Sometimes just upgrading your stereo components isn't enough. Sound deadening your car is the next added measure. Eliminating resonance, road noise, rattles, & squeaks. These measures improve midbass, overall sound quality, acoustics, and efficiency. Contributing a lot to an enhanced audible experience in your vehicle.
Sound travels in 2 ways:
-Through the air
-Through the structure
Sound traveling through the air is relatively easy to understand. Any openings you have are sound leaks and transmission paths.
Sound moving through the structure is much less understood. Sound vibrates your doors, your roof, your floor, frame, etc. All of those things are connected to other parts of the car. When they are caused to vibrate by the sound system in your car, what they’re connected to also vibrate and resonate.
So, how do we deal with these issues of soundproofing and sound deadening? The answer is “It depends.” It depends on your budget, your situation, your time, your abilities, what your projected level of performance is, etc.
The overall plan involves...
1. Controlling resonance and vibration.
2. Blocking air born sound.
3. Decoupling objects to eliminate rattles
4. Absorbing unwanted frequencies.
Controlling resonance and vibration.
The first major step involves applying a Constrained Layer Damper (CDL). Popularly referred to as Dynamat or a Dynamat alternative. It's basically a layer of butyl rubber and thick foil. This is applied to control resonance and vibration. Despite old popular belief, the more of this you use (in one area) is NOT better. It's simply wasteful. 100% coverage and layering has proven no better results than strategic placement of 25% coverage per surface area. Since CDL can be quite expensive, use it wisely and not wastefully. Apply a border around the metal of where your speaker is mounted. Then apply small pieces sporadically on flatter spots of the door, as well as other sheet metal areas of the car. Including the outer and inner frame walls, floor, and roof. Even use it on the inside of trim panels if you have enough. CDL comes in sheets or rolls of various sizes and thicknesses, from many vendors. Varying by quality and price. There's also vastly cheaper and inferior CDL alternatives like Peel & Seal, which I suggest avoiding.
A decent and more affordable Dynamat alternative is Noico 80 mil.
Don't stop there.
Back in the day, it was the belief that "dynamatting" your car was the only step required. It was the end all be all of soundproofing your car. That's no longer true. CDL is just one step. You will notice improvements with just that, but will notice even more by going further. Applying the CDL only eliminates or reduces resonance and vibrations to the parts of the car that has been treated. You still have outside noise, rattles, weak midbass, poor acoustics, and other things to deal with.
Blocking air borne sound.
This mostly involves reducing outside noise, while also keeping sound in your car. The sound proofing step. Foreign noise intruding on your sound system can be unpleasant. You can't 100% eliminate the sound of a Harley passing you, but you can combat it and other typical road noise. Reducing noises like the wind, your engine, your tires, your exhaust, (etc) from entering your cabin means you can listen to your music less disturbed, and at more efficient volumes. Keeping the sound in your car also is more efficient. I know some folks like to announce there presence with their blaring tunes, and you can still do that with your windows down. Having a better system with soundproofing measures means the system sounds better to you, the driver, more often. Your sound wont escape as easily. You wont have to push your system to clipping points to hit those high decibels. Music will sound louder and clearer to your ears inside.
To effectively sound proof, you need barriers. The best method is using Mass Loaded Vinyl (MLV) decoupled with Closed Cell Foam (CCF). MLV is used in various contruction applications to reduce noise transmission. Though to be effective, is has to be decoupled with a layer CCF from metal or plastic surfaces. Sandwiched. Example: Door sheet metal-CDL-MLV-CCF-Door trim panel. Unlike CDL, it's effectiveness relies on as much coverage as possible per area. A wall will block, but a crack in the wall will leak. That's where the actual physical application can become tricky. Creating templates may help in the application process. Effective surfaces to apply MLV+CCF include doors, floor, & walls. Areas below the window line. Treating a roof isn't as ideal for several factors. First, MLV isn't cheap. Secondly, little noise is coming in at a car from above. If you disagree, and feel you wish to eliminate any possible noise from above, by all means go for it. Though, if you have a sunroof, it would be 100% completely pointless. The ideal MLV you want to use in a car would be of virgin origin, 1 lb/ft², 1/8" thick. It's sold in rolls. Most MLV doesn't have a self adhesive layer. Securing it is up to you. Velcro is advised to make it easily removable for possible repairs on your vehicle. Rubber cement can then be used to secure the CCF to the MLV.
Decoupling objects to eliminate rattles.
