Danny, Some of that depends on how you define "outside noise". If you mean noise that doesn't come directly from your vehicle or its interaction with the environment, doors would be first since they let in most traffic noise. That leaves noises generated by the vehicle - engine, exhaust and its interaction with the environment - tire/wheel and wind, maybe rain too. Engine, exhaust and tire/wheel noise are complicated. The firewall and floor are where they penetrate, but not exactly as we tend to think. Engine and exhaust noise do hit the passenger compartment in a direct line, but they also reflect off the pavement, meaning they even enter through areas from which you can't draw a straight line back to the source. Tire noise is especially interesting. Most of us think that most of it comes in through the wheel wells. That's led to all sorts of exterior treatments that are supposed to avoid the complication of getting to the interior surface of the floor. It seldom makes an audible difference. The important point is that tire noise starts at the contact patch with the pavement and radiates up in every direction from there. A lot of the sound that would otherwise pass through the wheel well is blocked by the tire and wheel. The reality is that tire noise enters through most of the floor. Specifically on the "noise reflection from pavement".. I was tempted to look more into adding Cascade to the outside of my vehicle, under carriage areas that were as flat and had seams joining them together... Basically it is like Second Skin's sludge but a little thinner which could be applied by aerosol and was able to withstand the elements of outside qualities... This was the idea that adding mass to the outside panels would seemingly "deflect" alot of the road noises and sounds similar coming from random directions which could not be measured or tested specifically... Some reported improvements which provided no audible increase in audio equipment but neutralized the added environment noise more than just adding MLV to the floor panels... True or not I cannot confirm, but it could be possible... Anything truth to it? Would it be a waste of labor and money to try this theory out?