Sound or noise control is a big subject in the exotic 4x4 world. Home-grown and random ideas as to what might work to reduce noise often hurt the value of these vehicles, or is not very effective when the costs are weighed out. Reducing sound has been a sort of weird hobby of mine, as well as business in the past when building up custom 4x4s for a paying clients. I'm going to share what I know here based on actual experience, training and exposure to professional sound control products and their application in real world vehicles.
You will never get to the level of a quiet car or modern SUV but you can at least get half way there, or maybe better. Noise causes stress and is a major safety issue! This is why it's important and why noise pollution overall is vital to controlling in all of our environments.
THE NOT SO GOOD METHODS:
- My own #1 rule is to never use anything that could promote rust or hide a corrosion problem. I think it's very dangerous to use any "bedliner" type of material on horizontal surfaces. I have seen many 4x4's ruined this way, starting back in 1992 when I dove deep into the world of older Land Rovers. The Series and Defender models are very loud and some people tried to quiet the interiors this way. All it takes is a microscopic hole in that "paint" to let O2 and H20 do their thing and a few years later, you can be looking at a very costly and time consuming rust repair, or you may even end up with a parts truck! Have you tried removing that stuff before?! Don't make the next guy have to do it.
Also, rust often starts on the other side of that piece of metal but you may never know it because the bedliner is covering it up, as well as making it appear to be structurally sound.
- I have seen a lot of "stacking" over the years as well. This is what I refer to when guys will buy a ton of vibration dampening sheets (Dynamat, Rattle Trap, etc) and put it on everything in layers, which is kind of like trying to put out a brush fire with loads of gravel. Along the same lines, rubber mats and vinyl trim have been applied in the same way but none of this usually satisfies. In theory you can get some results from this stuff alone but the final results, weight addition, money spent and risk of hiding corrosion isn't usually worth it.
I'm sorry but it has to be said: Nothing made for the roofs of houses or hotwater heaters is going to be practically effective in managing noise in a 4x4.
WHAT REALLY WORKS:
The engineers who do this stuff for big auto companies and other industrial applications know how sound propagates in a vehicle and over the years materials and techniques have been developed to deal with this specifically.
Air is what allows sound to move. With no air, there is no sound. All noise control techniques involve blocking air or keeping something from making air move. We generally talk about noise in a car by splitting it up in frequency. Lows, which you feel in your chest and by touching the surfaces that produce them, and you can hear some of them as well (tire rumble, vibrating metal, bass).
The mids, where a lot of voices are and you can hear all of (road noise, wind, engine).
And the highs, which can really annoy you (whistling and gear whine!).
It's proven that some sounds on the extreme low or high can affect you subconsciously even though you can't fully hear them.
1- Vibration dampening This is the foundation of noise control treatment and really helps lessen your lower frequencies. There are a ton of these materials on the market. I use the Rattle Trap or the Dynamat brands. The bad thing about this stuff is the marketing. These are only one part of the process but they often make it sound like this is all you need; absolutely ridiculous!
Above is a Pinzgauer 710K ceiling being treated with Rattle Trap. This was covered over by custom trim panels which were then auto-carpet covered.
These dense sheets of flexible, rubber/asphalt type materials effectively increase the mass of whatever they are applied to and will keep them from vibrating so much, thus reducing the noise they produce and re-transmit. They do this well but they add weight and hide corrosion. They should not be applied to the floor. I use them on ceilings, walls, engine covers, some wheel wells, behind dashes and door skins. Always make sure they are applied over perfectly sealed/painted surfaces and use a heat gun and roller. Total coverage is not needed. Around 30-50% coverage is the goal. I double up on troublesome spots.
