What is sound deadening and how does it work?
Of the few things we vintage car folks have to concede to modern car lovers is the fact that our choice of locomotion is loud. All the noise is worth it for the experience, but taking out some with the use of some hidden modern materials is about as controversial an opinion as upgrading to disc brakes. For all the support for sound deadening and damping, there is an awful lot of confusion around the web regarding usage and expectations.
On modern cars it is called NVH: Noise, vibration, and harshness. These three aspects factor into so many points of vehicle design in the modern era. As cars became a near-mandatory part of society, the time we spent in them increased and brought along an expectation that we are comfortable in them. Even a Model T is comfortable when you consider the time it was built.
“What did you say?”
Let’s address the first of the trio: Noise. This is the sound waves emitted from various parts of the car as they function. The lifted Jeep Gladiator next to you on the highway chose those mud terrain tires for function but the side effect is a lot of noise, especially when the tread gets worn. Even highway tread tires make some noise, so no one is immune. There are a multitude of other items on this list including exhaust, heater blower motors, suspension creaks or squeaks…
New cars have all the same noises that we notice on our vintage cars. They are just muffled, insulated, or disguised. Some are just plain quieter too. Insulation for this part of the NVH formula is usually a product that is designed to cut the transfer of sound waves from where they are created to where the driver sits. Paddings and foam layers do the task well but coverage and placement are key as the goal is to cut down on the noise but also not end up in a tiny, mobile recording studio. Even a Jaguar isn’t meant to be Abbey Road.
The whole goal is to cut the oscillations of the sound waves. The power of a sound wave diminishes as it passes through a material, and certain materials like foam, cardboard, leather, and synthetic fabrics, and even the passengers ourselves, absorb and reduce those sound waves before they can get to our ear drums. That is what we perceive as a reduction in noise.
Another huge part of noise control is sealing the outside, well, out. That means weatherstripping. The foam rubber that seals the window to the door and the door to the body are all places where sound and therefore annoying noise can leak into your car. Sound waves will diffract after passing through an opening which causes them to bounce around inside a car and feel even louder.
Why is there a Home Depot link in your car forum?
That’s pretty much it for stuff that makes noise as it functions in your car, but what about stuff that makes noise just from the nature of driving? Sounds funny to say, but we are talking about stuff like the sheet metal of door skins, floor panels, and firewalls. Things like this that have any large flat panels become defacto speakers when the vibrations that come with putting hundreds of spinning parts inside a larger bunch of spinning parts all going down a bumpy road come into play. Things are going to vibrate.
But similar to the thoughts above about changing the sound wave size rather than eliminating it, adding mass will slow the movement. Less shaking, less noise. This is why there is a great debate about sound-damping materials. The most popular options are your Dynamat and Husthmat types brands and the idea is to add mass. Those products come in a foil-backed butyl rubber sheet that can stick to just about any part of the car that can vibrate. If your hands are of the roofing callous kind, this material sounds familiar, but don’t be tempted to go to the home center and grab a stack of asphalt insulation tiles and start sticking it on the floor.
Butyl rubber is less susceptible to both heat and off-gassing. If you think new car smell is bad, imagine the smell of a warm asphalt roof each time you get in the car on a warm summer day. Asphalt would also likely slide down any vertical surface if the car got nice and warm like a summer day in the sun. That could seal off drains in door panels, pool up and make for uneven floors under the carpet, or melt onto wire harnesses and cables making working in the future even more difficult.
The second big debate in this space is coverage. Listen to some people and every square inch covered is the only way it’ll ever work. Here’s the thing though, the science doesn’t back this up—and neither do the manufacturer recommendations. The very companies trying to sell you the stuff typically say to cover 30 percent of available surface area for any noticeable effect, and past 60 percent there is a diminishing return. That’s in direct competition to their bottom line. Why lie?
The science is even simpler. A large panel catches the right harmonic of vibration and is now emitting tiny pressure waves that hit your ear drums. All we have to do is change the frequency that the panel vibrates so it is outside of most normally appearing driving conditions. A thin sheet of rubber and foil puts enough of a kibosh on the party vibes that sheet metal has a dull thud rather than a tinny sound when hit with a stone like in a fender well or floorboard. Larger panels like roof shells or firewalls will quiet down significantly as the panels will move less. Less movement, less noise.
Harshness
All cars fall somewhere between the Beast of Turin and a Rolls Royce Phantom. Anything can improve if you want to really engineer a solution. Does it always make sense in putting a band-aid on a bullet wound? Are any of the add-in sound damping and deadeners sold for vintage cars magic bullets to modern comfort? Hardly, but with proper application and expectations you might enjoy driving your car even more than ever. But remember, that Rolls Royce Phantom has hundreds of pounds of insulation material and engineering that is all focused on separating the occupants from any sign that they are indeed driving an automobile.
