they will not last.

Results:
Adding SDM and SAM was not a huge improvement but is noticeable. Unfortunately, we did not have a decibel meter to quantify the results but it is better. The noise reduction we did get was the low hanging fruit. We can now identify areas that will need further attention, those being:
The bulkhead between the fuse box and the street side outer wall (behind the drive seat). While we looked at this area when addressing the rest of the engine compartment, it was skipped. The area looks fairly easy to install SDM and SAM.
The doghouse cover seal. This area was somewhat of a surprise regarding noise (and air) transmission to the cab. We have solid rubber (1/2" x 1") gasket for mating with the doghouse knife edge. This will take a little effort to get the knife edge to contact 100% on all three sides. Right now, the back edge contacts before the two sides. With this, the vertical edges at the hinge are slightly open which lets in the air and sound.

For us, the quietest ride is at 40 to 55 mph. This is the range where the fan clutch disengages but before all the other road, engine, and air noises escalates. If we could find a 5 or 7 blade cooling fan, we might attempt another go at changing out the fan clutch (going from the 'heavy' duty fan clutch to the 'severe' duty).

One other benefit is the thermal insulation on the doghouse. Prior to insulation, the top of the doghouse would get noticeably warmer while underway (say 15-20°). With the insulation, there is no real temperature difference from ambient.

Cost:
SDM ranges from about $2.00 to over $5.00 per sq. ft. RattleTrap was the least expensive. RattleTrap appears to work as intended and it does stick very well.

SAM: BoomMat and RattleTrap SAM cost near the same at about $65.00 for 12 sq.ft. As we had bought both and needed more than 12 sq. ft., both were used.

Our total cost was about $235.00. We have about 30 sq.ft. of SDM left over some of which will go in the generator compartment lining. There will about 6 sq. ft of SAM left over once the bulkhead and a few small areas are insulated. With good layout and preplanning, there is not a lot of wastage although I don't like piecing together several small pieces to cover one single flat area.

We took some pictures of the work in progress, but because of the glare from the foil, they aren't very helpful. But I've attached 3, one of the doghouse before foil tape was added, and one after and also one of another panel showing some detail of the one product installed over the other.

If you have any installation questions, let us know and we'll offer what we can.
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Old 06-26-2018, 12:51 PM   #2
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Great post Belle, very informative!
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Old 06-26-2018, 01:52 PM   #3
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Hi Martin - thanks for the feedback. At first I was going to add this to my original post where I asked about soundproofing but then decided a stand-alone might be better as it would be up on top instead of buried in the other comments.
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Old 06-26-2018, 05:38 PM   #4
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I tried to edit my post but couldn't figure out how so will add a correction here:
For the SDM, I had said:
"I found that cutting with a good pair of scissors was easier than a utility knife. "

Having just cut a piece I need to correct that. For the SDM a utility knife works better.
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Old 06-27-2018, 06:30 AM   #5
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Nice write up. I appreciate the information as I will need to deal with my doghouse in the near future. Also, my two wheel well covers and generator cover will likely benefit from some sound proofing as well.

Thanks,

Brad
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Old 06-27-2018, 11:57 AM   #6
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Nice - need mechanical attachment too?

I like this idea a lot. Any concerns about material losing adhesion due to heat, grease, vibration etc? Wondering if a mechanical attachment might work as back up. Thanks for posting!
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Old 06-27-2018, 01:50 PM   #7
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For the cost of an AI, this should have been done already at the factory. IMHO
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Old 06-27-2018, 04:04 PM   #8
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Quote:
Originally Posted by Doc Foster View Post
For the cost of an AI, this should have been done already at the factory. IMHO
It was insulated at the factory, the problem is these coaches are all old now and the insulation has for the most part fallen off and/or deteriorated so badly it's about ready to do so.
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Old 06-27-2018, 05:09 PM   #9
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Quote:
Originally Posted by Doc Foster View Post
For the cost of an AI, this should have been done already at the factory. IMHO
Doc, these are very old Airstream-built motor homes, not the 'modern' ones built on a Sprinter van chassis. Completely different item that LOOKS like a "real" Airstream...
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Old 06-28-2018, 05:27 PM   #10
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Sorry, I guess I wasn't paying attention
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