Awful Trunk Space/Fuel Tank Vibration "Delay" from Hardon Kardon

I thought I'd posted a complaint/inquiry about hearing a slight delay from the dual subs in this forum, but can't find it if I did. I did post it in the Challengertalk.com forum. I found it and posted the below update/resurrection of my original post there (make a couple of changes - this version is better). But the regulars in this forum have offered some really strong advice over the past five years on several topics. I'm interested if anyone here has experience with vibration damping their trunk sub-woofers. So here goes...

Update - "I didn't do anything about my perception of hearing a slight delay from the trunk subwoofers since posting this right after I bought the car. I've only driven my car 8,000 miles since new, and its up for winter five months every year. As I said, I always wondered if I was hearing something that wasn't "there". Never got around to bringing it to the dealership - too late now.

Got the car out of storage this month and yesterday a friend came over to see if he also could hear the slight delay - the poor quality of these expensive subs - I've always thought I'd heard. Holy Moly (or words to the effect)! What was causing the horrible racket that my wife, upon entering my detached garage while my friend and I were still inside the car with windows closed confirming there was indeed a weird bass delay effect, described as sounding like there was a person in the trunk trying to bang there way out? Inside the car we couldn't hear what she was hearing. The Challenger wasn't running, the stereo was playing a Led Zeppelin song from an iPhone with good bass, the EQ was near flat on the UConnect, the volume was at 31 (loud - louder than I listen when driving - but not maxed out) - and the components under the trunk mat were jumping up and down in time to the bass beat! The flat-fixer in its spot, the entire tray containing the subs, the subs themselves - all vibrating up and down badly. Only the battery seemed visibly anchored, but touching it I could feel plenty of vibration. Folks, you had to be there to appreciate how wrong this was!

We got out and opened the trunk and saw all the above. But what was the banging noise my wife had reported and we could now hear as loud as the music? When I looked under the car I saw the entire fuel tank visibly vibrating. It was where most of the awful noise was coming from. Really shaking, as in my first thought was the bolts/retaining apparatus holding the fuel tank in place wouldn't last long due to metal fatigue if I kept subjecting it to this kind of vibration. And the delay effect? It seemed to be from all the stuff slapping around, probably mostly from the fuel tank itself bouncing forward a bass-like sound and sending that sound to the front seats just behind the music beat.

Unbelievable, was my first acid thought. How could Dodge sell an upgraded audio system for $2,000 in a $90,000 car with dual sub-woofer vibration characteristics like this? Then again, maybe I just got a car without something in the trunk that would dampen out the sub's vibrations? My friend, who has installed sound damping products in a few cars pointed out there was the thinnest of damping products installed under the subs - we could see the edge without removing the sub woofer tray. He said he can remedy the vibration problem if we install Kilmat, which is a brand product (there are similar ones - Dynamat, for one) that is intended to dampen unwanted vibrations under the subs - so I ordered some Kilmat yesterday. He says this stuff "won't come up" so you have to put it down right the first time. He said, "You have an investment in this car. Do you want to change it from stock?" Well, not a big concern if it resolves the noise like I've kidnapped somebody, the bothersome delay, and potentially the fuel tank dropping off. Anyone interested in buying my car some day should appreciate/overlook my few reversible upgrades, like the catch can and the AAD suspension arms front and back. And, whatever I do to make the Harmon Kardon audio sound right could, with effort and a gallon of Goo Be Gone, be reversed.

Anyone else observed this crazy vibration? Any advice about trying Kilmat inside the car trunk where it will hardly be visible under the subs? My friend said also sticking some under the fuel tank would work, but I'm reluctant to try that. I'd rather work inside the car.

Thanks in advance!

The sub in my Demon 170 was shattered from over tightening. It rattled and vibrated. Replaced sub under warranty and all's good.

Hadn't thought of something like that. Will check their integrity before launching into applying anything.

The system is plain garbage. I would replace everything except the head unit TBH. Its a $2000k upgrade for a $100 in parts smh.

The stereo is NOT on par with the performance of the car so forget thinking there is a nexus between the two there isnt at all it sux it was like the worst radio ive had in any vehicle in the past 20 years!

Mine sounds fine.

Mine sounds fine also. Yes at #31 volume playing a big bass song that woofer can produce some good vibrations. Ha, I have been known when driving alone anyway to lower the passenger access door to the trunk to even have a better experience of hearing more clear bass.
But my ears ring constantly and fairly loud due to the abuse I have allowed listening to loud rock music for about 55 years now.

Not to mention road course driving with windows open and hearing the howl of the supercharger and the roar of the exhaust!

Ha true my friend but the helmet does dampen some of that. The other day I actually turned the Durango volume down because it seemed my ear drums were vibrating. Damn!

I wish mine didn't have an obnoxious sound-system, at all..... Or a back seat, all the nanny electronics, sunroof, etc.

I appreciate your post and I've seen other general complaints in forums. My friend who will do the Kilmat installation is of a similar opinion about all factory car stereos.

