Sometimes it's a single problem, like drone from an aftermarket exhaust. The more challenging projects are those vehicles for which NVH (noise, vibration, harshness) wasn't a priority, either for economy or to minimize weight for performance. In this case the vehicle's occupants are submerged in a cacophony of noises from multiple sources and directions.
Tire noise is one of the most common problems we face and one of the most difficult to mitigate. Noise originates where the tire contacts the road and radiates out from there. That makes the entire underside of the vehicle a target.
It reflects off of adjacent vehicles and obstacles along the roadway and enters your vehicle through the doors and side panels. This is why your tire noise is so loud when you drive close to a wall.
The complexity of this noise sources requires treatment to most of the vehicle to be effective …
… tire noise is the only noise you are concerned with. In that case, new tires might be the best option. Tire replacement may also be part of an overall treatment. Tires may get noisier with age and wear. Some tires are just much noisier than others, even when brand new.
Primarily enters through the lower firewall and front floor. Can also enter the vehicle through the front edges of the front doors and the center tunnel.
This is quite easy to test. All you need is a heavy blanket - old style moving blankets are ideal. Sit in the vehicle with the engine running and have a friend lay the blanket over the hood. If you hear a significant reduction, it's worth pursuing. My advice is always to treat the passenger compartment first. Reducing overall noise levels will make it much easier to hear what's going on.
Another complex under vehicle noise that interacts with the environment.
Treating the roof and outer door skins can help.
Noise that originates from nearby vehicles or noise that ordinates from your vehicle and reflects off of nearby vehicles.
I include this one because it's cool not to be startled by the sound of a sudden downpour hitting the roof. It's somehow really nice to have the loudest impact sound coming from the windshield.
How Quiet can we Make this Vehicle?
Hard to predict precisely. Far too many variables. It is generally true that the noisier a vehicle is and the less the manufacturer addressed noise issues, the bigger the improvement. Conversely, a luxury car will have very low noise levels to start with and most of the space we're counting on is already used.
I will guarantee that you your vehicle will not be silent. That's barely possible if you were designing the vehicle from scratch and there were no budgetary constraints. We have to work with the existing suspension, drive train and glass. Can't get to silence. It's a lot of steel, glass, rubber and composites rolling on a rough surface, propelled by exploding petroleum, (unless, well you know......hybrid or plug-in). You are going to put a lot of time and money into the project and somewhere along the line silence can become the target.
Just because it isn't silent doesn't mean it isn't drastically better. I consider a project a success if the vehicle is transformed from unpleasant to drive to pleasant to drive. That's a really big deal. If you can speak to a passenger without shouting, if your music sounds better and louder than ever before with the volume knob turned down from previous standard levels. If you can drive to work without getting a headache. All really big deals in my book.
The most interesting reactions by far are from people who parked their car and didn't drive it again until it was fully treated. It's harder to get the full impact when you've worked on the vehicle yourself since you adjust to the changing acoustics as you go. A really common reaction is for people to spend a lot of time listening to the sound of their own voices. They really can sound much different. It's also common to hear things that you couldn't hear before.
Reducing noise levels is always a good thing. What we are doing is re-engineering some luxury into the vehicle. There are all sorts of vehicles that have desirable characteristics - performance, economy, utility - but are just too noisy. Being able to influence noise levels with aftermarket treatments significantly lengthens the list of great vehicles.