2- Sound barrier or blocking This is probably the most effective overall material in the process but I think it's the least applied and understood. This stuff is basically a dense "mass loaded barrier" sheet that is "decoupled" from its surroundings by a layer of foam. Dynamat makes some under the name DynaPad. Cascade Audio also makes a series with names like VB-4, VB-3, etc.. they all work the same and literally block sound waves from coming at you. They do this by suspending a heavy, dense layer of vinyl (or lead) with foam which is designed to lay on the floor with no need for glues, etc. It works really well when carpet is glued to the top of it. It usually weighs 1lb per sq. foot so the weight additions can add up fast. Cascade makes one called VB-4.5 which I used in part of my Gwagen. It is .5lb instead and is very flexible. Their lead-lined stuff is great but its very expensive.
Cross section of the VB-4 sound barrier.
Above is a 300GD Gwagen getting a full floor treatment of VB-4.5 and DynaPad. The shifter area was covered as much as possible to reduce sound from getting past the control openings. The materials on the floor should always overlap so no air gaps are possible.
The key is total coverage and an "air-tight" approach. It should cover every bit of flooring you have, including under the seats and should not have gaps or holes except where 100% necessary.
Another older but effective material I use sometimes is Thermo-Tec. It is about 3/8" thick and is a mat made with syn. fibers with an aluminized outer layer. I use this as a backing for carpet and works great when you lay this over a high-end material like Dynapad. It's also a great heat barrier and costs very little compared to the other stuff.
Above is a Pinzgauer's floor after being covered with a product similar to the VB.
3- Sound absorbing This is rare to find done correctly in auto builds. It's used extensively on planes and the new bullet trains and should be part of any 4x4 build I think. The most effective material for this is 3/4" to 1" Melamine foam which can absorb 4K Hz and above. If you can find it, Hydrophobic Melamine is the best since it won't absorb water. This is ideal for cavities in walls and behind trim, etc.. Unlike any other material, it will actually help to absorb the mid/higher sound waves around it.
In the sound absorbing category you also have carpet or any porous outer trim, like headliner fabric. This is very important!! These materials are the final straw for annoying noise. They very effectively keep sound waves from bouncing all over the place. Your ceiling is a major factor in how much noise gets to your ears. Ever notice how quiet a soft top is with the top off (other than the wind and outside sounds)? All that noise we are trying to control just beams up into the air when there's no top to reflect it.
Above is a 710K that had a custom headliner panel made. Under it is Rattle Trap to keep the roof from vibrating, in the middle is a sound absorbing foam (in this case, Dynamat's HoodLiner foam) and an auto-grade carpet was applied over the panels for a night and day difference in the noise levels.
Above is the 710K's rear ceiling finished.
Above is the 710K's finished front cab. The floor bottom was left alone in this case but the area behind your leg was treated with Rattle trap and covered with carpet. A small patch was added beside the transfer shifter. I had preferred the shop make that a little more finished looking but the budget was very tight.
Above is a before and after of a Mercedes G-wagen 300GD. The stock headliners are usually a hard plastic material and there is no vibration dampener on the massive, flat metal roof. Rattle trap was added, 3/4" foam was put in-between and a 1/8" padded headliner cloth now covers the old headliner. The reduction is sound was easily noticed.
So these are the basics. You need to reduce your lows, mids and highs by dampening, reflecting/blocking and absorbing, and you want to do this without sealing up your floor where rust WILL hide out.
The Pinzgauer, especially the M models with their soft tops, have unique issues.
The ceiling really needs something to absorb the sound. Aftermarket tops are the best but an expensive solution. Some people have made their own, soft under-layer. Anything you can do will help massively in keeping those mostly high-frequency sound waves from bouncing back at you.
The floor in the back: A sound blocking material goes here. I used the 1/2" Barrier matting material from http://www.heco.net/noise.html with great results before. The key is also to cover the sides as well, and keep that air-tight coverage thinking in mind.
The major treatment areas up front are under your seats, the engine cover and the doors. I lined an engine cover on the inside and then had an upholstery shop cover the outside with great results.
You can also block a lot of noise by making a barrier to block the bed area from the cab. The stock flap is thin and doesn't do much. You can modify that or make your own. Again, keeping that air-tight approach in mind.
And if you haven't already and for the sake of noise and safety, GET RID OF THOSE MILITARY BIAS PLY TIRES.
Technical articles by owners, for owners.
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