CCF (or Neoprene/EPDM Close Cell Foam) has ideal properties in sound deading applications in vehicles as a gasketing material. It's waterproof, compressible, and resiliant. It doesn't absorb or block sound, but can combat vibration. Beyond a layer decoupling the MLV, it can be used to treat several other things. Mostly rattles. Preventing objects from making contact from one another. For example, put a layer of CCF on the back of your license plate. No more rattle. Put some on loose door mechanisms. Between joints of trim panel. Loose points where trim paneling hits metal. Rattles can be the most annoying aspect of car audio. Often hard to track down and tend to. Some would prioritizing this procedure over soundproofing and blocking airborne sound. I wouldn't blame you for skipping that step, but please tend to those rattles before trying to impress someone or yourself with your system. CCF comes in varying forms. Sheets are sold by the roll in square feet. An ideal thickness layer for MLV decoupling would be 1/8". Common weatherstrip tape is CCF and has a self adhesive layer and come in small rolls of varying thickness. This is the stuff to use to combat isolated rattle points. Found at many home improvement stores.
A good CCF product for coupling MLV is 5mm Uxcell Liner. But as with any product or item I link here, do your own research for best prices. Or cheaper functioning alternatives.
Second Skin Audio offers a MLV/CCF combo product call Luxury Liner Pro. It may be more convenient to work with, but also may be more expensive.
Absorbing unwanted frequencies.
The next step in Sound Deadening is absorption. To further combat foreign and unwanted noise, and lingering resonance. Also assisting in better efficiencies, improved acoustics, and furthering the sound proofing aspects. This step mostly involves filling in air gaps and voids. Those empty areas behind trim paneling, and above roof liners. There's a few materials to choose from to use in this procedure. Hydrophobic (or regular) Melamine Foam (HMF). The Hydrophobic variant is water proof, and much more expensive. This can be used in door panels or areas that aren't water tight. Regular MF can be used everywhere else. This is a case of the more the better, or the thicker the better. The thicker the foam, the better frequency absorption rate. Keeping in mind, that the more the foam is compressed, it function's worsen. MF has great sound insulating properties and is used as such for various applications including on bullet trains. It's also fire resistant. MF is usually sold in bricks or sheets. Fun fact, Mr Clean Magic Eraser is actually a brick of Melamine Foam.
Other materials that serve the same purpose include common forms of fiberglass alternative insulation. (Though Fiberglass will work as well, it's just more difficult to work with.) Products like Rockwool Safe 'n Sound, and UltraTouch Denim. Rockwool, like HMF is water and fire resistant. Easy to work with and cut, and isn't harmful like fiberglass. It has very good noise reducing properties, and is used as such for sound proofing various constructions. It can be obtained at many home improvement stores. Sheets are 3"x15"x47", sold in 38lb bags. UltraTouch is basically recycled denim. Not water resistant at all, though can be sealed in water tight bags if need be. UltraTouch has even better noise reducing properties than Rockwool. Not sure how it compares to MF. A similar form of this material is widely used in the automobile industry as insulation already. It's even easier to work with in most water tight applications. Just tear apart and shove handfuls in where needed. Like Rockwool, it's available at many home improvement stores. Sold in bagged rolls of 16" x 48".
Magic Eraser (MF) can be found at many stores. Generic MF bricks can be bought in lots at Amazon and eBay for cheap.
Hydrophobic Melamine Foam is much more expensive. Do some googling to find the best deal if you must have the water resistant version.
Sounds like a lot of work right? Sure is. Whether it's all worth it to you is your decision. At the vary least, you can whittle some of this down. Cut corners here and there, and still show improved results over doing nothing at all.
Going even further, and tips.
Lets go over some other related improvements, and finer details we can do to improve the sound quality in our car.
Can't I just bring my car to a shop to get it done?
Possibly, but not likely. Many stereo install business wont even install Dynamat. You'd be considered lucky if you can find a place that would do all this, and do it right. At that point, your also doubling your costs (at least). Labor and mark up is big $. Sound deadening is mostly a DIY project. If you can't do it yourself, or with the help of a buddy, this project aint for you.
It aint cheap. MLV and CDL will be your most expensive materials. If your going to do this, gets some measurements before hand, and do the math. Shop around for best prices. MLV is also heavy. Shipping costs can be horrible. Try to find a local supplier. Good planning goes a long way to save $. Figure out what all you want to do. See what that's going to cost. Then maybe re-examine where you can make sacrifices, or cut corners.
Beyond treating the doors with CDL, MLV, CCF, and filling in voids with insulation, here's what else we can do. Seal the door. I'm not suggesting making it air tight. We still need water to escape. (Which by the way, keep in mind during your application of all this stuff. Water still needs a exit route.) I'm talking about making the door more like a proper speaker enclosure. The door's inner walls have maintenance openings that we can effectively cover. Creating a sturdier wall and improving mid bass. There's a couple ways and materials to go about this. Keeping in mind, that you want something removable in case of required maintenance. I personally bolted on a panel made of roof flashing. Coated with CDL, with a decoupling gasket made of Butyl Rope. Alternatively a thin sheet of Plexiglass or ABS may actually work better. But I went with what was easier for me to cut and deal with. Take precautions if drilling holes, and treat new holes with bit of primer.