Harshness is the combination of how the sounds produced by the car wear on the operator. This one is less quantifiable and more personal. While the noise level can be measured in decibels, harshness is unique to each person. I’ve been able to tolerate weeks driving Hagerty’s 1917 Peerless Green Dragon which is one of the harshest vehicles I’ve spent real time in, yet I find my 2015 Chevrolet Express van in need of more sound deadening and insulation. Different expectations.
This is all to say that insulating your vintage ride can be well worth the effort but be sure you understand what products you are using, how they are designed to work, and what you should expect from them. A full layer of Hushmat brought down the noise in my 1965 Chevrolet Corvair, but my wife still finds it too harsh to enjoy. Her loss, but more importantly not my hearing loss—at least not anymore.
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Dynamat on the metal surfaces made a HUGE difference in my 67 Mustang Fastback, along with mass back carpet and insulation above the headliner. One controversial aspect of the adhesive sound deadener is some feel it could hold moisture / water and promote corrosion. Personally I think the reduced sound level is worth any risk…plus my car rarely sees wet conditions.
A quiet car does make long trips more comfortable.
And short trips as well…
When I had some small door dings removed from my older 911 some years ago, the technician mentioned that he was glad I hadn’t installed adhesive sound-deadening material inside the door panels, as that would have made his task impossible.
An interesting point. I have always been careful about when and where to apply just based on how difficult it can be to remove if there is ever any work to be done in that area. PDR hadn’t even crossed my mind.
That PDR is a good point, however the inside of my 67 already had a factory applied sprayed on sound deadener that would have prevented PDR.
Plenty of PDR tricks done from the outside, so all is not lost!
In my modded ’68 mustang I have a heavy Holiday Inn rug (don’t ask) cut up and covering the floor under the carpet. Even with glass packs it’s a nice quiet ride. My ’65 is factory with only the dynamat that you get from mustang venders, and even with the factory exhaust and no go fast parts, it’s louder inside than the ’68. And it’s easy to guess which one gets driven more.
For those of you on a budget or just plain cheap. Get yourself some of those windshield sunscreens. The ones with the foil on both sides. You will need more than one. Cut to fit the floor pan or roof if you are doing the headliner. You can glue it down in spots if you want with a hot glue gun. For the roof I just tuck it under the metal bows. Every time I saw one at a yard sale or second hand store I bought it. I think every one was brand new and never used and never paid more than 2 dollars for them. No need to pay big bucks for dyna mat. Your welcome.
I like saving money were possible, but I generally hesitate to use something not designed for an application in a location (like above a headliner) where it would require removing my windshield, back window to remove the headliner to fix if it failed.
Why in the world would it require you to remove the windshield or backglass?
…a better question might be “how does a sun-shield fail?”
Do those not crinkle when stepped on if they are under the carpet? That doesn’t seem like it would dampen sound much if at all since it would just be set on the surface rather than adhering to add mass and thus limit the sound.
I found that these sun visors deteriorate badly as they age. Did you ever have to go back to an area you put them? This is a great idea I may try !
Sound deadening has come a long way. I am a OEM supplier, and provide parts used throughout a vehicle to reduce vibration . Most are variations of foam with PSA to hold them in place. Thinsulate is one material. Zero torque foam is another. This includes body panels, inside foam pieces of elastomer that expand ten times when activated through a heat tunnel and turning into foam, between the inner and outer skins of a vehicle…foam behind the dashboard, and even carpet on the inner roofs of Broncos…..all sound absorbing. Sound deadening , like most upgrades, starts from the top of the line and works downward…..some manufacturers are fanatical….ie Rolls Royce…….:}
Not sure if you’re “Dearborn Lee” but if so, ALWAYS a treat to see you and your cars at the village. You’re awesome!
As to your post, couldn’t find anything on Zero torque foam – plus assume PSA is Pressure Sensitive Adhesive tape?
I too believe in top down approach, and currently doing a large-scale NVH project (specifically a 34.5ft ’87 Airstream Chevy P30-based motor home) where I’ve beefed up lining around the biggest noise zones plus also considering using 1/4″x 1’x 1′ square felt panels to line front and back ceiling caps plus a 3 ft wide strip down the center to reduce the booming effect. Any recommendation on adhesive to keep these felt tiles from dropping from the ceiling? (Trying to avoid using high smell/VOC contact cement or additional holes if using metal ribs.)
