Improving audio in Mercedes GLK350 (X204), Part 2

In Part 1 of this post I explained how I replaced the "basic" speakers with the ones from Harman-Kardon Logic 7 configuration and studied their behavior with AudioFrog GSC610C passive crossover. Now it's time to install the crossover into the car and tune the system.

Installing the Crossover

Initially I was planning to fit the crossover somewhere inside the door—affix it to the middle steel panel. However, the panel has turned out to be quite uneven, to improve its rigidity, I guess—so the crossover unit couldn't really fit anywhere on it. Then I went with the plan B—installed the crossover into the door pocket at the bottom of the door panel (also known as "map pocket"). It took about half of the space, still leaving room for a water bottle. Learning from the hardware designers of Mercedes, I used aluminium rivets to mount it.

Obviously, I had to tap into the wire harness that goes from the cockpit into the door to insert the crossover. And since I had installed the crossover into a removable part of the door, I had to enable the possibility to detach the connecting wires of the crossover. I ended up with the following arrangement:

There is a hatch in the door panel which is used to get access to the electrical wiring of the door without actually removing the former. There was still some room there so I put a screw terminals block that joins elongated wires from the door harness with the wires of the crossover unit. Would anyone need to remove the door panel, they will need to unscrew the crossover wires first.

Checking Tweeter Polarity

Now the most interesting part—squeezing the best possible sound from this configuration. First I decided to check the sound of the door speaker as a unit. The main problem with car door speakers is that the drivers are placed very far from each other. Ideally, one would need to listen to such a speaker from a good distance in order to achieve "blending" between the tweeter and the woofer. Unfortunately, this isn't an option in the car. The other problem is that the speaker components are not equidistant from the listeners' ears. In sophisticated car audio systems this problem is partially solved by installing a lot of speakers and tuning the delay of each speaker driver individually. In my case this wasn't an option though.

One thing I wanted to decide on is the polarity of the tweeter. Initially I just followed the color codes of the wires, attaching brown wires to "-" terminals of the crossover. However, this didn't exclude an option that the tweeter could be wired in reverse polarity at the factory to achieve better integration of the drivers. In order to check which polarity works the best, I opened the door and set up a microphone to be equidistant from the drivers (about 55 cm).

Then I captured the impulse response with the tweeter connected in the initial wiring and inverted. The frequency response didn't differ much:

The response shown is FDW-windowed at 7 cycles to get rid of surrounding reflections. Note that in both cases there is a huge notch between 500–600 Hz. Perhaps, it's caused by some reflection inside the door—the quarter-length wavelengths involved here are 17.2–14.3 cm, and the door isn't fully stuffed with absorbing material, so this seems to be a natural explanation.

However, the frequency response doesn't tell the full story. Here are the impulse responses:

As we can see, they look very much like mirror images of each other, and the inverted one (black) looks more correct to me as it's main peak goes in the positive direction. So I ended up inverting the tweeter polarity. I think it's actually the "natural" polarity for the tweeter, thus it seems that in the "basic" configuration the tweeter polarity was inverted, and I just restored it back.

Taming the Rear Door Speakers

As I mentioned in Part 1 of this post, I didn't replace the rear door speakers due to their peculiar mounting. However, I wanted to find a way to minimize their negative influence on the sound of the front speakers.

Fortunately, Audio 20 unit has an engineering menu (accessible by pressing "Hang Up", "1", and "#" buttons on the keypad simultaneously) that gives access to the equalizer and low- and high-pass filters for each door driver individually. I decided to leave to the rear door drivers the role of low frequency extensions, so I set up for them a low pass filter to 100 Hz with a 12 dB / octave roll-off:

Besides eliminating incoherent sound, this also leaves more current for the front speaker channels in the power amplifier.

Aligning Front Door Speakers

While verifying the polarity of the tweeter, I also checked how similar the resulting door speakers are. They were pretty close, which was good. However, in the car the speaker setup is highly asymmetric due to the presence of the steering wheel and the driver's body.

I was thinking for some time how to approach the alignment. I've found the paper by M. Ziemba "Test Signals for the Objective and Subjective Evaluation of Automotive Audio Systems" which suggested the use of a stereo spherical microphone which simulates human head better than a regular measurement mic or mic array. What I realized is that a real human head with in-ear microphones constitute an even better measurement device. Another advantage of using a real human is that we also perform exact measurement of the actual acoustic crossover formed by non-coincident door speaker drivers and the driver's body.

