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I've always found that I get a lot of road noise through the front doors (tire noise, rain, other vehicles, etc.) and it always seems to be worse with coupes as the door is longer creating a larger sound chamber.
Caution - please investigate the risks of what I suggest below. Fiberglass is a carcinogen and this may not be for you.
Yes - I install fiberglass in my doors. Compressed fiberglass is a great way to control noise - I originally installed with peel and stick to enhance sound quality of the audio system (which it does) and quickly found a welcome benefit of minimizing exterior noise.
The insulation I use is from Menards (hardware chain) and it's fiberglass ceiling tiles. I peel the white pebble coating, cut into appropriate sized pieces, thoroughly wrap in thin plastic drop-cloth, seal with packing tape, then secure with aluminum tape (HVAC). The drop cloth and packing tape seal out moisture and seal in the fiberglass - allowing loose fiberglass to enter the cabin (and your lungs) would be extremely dangerous!
There's a build thread on the Accord forums where this is done and explained buy an acoustical engineer.
Awesome. I am going to do this on my M35. I want the cabin noise to go down a bit and I have also read that some people apply thin sound deadening (50mil) sheets to the inside of the plastic wheel well covers. This would further bring down the vibration levels caused/absorbed by the wheels. I am going to do the OP's suggestion plus the thin sheets under the plastic and take before/after readings.
OP, did you use a phone APP to measure noise levels? There's a lot wondering if one is better than others.
Good to see the various technqiues used by others! Will consider doing this for all of my future cars as well.
As for the iPhone App, there's several available but the one i used is called: dB Volume. Simple and effective. Not sure HOW accurate it is but at least its consistent during each stage of the experiment.
I also used an App called: "Spectrum" that is a spectrum analyzer to measure sounds across the spectrum to identify noise of the drone on my 300zx Magnaflow muffler. That was a fun experiment to identify the offending frequencies (108Hz!!!) to eliminate using a Helmholtz tube. Ended up going back to stock muffler bc the drone was too much for my 56 year old ears.