My complaint for years has been that there is no way to know which brand is better than the next. They all say theirs is the best. Why can’t the industry develop standards such as used with house insulation, i.e. R12, R24, etc. so we can compare and not listen to a bunch of BS. I have never seen any scientific proof of how good, or bad, any of the insulation products are.
Some but not all products talk of decibel reduction as a means of measuring, but that is really tough because decibel measurement is logarithmic. A 10 decibel difference is ten times quieter/louder. A 20 decibel difference is 100 times quieter. Since most don’t understand this, they see a 3-4 decibel difference advertised and think “well that’s nothing, why would I do that?” When in fact it is a big difference. Elevating public knowledge to understand logarithmic sound values would be a sizable uphill battle.
I see your point, but at least they could put some kind of standard value that we could see/read and make our choices that way.
You can see this web page for sound deadener testing that gives you the exact info that you are looking for.
https://resonixsoundsolutions.com/best-sound-deadening-material-independent-testing-data/
I agree on the ‘standards’ idea..but sometimes the purpose varies; in my chevy work vans I used 1/2 to 3/4 white foam board scored and glued to fit the roof… nothing worse than working in the back of a ‘tin box’ when the snow/ice/ minus 30 hit.. talk about an ice cream headache! [ similar effect in the old ‘glass sunroof’ cars or yore’… no sliding covers back then to block cold/sun ] … but I have used ‘mat’ material in several vehicles.. my GR 86 especially on select panels, same in my Prius V, and loaded up my Tacoma with it… all are ‘drone boxes’ on our crap roads. And as a stereo fan I have ‘tuned’ the nvh on most of all I owned… first one? a used mid / late 80’s Ford Courier pickup.. by far the loudest tin box I ever owned… the product needs to fit the desired outcome.. nth? heat? cool? all? or ‘luxury feel’? ~ keep driving!
I am surprised at the omission of KoolMat in the article. It not only cuts the noise but is great in relation to keeping heat out of the cockpit. Ask anybody who has used it on an E-type Jaguar. Also easy to layout.
@Jerry- I was going to ask the same question. My C3 Corvette (No air-conditioning, manual transmission) was miserable in stop and go traffic on an 80F day. The engine is a foot in front of you, the transmission is by your side and you are sitting on the differential. An interior made from KoolMat might have been a solution.
I own a 1984 C4 with a 6speed trans. When 6speed was installed, it appears heat and sound shields weren’t installed. My console gets hot when car get warmed up. It gets cold when I run AC. The trans whirl is very noticeable.
I have to believe there is something simple that I can do to control heat and sound.
Any good suggestions?
Brown Bread is another. Used it with my Dodge Cummins.
As the owner of what is possibly one of the nosiest cars ever made (an Opel GT) I attacked the problem in several ways. First was when the car was completely stripped of paint I did the interior with truck bed liner as well as the entire undercarriage with color match liner. I used 2 self-adhesive “sound mat” products when putting the car back together that made a world of difference in how quiet it is now. Those were: Siless Liner 157 (4 mm) mil 36 & Noico 80 mil. Both available from Amazon.
I noticed everyone talking about cars, I am interested in buying a 49-50 chevy pickup. If you ever closed a truck door from these years it sounds like a large ti can. Was wondering if placing a sheet of sound dentening inside the door as well as head liner if it would make a big difference. And wereelse would make a difference!
I have a ’92 Olds Custom Cruiser, big engine, loud pipes.
While changing the interior to a ’73 442 motif (buckets/console/shifter), I also replaced the carpeting.
I installed a repro early ’70s era square weave carpeting kit (after removing the original style jute backing) right over the original rubber-backed GM mouse-fur.
I shaved the fuzz from the original rubber backing that would be under the sillplates so it would not be too thick with a double layer.
The inside is now much quieter, and the drumming of the exhaust is no longer an issue that intrudes into he car.
Have any of you used the spray on Lizard Skin ? I’m curious to know how it compares to adhesive type foam/pads.
I purchased a 68′ El Camino with rotted-out floors and Cherri Bombs that sat directly under the passenger floorboard (what was left of it). Talk about loud. Replaced the FBs and added a layer of Coolmat topped by a 1/4″ foam pad (Lowe’s grey stuff) finally covered with new carpeting. The doors, roof, and back of the cabin had a layer of Cool King insulation foam (aluminum backed, again, Lowe’s) under the carpet, and headliner which cut the sound and heat dramatically. Several folks commented on how solid the doors sound when closing. 7 years later and all are still working great. Also, only covered the largest areas, not every square inch. Not needed IMO.
Lizard skin works great but it is a spray-on product so plenty of prep and masking off what is not to get a coat is required. YMMV.