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canna change law physics
 
red-beard's Avatar
 
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Sound Absorbtion

With one of the other topics, we got on to sound deadening. I'm looking to reduce the sound level in my "new" 914/6. I'm going to get a bunch of the "dynamat" from McMaster-Carr. But...

I found that they will custom make Double sided vinyl covered fiberglass quilts for $5.41/sq ft.

http://www.mcmaster.com/cgi/loadpagerange.cgi?pagerange=*3221,3223&descid=12450

Does anyone know the dimensions of the firewall inside the Engine bay?

James
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Old 04-05-2002, 02:45 PM
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I went the Dynamat route and did the engine-side firewall with two of their "DynaPak's", which I think are 4 sq ft each. I think the rough measurements are 20" x 50". Whatever their short dimension was on those sheets was just right. You're gonna be doing some cutting and pasting, it's not acoustically important if you have joints in the stuff, as long as there are no gaps.

I don't know the McMaster Carr stuff, but something that faces metal towards the engine is a good idea. I covered both sides of the firewall with 2 different types of Dynamat, and it improved the sound in the cockpit, but it's still probably above female-passenger-comfort-level. Probably due to the bulkhead engine mount and reinforced concrete suspension bushings.

I have since been counseled that a LOT of engine noise comes up through the engine lid and resonates the rear window, especially if you are doing one of those GT engine grills. This information dampened my enthusiasm for the GT engine grill, and I'm thinking of pasting my leftover Dynamat scraps to the underside of my engine lid (solid part, that is...).

I've since found another interesting source of similar but cheaper stuff at www.partsexpress.com, no direct experience with that stuff though.
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Yellow '76 914 3.2
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Old 04-05-2002, 05:50 PM
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canna change law physics
 
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I saw some interesting stuff which might help. This stuff is water resistent, and would let water through, and it looks like it would let air through too. It could be mounted under the engine grill to absorbe sound from the compartment and the Carb velocity stacks.

http://www.mcmaster.com/cgi/loadpage.cgi?pagenum=3221&descid=38298

James
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The pessimist complains about the wind; the optimist expects it to change; the engineer adjusts the sails.- William Arthur Ward (1921-1994)
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Old 04-05-2002, 08:45 PM
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Couple of things:

The link above to http://www.partsexpress.com

has a comma before the / so it doesn't work. Try this link:
http://www.partsexpress.com/pe/showprod.cfm?&DID=7&CATID=34&ObjectGroup_ID=37&OBS=10&Move=Next&Pcount=34&CurrentPage=1

I found some stuff on ebay (search under Dynamat).

Questions to the floor of experience (that makes you all "Mavens"):

1. Engine pad is rubber backed by some weird hair-like stuff. It holds water and I scraped it all off. Pad is heavy.

2. Inner pad in passenger compartment is rubber and heavy.

3. I recall from my physics of high fidelity class in college that random pockets of air space would "absorb" sound by preventing it from bouncing around, etc. We did some cool experiments with live rooms (hardwood floors, no pictures on the walls, etc) and then deadened the room with stucco paint on the ceilings, area rugs, pictures, curtains, etc. So flat rubber that weighs a ton is not the way to go. Your car can be lighter and quieter. Take the weight savings and do the doors too.

4. The Engine firewall can be painted with undercoating (Schutz) or bed liner to get the random patterns that will deflect the sound. It is lightweight as well. Adding pads over that would be even more effective.

5. Under the engine lid and back window? Moisture evaporation flows upward, so be careful about air pockets that could trap water. Treatment before insulation, with epoxy sealers, POR-15 or similar couldn't hurt.

So, does anybody have experience with these ideas and such?

Joe

Last edited by retro74; 04-06-2002 at 06:47 AM..
Old 04-06-2002, 06:25 AM
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...in the engine compartment I used the thermo/sound dynamat. Then I took My original rubber mat and stripped of the funky hairy stuff adn install teh rubber over the Dynamat...Works very well and still tllos stock...
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Old 04-06-2002, 09:36 AM
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Joe,
Sorry about the extra comma at the end of the link, guess the BBS s/w decided it was part of the link when I was typing.

Not sure what your question was, but the dynamat (and similar stuff) embodies the principles you mentioned. Depending on which product you use, The stuff is a sandwich of closed cell foam and some really gooey neoprene or PVC stuff (which is much more compliant than any undercoating I've encountered). You can get foamA/lead/foamB or black goo, or black goo with metal heat barrier. That goo is like bubble gum on a hot sidewalk. Anything that will dampen the oil can resonance of the sheet metal is a good thing, but I'd trust the geeks at dynamat to have tuned the "gooey-ness" for wide spectrum attenuation.

I don't think coating the underside of the engine lid with this stuff is a rust risk, neither the goo nor the metal absorb water and ya gotta not trap air when you install it if you expect it to stay put.
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Old 04-06-2002, 09:56 AM
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Two questions, if anybody has a minute:

1. I read the info at the Dynamat site, and my head is still spinning with the many choices of types of Dynamat, liners, etc. Anybody remember exactly what product(s) they used with success?

2. I remember reading *somewhere* a couple of years ago about somebody who had installed two stock rear 914 windows "doubled-up," one pressed against the other, and raved about the cockpit noise reduction that resulted. Anybody have any details on this? Any trick to it? Wouldn't moisture form between the two layers of glass?

Any info appreciated.
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Old 04-08-2002, 06:57 PM
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Lightbulb

Hello,

Check out this product. . .

www.b-quiet.com

Their prices are much more reasonable than dynamat, and from the reviews, looks like lots of car stereo competition folks are using it. I'm going to buy a big roll for my 914 and a Datsun Z.

