Broadly speaking, noise reduction as it applies to the inside of a vehicle, falls into
THREE literal layers —
1) Reducing flat panel resonance … using CLD (Constrained Layer Dampers) which is a self adhesive, peel n stick product, made from a layer of butyl rubber laminated to a layer of aluminum. Think of products like Dynamat. CLD reduces panel vibration essentially by converting any sound waves that hit the panel into heat.
It is important to note that all CLD products are NOT equal and some are more effective then others at lowering panel resonance. Even though higher end products will cost more initially, you can actually use LESS of them, which can offset the cost both in terms of amount of material used and the labor and time to install it. Also, importantly, higher end CLD products will have NO smell or melting at the high internal temperatures that any vehicle can attain in some parts of the country.
There is an entire FB group called "the Deadening" that tests and compares different CLD products in a scientifically rigorous manner. IF you do FB, it might be wise to look at their "master list" of CLD products tested. BOTH Dynamat Extreme and Amazon Basics CLD make the top ten, but the clear winner is also the most expensive, a CLD distributed by Resonix. Resonix is also an excellent source for ALL high end deadening products as well as some high end SQ equipment, though they ARE expensive products.
FULL disclosure, I have used some Resonix products, as well as those from other manufacturers, and have NOTHING to gain from your decision to do so or not!!!
IMPORTANT NOTE: Do NOT try to save money by using asphalt/bitumen based construction products in place of a bonafide CLD…you will come to regret the decision. Even if your doors pass the "knock" test, asphalt backed aluminum flashing products are nowhere near as effective at reducing panel resonance as butyl rubber based products are. They also have a substantially increased chance of mechanical/adhesive failure due to heat…in other words, the MESS and SMELL that they make in your car when they fail will have you swearing for ignoring this warning.
One should apply CLD materials to the approximate center of any flat or unsupported panels at approximately 50% coverage. Note that while I am saying "flat" panels, what I really mean is any panels (flat or curved) that have no braces, ridges or extrusions to reinforce them. For maximum effect, one can also apply it to the center/open area of any panels that are reinforced, but this is less necessary to do, as the more rigid panels are already less prone to vibration. Other then OCD, there is ZERO rasion to apply CLD OVER bracing or other panel stiffening extrusions. ALSO note that while you can apply CLD at up to 100% coverage, it is not actually necessary … what you are trying to do is to reduce audible resonances from the panel vibrating with acoustic energy and 50% coverage with a QUALITY product is enough to do that effectively.
Applying CLD involves accessing the bare, painted metal by completely removing plastic door interior and/or vanity panels and/or also floor carpet in order to access the areas that you want to treat. You also need to make sure that the metal is clean enough for the butyl rubber "peel n stick" to adhere to. Once the panels are accessible and clean, the cutting and installation of peel n stick CLD is relatively a easy breeze using scissors and a roller. That said a wooden or smooth steel roller is absolutely crucial to properly apply the CLD. Resonix, at least used to, include a very good roller with a minimum purchase of their CLD tiles.
At this point, it is also a good idea to seal any large, medium or even small medium sized "access holes" within the inside door skin. For this, it is simplest to cut and use a piece of 1/8" thick ABS plastic slightly larger then the hole you are covering, but also with a piece of CLD stuck to one side of it, to stiffen it. Then cut the correct length of butyl rubber "rope" to stick the newly made plastic panel to the OEM door panel. Smaller holes can simply be sealed with a piece of CLD slapped over them.
Sealing these openings will add potential volume and clarity to the listening space of the cabin as it will prevent any sound waves emanating from the BACK of the door mounted speaker from entering the cabin and causing interference and cancellation of the actual sound waves that you want to hear from the front of the speaker. That said, the butyl rubber rope will also allow you to easily remove the newly made panels for future interior door access and service. Note that butyl rubber rope can also used to press into the spaces between OEM panel braces and the panel itself to also help deaden the panel.
2) Blocking Noise
This is a HUGELY important, and the most labor intensive part, of quieting the interior of a vehicle.
The goal here is to actually BLOCK outside noise from infiltrating to the the inside of the cabin, degrading the listening environment and competing with the sound from your speakers that you WANT to hear.
The DENSITY of the material you use here is of utmost importance. as well as partially the quality of the CCF that you use in between the CLD and the MLV.
Some people (myself included) opt to use thin (1/32") foam encapsulated LEAD sheeting that weighs 2#/sf. That said, the majority use easier to obtain and MUCH less expensive MLV (mass loaded vinyl) that weighs approx. 1#/sf and is slightly over 1/8" thick. The MLV is obviously less toxic to use, and also easier to install on vertical services using indistrial grade 3M adhesive velcro strips. The velcro allows easy installation, as well as easy removal for eventual later access to the door components. That said, it is harder to get the thicker MLV material to conform to things like transmission humps and floor pans where the seats mount, which is where the sheet lead, shines in ease of application. You DEFINITELY want to use the virgin vinyl variety of MLV as it does not make your care smell "plasticky" like the recycled versions will.
Another step on blocking noise, is to use 1/8" thick CCF (closed cell foam…think yoga mats) to isolate the MLV from the vehicle underneath. The CCF layer keeps noise or vibrations from penetrating or vibrating the the MLV itself and it is extemely effective in quieting the vehicle, though it is important to note that it us the MLV (or lead) sheeting that is doing the heavy lifting by simply blocking sound waves due to its density.
There are high end dual laminated products, again available from Resonix, that incorporate the foam and MLV (or even sheet lead)in a single product, saving installation time, but you end up paying MUCH more for the materials, and it is no more effective then separate layers of CCF and MLV.
The MOST important thing to realize about sound blocking is that 100% coverage , or as close to that as you can reasonably achieve) is CRUCIAL in order to BLOCK the sound waves (road noise, wind noise, tire noise) since sound waves travels omni directionally and will find and infiltrate anywhere they can.
TLTR — Think about the difference in volume of someone shouting at you from outside the house with the window open vs closed … just the thin pane of glass makes a huge difference.
Seams between pieces of adjoining MLV can be lapped over each other by a couple inches, or they can be butt fitted and then another 2 or 3" strip of MLV can be used on top of the seam to join them using Vinyl Glue.
Also important to note, MLV and CCF are water proof and with the right adhesives, can be applied to the back of wheel well liners in order to try to quiet down tire noise.
3) Reducing Rattles … caused by cheap OEM construction, or too much bass, or BOTH.
This is typically done by SQ purists or those installing multiple subwoofers whose high output, low frequencies can make any panel rattle or any windshield bulge, and ALL loose door or dash hardware rattle. Treatment can be as simple as wrapping the door latch hardware in Tessa tape or adding some CCF behind your license plates. It is pretty much done on an as needed basis and peel in stick CCF, butyl rubber rope, tessa tape and also 3M acoustic Thinsulate, are all your friends here.
The 3M Thinsulate is also useful for stuffing follow cavities, think A pillars, as well as placing a sheet between the headliner and roof to quiet wind noise from the roof. This is especially useful if you have a roof rack on your vehicle.
The commonality between all these steps is LABOR and material quality selected … It takes time and dedication to cut and apply the materials in a way to properly achieve your SQ goals and make the $$$ spent on materials and equipment worth it.
Just my .02