When I first noticed this slight bass delay in May 2019 I called Harmon Kardon's technical support line, thinking maybe they would have an idea why I was hearing it. The person I spoke with said Harmon Kardon only supplied the speakers and really had nothing else to do with the system. I don't know if that is true.

At lower volumes than 31 I hear the slight delay - things have to be vibrating back there to some degree - but with a little EQ I wasn't totally unhappy.

We may tackle installing the Kilmat Saturday. I'll video the subs and the fuel tank vibrating and the noise before and after. Maybe I can figure out a way to share it here.

They could put a mid range aftermarket quality system in the car for less than what they charge for the up-grade HK system and get better performance, I have the HK upgrade and I am ok with it for now but will change everything out as GNBRETT said except the head unit after the warranty is up.

One of the biggest regrets I have in audio upgrades is changing out the factory HU on my Road Glide (went with Sony) as the factory unit was a good unit but it made adding the amps easier and an overall better performing set up but I miss the features.
None of my vehicles except my wife's 21 Cherokee have factory systems simply because aftermarket is easy to upgrade to and reasonably priced for the most part.

Yes, they only supply the speakers. They work with manufacturer to build a speaker that meets the demand for the desired location but that is it, after that it's on the car designers on installation and system set up.
Ours come in in big shipping crates and get assembled into sub assemblies and the put on line to go in the vehicles

Guys if u just heard how much better a new set of front speakers, an amp and a JL stealth box would sound EVERYONE would do it. Its just not that much money and will sound a 100x better!

Lots of u own cars that cost as much as a small condo. Some payed a $100k for their baby. I just cant wrap my head around cruising it with such a lame stereo system. I like to go fast like everyone but I also like my tunes!

Admittedly I am an audiofile so I have a different expectation then most but its just one of those mods that after u do it ur like I cant believe I waited this long to have a real sound system. It just makes the driving experience way better. Lets be real here we go WOT like maybe 4-5 times every time we take the car out if that much with some shorter blips..... The rest of the time ur just cruising listening to tunes!

Speakers, wires, JL Sub box and an Amp installed is probably less than $2k. $2k..... cmon guys thats nothing to have a REAL sound system. Its a tiny foot print u dont need much the air space is really small in there.

I have the Harmon n kardon stereo in mine. I have the 6 speaker cheap radio in my daily. I think the Harmon n kardon really sounds great, but maybe I'm biased from listening to the crap one everyday?

Garbage in garbage out. One can't stream music and have it sound good let alone great. Mine rattles in the trunk. And most of it comes from the floor pan block off covers.

We all hear different just like we see. My 13 mustang had 4 speakers sounded really bad. I replaced them added a sub woofer and 2 amps. Sounded way better but not much different then the beats system in my daytona or the hk system in my hellcat. The challenger has the alpine system and that one is just ok. I would not upgrade a hk system, sounds great to me

Yep same here. Compared to the 90 and 2000s cars I've had, my hk sounds the best. Rather spend the 💰 on a E85 tune

IDN bout sounding great but like I said im an Audiofile so I have different expectations then most but when I compare my 2020 Redeye to my 2020 Ram Limited its a night and day difference. I never even upgraded my factory sound system in my Ram it sounded that good!

Why the Challenger got such a shit radio is beyond me smh. Again, its one of those things that just doesnt mean much to most ppl so they stick with whatever came from the factory but if one wants good sound that stock system has to come out there is no "Adding" a subwoofer to make it sound good the whole thing sux.

CA
Where are these floor pan block covers located? Can you see them from under the car?

Sound dampening update...today spent several hours installing Kilmat product. My friend did most of the work and I tried not to get in his way. We applied sheets and cut to fit pieces inside the entire trunk cavity.

There is Kilmat under the trunk lock, and we put some around that silver screw to the left where the sub-woofer tray has a hold down nut. I also appropriated my wife's yoga mat and cut it up to put its rubbery thin material under the fix a flat holder, inside the fit a flat holder to secure the thing, and under the tray itself. And we pulled the trunk lid liner and put some Kilmat under the trunk lid. Only a little bit of this Kilmat is visible with the trunk liner removed looking at the sub tray and battery. I'll take a picture tomorrow.

A lot of effort and...only marginal improvement. At the same 31 volume level there still intermittent rattling to the same Led Zeppelin song. It might be one of the subs is defective. It is definitely coming from the rear and underside of the car. With the car up on a lift the last thing we did was put some Kilmat on the underside of the trunk up on the sides where it can't be seen, and where it probably won't degrade much.

Long day...will sound check tomorrow and update progress again soon. At my normal listening levels I think we may have addressed the rattle/delay effect coming from the underside of the trunk - I may not hear the delay because the rattling won't be happening. I hope to hear improvement when road tested. Hopefully those last few Kilmat applications on the exterior of the underside of the trunk space under the car will help ever more.

So this thread still open and I'm interested in more comments.