Recalling my visit to the AES Conference on Headphones I decided to buy Sennheiser Ambeo headset. I learned that Sennheiser has sunset this product, but I've managed to find it through 3rd party sellers. Yet another technical obstacle was that the Ambeo headset was only available for iPhones and thus has a Lightning connector. Thankfully, Anker has developed a Lightning-to-USB adapter which explicitly lists the Ambeo headset as one of the supported products. This allows using this headset with laptops. The Ambeo headset has turned out to be a very useful audio measurement instrument, although not free from issues. I plan to do a separate post about it.

Now my strategy was to align the frequency response of the left door speaker as measured by the left ear microphone with the frequency response of the right door speaker as measured by the right ear microphone. I used Rational Acoustics Smaart v8 in dual-channel FFT mode to be able to look at the correlation graphs for the measured transfer functions. Yet another useful feature of Smaart is the ability to apply gating in the dual-channel mode. Since the car cabin is full of reflections, it's important to ensure that we are equalizing the direct sound within the integration period (the first 10 ms of arrival). I was using an 8 ms window in my tuning process.

The tuning was done by manual real-time tweaking of the Audio 20 parametric equalizer. I didn't use REW because it isn't aware of the correlation between the test signal and the captured audio output and thus it might attempt to correct areas that are simply not correctable because the dips are caused by destructive acoustic interference or other acoustic interaction. Also, by tweaking PEQs manually in real time I was able to see the changes instantly on the analyzer.

This is what I've got for the left speaker as measured by the microphone in the left ear vs. the right speaker as measured by the microphone in the right ear. The levels are matched for the purposes of comparison. In reality the left channel is about 2 dB louder due to proximity.

This looks a bit scary, however this really demonstrates how challenging the car cabins are for accurate sound reproduction. I couldn't make the region of the left channel from 600 Hz to 2 kHz to match the right channel—it seems that the dip is caused by acoustic interference. Also note the 5 kHz dip which is an artifact of measurements with the Ambeo headset.

I've also made a moving microphone average measurement with both left and right channels playing, in the area where the driver's head is (w/o the driver being there, of course) using Beyerdynamic MM-1 microphone with diffuse field calibration profile. Here it is compared to the target curve recommended by AudioFrog:

I've got slightly more bass, but this is easily adjustable using the tone controls. Also, the bass in the car anyway needs a bit of boost while on the road due to engine and wheels noise. As we can see, there is no dip at 5 kHz we've seen on the measurement acquired via the Ambeo headset. However, there is a wide dip at 500–700 Hz likely the same one we've seen on the door measurement, and also a narrower dip at 2 kHz. It can also be seen on the door measurement, and it could be a result of an imperfect crossover overlap. If we recall the measurement showing the effect of the crossover on the HKL7 drivers:

I've set GSC610C to "-3 dB" setting to tame the high frequencies (yellow graph) and we can see that it has the fastest decay at the crossover point, however it's still a bit higher than w/o the crossover (green graph, the topmost in the right part).

Listening Impressions

I checked my usual "spatial" test tracks:

• track 28 "Natural stereo imaging" from "Chesky Records Jazz Sampler & Audiophile Test Compact Disc, Vol. 3";
• track 11 "LEDR" from "Chesky Records Jazz Sampler & Audiophile Test Compact Disc, Vol. 1"

For sure, the localization is much worse in the car cabin environment with lots of reflections and asymmetric seating. On the first track, there was no difference between "left" and "extreme left" (and right) drum positions, however, the drum running around the listener still could be imagined well. On the second track, the "going up" sound wasn't actually moving, but the one making an arc from the left to right speaker was perceived realistically—to my surprise.

I also checked some musical tracks and compared them to the sound in Shure SRH1540 headphones matched by volume. The car sound was more "relaxed" and even had spaciousness, it sounded more "enjoyable" than in headphones, which sounded more "forward" and a bit brighter.

Costs

Here is what this upgrade cost me (excluding tools):

• HKL7 tweeters A2128201002, pair (from a disassembled car): $30
• HKL7 woofers A2048202102, pair (new): $318
• Door trim retainer clip A0009917940, pair: $5
• AudioFrog GSC610C crossover, pair: $194

Total: $547

Conclusions

This has turned out to be an interesting and challenging project, requiring a lot of research (at a hobbyist level). I'm glad that I started it, and like the result. It's a pity that basic car audio packages do not sound well. Next time I will be buying a car, I will definitely look into "premium audio" options.