Don't wait years to treat door speaker backwaves/reflections like I did.
Pictures and measurements will be linked at the end.
Before I get to my findings I want to share the backstory that started me on this learning experience.
I am far from an expert installer/measure/tuner and recently changing my front stage back to a high power 2-way (from 3-way) really highlighted my flaws. Specifically I have Steven's Audio MB-8s in the doors of my 2015 F150, and I moved the lowpass up to 2000hz L-R 24db slope. I'd been wanting to try them in 2-way because they'd been serving as absolute distortion free monsters in 3-way and certainly could outpace the midrange I had been using. The MB-8s are running on a JL Audio HD600/4 bridged to 300x2. The treatment in my doors consisted of cld deadner inside & out of panels and covering all holes including the access hole with painted 1/4" MDF.
I did an initial tune and got a decent sound but there was significant harshness coming from my doors that would peak enough to hurt my ears on certain notes/sounds. And yes the curve I tune for includes a slight dip in the 1500-3000 range. But the MB-8s should have no problem playing that high so I was determined to figure out my issue.
I reached out to Steven's Audio facebook page for help and they were super helpful with aspects about my install to fix. First was making sure the front of the driver is completely sealed off into the cabin with closed cell foam, I had that done already. Second was backwave/reflection treatment, which I had always seen pop up as a topic here but neglected to ever take seriously until now....
There are many threads about this topic already so I'm just going to share my experience with the process since it was my first attempt and certainly don't claim the method to be better than others.
I opted for the "DIY insulation bag" method and followed Richard Vedvik's video:
For the insulation panels I went with Owens Corning 703 acoustic panels (expensive I know) but I plan on using the left over panels in my home installs. I purchased a 6 pack of 2" thick (24"x48") panels which proved to be an overall decent thickness for my doors, I used just under 2 full panels. The 2015 F150 outer door panel is broken into 3 sections by 2 horizontal bars. I measured and cut 3 pieces for each door (Top Middle Bottom) and then split each piece in half to make it easier for installation. Certain sections needed to be thinned down by 1" to allow the window track movements. I cut my panels to fit very snug so minimal tape was needed to keep them in place but if I were to do it again I might trim them down a bit. I lost count how many times I torn a piece of the plastic covering sliding them into place because it was so tight and had to take it back out. Window clearance was not a fun step either but possible with patience. There are concerns with this method about moisture even with the bags so time will tell, I've set a reminder to pull my panels off and inspect later on down the road. I also ended up with no room left directly behind the speaker for the angled dispersion piece mentioned in the video, if that ends up being a problem I will revisit that section. But on to the results....
After installing the panels I did the measurements again, did my tune to the same curve I've been using. Played my first song and instantly heard the difference, harshness in its entirety was resolved. I was shaking my own head at myself as I flipped through my go-to tracks that I had not done this sooner or in any of my previous vehicles. While the harshness being the big win, the more I'm listening the more I notice the full bandwidth of the MB-8s has improved immensely. Richard mentioning in the video about "busier songs sounding muddy almost like distortion" is spot on as well. When I got to the metal/hard rock songs in my list which include lots of electronic guitar/bass distortion, they sounded 10x better now with the headroom the MB-8 has when properly treated.
I compared my initial measurements to the bag treated doors measurements, and while I don't claim this to be a scientific conclusion, it seems to visually represent exactly what I was hearing. Significant smoothing of peaks/dips above 900 with no EQ applied on either of these measurements.
See install pictures of the right door from link below and also the measurements mentioned. Please note these are no EQ measurements in 1/48 smoothing to show all the glorious peaks. Also these are from the front left seat listening position so I'm thinking that's what the funny business going on between the left and right around 150-250hz. And again not a scientific conclusion, I was holding the mic by hand in both cases.
Current setup for reference:
LG V30 rca > JL Audio TWK 88
Audiofrog GB15s in A pillars
Steven's Audio MB-8 in doors
Under rear seat subs
Dual ported JL 10w7s