I also suggest using Fast Rings. Use of this (or some diy version of it) helps project the sound from your speaker directly into your cabin. Reducing sound getting lost between the trim panel. It also helps to have some sort of rain shield protecting the back end of your speaker. Just something to divert water. I chopped up some XTC baffles for this.
Here's an effective way to treat your cargo floor. Remove the cargo mat & false floor. Take out the spare and everything in that area. Apply CDL (25% method) to the sheet metal around where the spare rests. Now, between the false floor and cargo mat, put down a layer of Stinger Road Kill Carpet Mat, Dynapad, or Luxury Liner Pro. Trace the shape of your cargo mat onto one of those materials, and sandwich it in. If your feeling up to it while your back there, remove some trim and slap some more CDL on the sheet metal, and stuff open air gaps with Melamine Foam or Denim filler.
Firewall Engine noise.
There's not a whole lot you can do to combat this. In fact, treating every other part of your car will actually make the firewall area seem louder. If you really wanted to tackle this, it would involve removing as much of the dash as possible. Treating with CDL, and some form of insulating. An MLV barrier would be near impossible without entirely stripping apart your car. Your better off at just shoving in some insulation and absorptive material behind the dash where ever you can. Removing some of the trim and the glove box will help.
Spraying or coating in your outside wheel wells isn't as efficient as you may think. It's not going to help much in sound deadening. It would require several layers to make any impact. The cost would outweigh the benefits. If your going to coat your wheel wells, do it for other reasons, and not for sound deadening alone. Focus on your inside floors and panels to reduce the road noise from that area. Your choice of tires can make a big difference as well. Some tires are quieter than others.
There is a myriad of wires hidden in our car. Including the wiring for the amps you just put in. Wires which can vibrate and make noise if they're touching a trim panel or what have you. Put care into decoupling these as well. Wrap or sandwich them in CCF, or Tesa Wire loom harness tape. Same applies for harnesses or anything you may come across that may cause rattle or noise from coming into contact with something.
Retention clips and holes.
The various places where a mounting clip is connected and secured, could be the source of rattle. Decouple these points with a piece of Tesa Wire Loom Harness Tape. Take care to decouple all points where clips meet another surface. Both on a trim panel, and on the frame of the car. Also keep in mind, you may break some clips while taking a panel off. Having replacements on stand by before you start your project may be wise. These Trim Panel Clips are Nissan OEM style but improved with a foam washer. If you're adding layers of material in your panels, OEM clips may not be long enough to secure your trim anymore. These Ford clips may work well. They have similar dimensions, but are longer. When I did my doors, I had to run to a hardware store to find something like the Ford ones to put things back together.
That plastic on your dash makes for a harsh surface for them tweeters to reflect off of. Consider a dash mat or cover to combat this.
All that plastic trim.
Like your dashboard above, plastic paneling isn't acoustic friendly. An optional way to remedy this is to wrap or layer that trim in a better functioning material. Something like, (but not limited to) self adhesive micro fiber pseudo suede.
Since you have that panel off...
When you plan on doing a speaker upgrade, heavily consider some form of the above treatments. At the very least some CDL. Save yourself some time and effort while you have those panels off. Instead of taking things apart multiple times. Similarly if for any reason you have to remove trim paneling, flooring, roof liner or what have you. Consider applying some form of sound deadening treatment while you have things apart.
Who doesn't like watching videos that show us techniques. Apparently I can only post one video directly per post. The rest of the videos will have to be links.
This first one is from Car Audio Fabrication and shows a great example of a full door treatment. However, this guy does this for a living. He has expensive tools and materials at his disposal. Don't let that deter you. You can likely accomplish this at your level, with what you have available to you. Your results may not look as pretty, but you can achieve the function. Nobody will ever see it. Soak in the gist of the information, and convert it to your application.
This short video is an example of why Peel n Seal isn't a suitable Dynamat alternative. I'm all for saving money, and cutting corner where allowable. But there's a line you don't cross.
The guy in this video utilizes fiberglass acoustic panels. Stuff like the Rockwool mentioned earlier.
The same guy showing his example of using UltraTouch.
Footnote: If you or I think of anything more worth adding to this post, I will do so.
Disclaimer: This knowledge and information is from my own personal experience, as well as information found on the internet from various sources.