Hertz Audio DCX 87.3 3.5" coaxial speaker in Jeep JL

Time to upgrade the dash speakers in the Jeep. This is how the idea started. My wife's old Sahara had these Mopar headliners for the removable hard tops. My Jeep is the Willys version which is bare bones when it comes to interior, but comes with the rock rails and shocks from the Rubi.

I decided to go with the aftermarket Design Engineering Boom Mat Kit at an el cheapo price on Amazon with Prime shipping. The Boom Mat Kit isn't as nice as the Mopar kit. The Mopar kit is some dense foam. However I feel the Boom Mat Kit is more effective in keeping the interior quite and cool (it also serves as great heat insulation). The reasons would be that the less dense foam (it's squishy medium density) of the Boom Mat does a better job. Also, the Boom Mat covers more area, following the recessed edges of the underside of the hardtop. The Mopar pieces are like two square pieces that just stick to the top.

Drivers and passenger front panels


Rear hardtop

 

Anyway, I got lost in thought. After these were installed, the inside of the Jeep got much quieter, or to use a phrase we often use here less grey "blackground". Sure there was the wind noise (can't fight this with a vehicle that is essentially shaped like a box) and tire noise (can't fight this either if you want big tires that can grip slate and mud), but I noticed that sound wasn't bouncing around all over the place inside like before. Conversations with my kids, kids' friends, my wife, became quieter and easier. And get this: the car stereo sounded better.

I guess this is a good argument for room treatments for home stereo. Or at least smartly laying out rugs and furniture so your listening room doesn't look like a rape chamber. And by room treatments, I do not mean the Shaki Audio Hallograph Soundfield Optimizer bullshit. I mean what the mix stages use.

Say no to this bullshit:


But yes to the stuff you see here (Formosa Group ADR Stage at Burbank)

 

So it got me thinking. I hadn't messed around with car stereo in ages, but with Crutchfield making it easy with install guides and wiring harnesses, I figured why not. I was already familiar with the typical names Alpine, Kenwood, Infinity, and Focal on the high end. One name which I started to hear of recently was Hertz. The word is that their stuff was easygoing, warm, not bright. To add, Crutchfield also has this car speaker comparo that lets you play the speaker back, allowing you to select your headphone (HD600 is on the list as well as many others that we would own).

So I ended up purchasing the Hertz Audio DCX 87.3 to replace the dash speakers. I have to say that I am pretty happy so far.

First things first, let's get a baseline of the stock stereo with the built-in EQ (bass, mids, treble) set to flat.

Jeep JL Willys
Frequency Response
STOCK speakers (with Boom Mat and no EQ)
(note target set to 1db/oct slope downward - microphone around where my head would be)

Note: ignore 17kHz+ dump - lossy pink noise is bandwidth limited

That's actually pretty good! TBH, I thought the frequency response of the stock stereo of the Jeep was pretty darn good. Maybe at times, I would notch up the bass and treble a bit, but with the Boom Mat in place, I found that EQ on flat sounded best. The extra midbass is needed in a car. The tire rumble is significant.

The one thing I would take with a grain of salt is the 40Hz. The stock stereo just doesn't go deeper than 75Hz. There are 3.5" dash speakers and I think maybe 4" or 5" speakers inside tiny boxes in the knee panels. Most cars will have 6.5" speakers in the doors, which provide more volume for bass. The Jeep has removable doors, hence there are no speakers in the doors. Think of the Jeep stereo as a small Harbeth nearfield monitor. Only a subwoofer will solve the deep bass issue - but could be another thread.

 

Here is the Hertz coaxial.


Test fit - had to cut the mounting brackets with the tin snipes.


Stock paper driver for comparison. It actually isn't bad. I have seen worse, cheaper. Like on a C8 Corvette with the base Bose. (I hope the old geezers on the Corvette forums don't see me writing this - people on Corvette forums are super duper sensitive - any kind of criticism against the Vette seems to be taken badly - especially on the larger forum, I think the one with more old geezers).

 

So F it. I am going to make it easy on myself. The windshield of Jeeps can fold down. I figured I would do this to make sure the hinges still work and don't seize up from all the salt in the air from living on a barrier island. The hard part were the wipers, which are friction fit into small splines. My neighbor lent me a old battery terminal puller (think wine bottle opener) that I used to pull the wipers (a step that is needed before folding the windshield down).



So during the test fit and listen, there was a bit of nasal and cavity resonance sound. I figured it was the air duct right under the mounting point. I applied some Siless 80mil deadening mat (too cheap for Dynamat Xtreme) on top of the air duct.