-'Chung
Old 04-08-2002, 09:17 PM
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Sorry to clog with more posts, but I've been looking at sound isolation recently because I'm building a recording studio. . .

3 ways to stop sound from coming in through a wall. . .

1. mass. . . takes more energy to move a heavy wall than a light one. . . that's why a lot of car sound deadening materials list weigth per square foot as a reference.

2. Convert kinetic energy into heat. . . stick something goey to the metal that will turn vibrations into heat.

3. Isolate. . . in a house or studio, it's floated floors, ceilings, doubled windows, and walls decoupled by rubber spacers. This I guess is the strategy behind B-quiet.com 's ultra-light stuff. . . isolating layer and then, another stiff aluminum layer. . . (but it's not flexible and harder to install). Must be careful that the 2 decoupled walls do not have shared resonating frequencies that would let certain sounds through.

The foams do more in the way of absorbing sound so it doesn't bounce around in a space than to stop it from travelling through a wall. . .

If you're going for super-quiet, it's going to be heavy, but 2 layers of sound deadener works much better than 1. . . which makes it doubly important to find something REASONABLY priced as opposed to dynamat.

Stepping of soapbox ,

-'Chung
Old 04-08-2002, 09:37 PM
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I had the interior side of my firewall sprayed with bedliner. That and the newly recovered backpad seems to work fine. I have just the red paint on the engine side. I only have a 2.0 four for now.....
here is a picture of the engine side
http://pelicanparts.com/swapmeet_pics/GermanAutoFest-01/Pic475.jpg
Bee Jay
Old 04-08-2002, 10:05 PM
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I used the aluminum faced Dynamat on the fire wall......still sounds like the hounds of hell are on the loose bout 18 inches from my ear....cause they are. Webers don't bark, they howl. I live with it.

Got a pic, somewhere.bad angle, tho.
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Old 04-09-2002, 12:34 AM
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beejay, where did you get that fine looking throttle cable holder?
Old 04-09-2002, 05:52 AM
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Geez J P, every time I see a picture of your car I feel guilty. If I had a car with a motor and engine bay that looked that nice, I'd probably spend at least an hour a day just staring at it.

Hmmm...you used the good dynamat and it still sounds like the hounds of hell right behind your ear? I think I'm going to have to think of a different analogy before coaxing my wife into the car for a roadtrip....
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Old 04-09-2002, 06:58 AM
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Bob,
Used the aluminum faced "dynamat extreme" on the engine side, and their "extremeliner" on the cabin side. Not quite the hounds of hell (mfi might also be quieter than carbs), but still not conducive to listening to the local classical station. I'm still trying to decide how much noise is due to the firewall vs the back window vs the firewall welded engine mount with the delrin bushings. Not much isolation there...

The double window idea sounds intriguing, shouldn't be too hard to sandwich two panes together with some of the gasketing used to make "thermopane" windows. I wonder how mount-able that extra thickness would be though.

BTW, nice '74! Wish I had some extra bucks...
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Old 04-09-2002, 07:47 AM
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Just a thought, I think one of the original uses of dynamat was in cars with big sound systems. They use it to absorb vibrations not sound. A good acustical material is open cell and textured.
Old 04-09-2002, 08:32 AM
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Vibration. Sound. Pretty hard to separate the two. Vibration is how sound is transmitted across a barrier, i.e. firewall or window. Different materials have diferent sound transmission characteristics, but eliminate vibration of the barrier, say use a 1 foot thick concrete wall, and transmission will be reduced significantly. Of course, so will acceleration! :-)

Mike
Old 04-09-2002, 09:19 AM
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I expect that when these guys are talking about vibration dampening vs sound, they are more looking at the frequencies than really trying to distinguish between the two.

At 6000 rpm, the crank "hum" will be about 100 hz. You would feel this more than hear it. And there are other noises of different frequencies to deal with as well.

I am seriously looking at the 1 inch "Speghetti" mat to place under the engine lid to reduce the carb noise. It should still allow breathing and ventillation of the compartment.

The McMaster Carr prices look much better than the b-quiet place. And I think I like the idea of a beige paint in the wheel wells and underneath the car instead of light blue...The Asphalt sound deadening material they sell is about a buck a square foot.

I was thinking about 50 sq feet...Floor panels, under the seats and behind the back pad and maybe the engine side of the firewall and the door panels. Then using the sound deadening paint in the wheel wells, front and rear trunks and under the hoods and in the engine bay. And a mat under the engine compartment lid. I expect about 50-60 lbs extra weight. But I think the six can handle it.

James
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Old 04-09-2002, 09:53 AM
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James, what is the spaghetti mat you are talking about? Where did you spot this stuff?

John
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Old 04-09-2002, 10:06 AM
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McMaster-Carr

Water-Resistant Sound Absorbing Sheets
Made of bound polypropylene beads, these rigid, flat sheets will not degrade when splashed with water or saltwater. Often used as a liner in moist environments such as marine air inlets. You can cut sheets with a razor knife and mount with a rubber-based adhesive such as 67015A on page 3134. Temperature range is 0 to 240 F. Color is gray.
Thick. Sheet Size NRC Each

1" 36" x 48" 0.50 9107T1 $52.21

2" 36" x 48" 0.50 9107T2 80.63

Go to the McMaster-Carr website ( www.mcmaster.com )

In the search field type "sound"

Then look for the sub-heading "Sound Aborbers (6 pages)"

Click on the (+) and then go down the list the the last item:
Water resistant sound absorbing sheet

James
Attached Images
File Type: gif 9107tp1s.gif (8.8 KB, 445 views)
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Old 04-09-2002, 10:15 AM
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Thanx.
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Old 04-09-2002, 10:21 AM
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