Then I stuffed the cavity with polyfill


And finally some quality rubber compound weather stripping that could be squished down to 1/16"

 

Jeep JL Willys
Frequency Response
Hertz Audio DCX 87.3 speakers in dash only (with Boom Mat and no EQ)
(note target set to 1db/oct slope downward - microphone around where my head would be)


Violet line is to get a baseline of stock woofer level in knee panels. From this we can see that the Hertz Audio DCX 87.3 is more sensitive to the stock part, especially from 2kHz on up. Also the Hertz part has a peak around 13kHz. I probably can't hear a pure tone at 13kHz at low levels, but the Hertz part did sound more airy.

After a few days of tweaking the treble EQ setting, I realized I just couldn't get the highs to sound totally right and was always messing around with the treble EQ depending upon the recording.

To be continued...

 

Funny you mention this. It turns out that the stock driver has a cap in line for a first order high-pass. It's entirely possible that other 3.5" coaxials also come with something similar. The Hertz piece does not, other than the cap for the tweeter. Someone mentioned the need for bass blockers in the Crutchfield reviews section and I confirmed this.

At this point, I cannot crank it up without excursion issues. Need to look in my spare parts bin to find a 100uF to 200uF cap. Will post impedance graphs of stock part and Hertz part in a bit.

 

Here we go.

Hertz Audio DCX 87.3
Impedance Curve


About 3.5-ohms at 100Hz and 500Hz. The Fs is over 200Hz, which would be in line for a 3.5" standing in free air with no damping.

Jeep JL 3.5" dash stock dash speaker
Impedance Curve (cap is bypassed)


Jeep JL 3.5" dash stock dash speaker
Impedance Curve - with cap inline


Wow, the stock driver is actually a bit over 8-ohms. Now I am wondering if I can stick a Fostex 3" in there with the stock amp.

 

Found a pair or 270uF caps. Larger than I wanted, but given the low impedance of the Hertz part, it should be OK.



Hertz Audio DCX 87.3
Impedance Curve - with 270uF cap inline


That should do. At 100Hz, twice the impedance as before, so would effectively divide the voltage by half or 6db. Even more below that. This should protect the 3.5" dash speaker.

Will post final in car measurements soon, right before my morning meeting with co-workers.

 

So to address the 13kHz problem? TP. Applied two layer TP under the grill to flatten the peak. I then set EQ on treble -2 and bass +2 to account for the greater sensitivity of the Hertz part and also because more bass in a car with rumbly tire noise is good.

Jeep JL Willys
Frequency Response
Hertz Audio DCX 87.3 speakers in dash only (with Boom Mat and no EQ)
TP over driver, +2 bass, -2 treble in car EQ.


Listening to a few tracks, I could probably have more fill in the 200-300Hz area, but whatever, minor complaints.

In a nutshell:

Not perfect, but limitations are due to lack of subwoofer or lack of larger door mounted speakers. The knee panel boxes are not gonna but it. As I mentioned, think of the system as small Harbeth nearfield monitors (with no low bass, but plenty of midbass). After the inline bass blocker, there is more disconnect between the dash speakers and knee panel woofers because of their physical distance from each other (and also because the dash speakers are much higher quality).

However these drawbacks are minor as this ranks up there as the best money I've spent on audio. I can't believe this was $99 for the pair including the harnesses and extras that Crutchfield threw in. I have chased 1% to 5% better sound for thousands, so this is why I am super happy. I listen to music in the car more than at home these days, running errands, going to Home Depot for gardening stuff, teaching kids to drive. Success in audio procurement is measured in how much you want to go out of your way to listen to music. Putting these in make me want to drive the Jeep more!

Future Ideas

• Figure out a way to put a Schiit MMB2 in there
• Mean metal bumper from Next Venture Motorsports (based on CO - buy USA).

 

Yup. In the late 80s in college, I was totally into car audio. Had x2 Boston Acoustics sub in my Honda and MB Quarts up front with a Sony deck and x2 small Soundstream? high bias class A amps with power running directly to the battery.

Since then I have not touched car audio... until recently, over 30 years later.

There's some pretty cool stuff today. I just found out that there's a module that takes the Jeep CAN bus from the head unit and outputs Toslink. That opens up huge DSP (Audiosource, etc.) and other possibilities. I was kidding with Jason that I would stick a MMB2 into the Jeep. Of course, because the Jeeps are so popular, there are also head units that are compatible with the Jeep's built in functions.

I don't think I will go crazy. However the Jeep does have an analog in, so maybe I can still stick in an MMB2 or Modi somewhere in there. Finally, I realized I would leverage the sound bar above (I think 5" or 6.5" drivers) for "sub" duties without adding a huge box. I use the compartment in the back to store air compressors, torque wrench, recovery straps, etc. It would be different if I used the Jeep solely as a mall crawler, but I like driving over shit.

P.S. I put a large square of that Silmat stuff over the rear wheel wells. Big difference with less road / tire noise. Again, not going crazy, just selective application.

P.P.S.

Next project:

 

Toilet paper.