In the Fall of 2019 I purchased a 2018 Leaf SL from a local Minneapolis Nissan dealer. In the 9 months I’ve had the car, I’ve been very, very happy overall, with the exception of the sound system. I’ve been an “audio guy” for more than a few decades, and most OEM (and aftermarket) car audio systems just don’t sound very good to my "audiophile" ears, so this is no big surprise to me. The Leaf came with the BOSE system, which sounds far, far better than the non-BOSE system (as it should), and I think the value of the system is fair for the added cost to the car. No car stereo shop on the planet could ever design and install a system with the same fidelity, features and integration into the car for the same cost. None.
The CarPlay head unit is generally good--even though my Leaf’s CarPlay is far from perfect (the unit has a penchant for not loading the phone every time when connected via USB) but because its integrated into the car’s electronics, it’s the only element of the sound system that cannot be replaced without losing functions.
2018 Leaf SL charging up!
Since buying the car, I’ve done quite a bit of Google searching and looking through various Leaf forums looking for detailed information about the BOSE audio system, with the hopes of finding posts from folks who did partial or system-wide audio upgrades. There are a few out there. But most are limited in detail.
I’d like to see if I can create my own here that will serve as a basis for helping readers with facts that they car use as a basis for their own upgrades. That’s the intent of this thread. I warn the casual reader that this write-up will be long and will contain technical data about the car and about the equipment I end up using, so if you like brief write-ups, this is likely not the thread for you. On the other hand, if you're a data person who likes process, testing, and experimentation, stick around. I’m at the beginning of what will likely be a phased, months-long upgrade to the sound system, and I’d like to take you, the reader, along with me as I document the process. If everything goes to plan and you have a Leaf you'd like to upgrade, my process should both answer questions and shorten your work.
The goal of my work is to finish with a system that is properly integrated into the car, with minimal loss of room and done in a manner that is electrically and mechanically of the highest level I am capable of accomplishing. There will be no alteration of the car's structure or electrical system. The completed system will have bass that is deep and natural-sounding, combined with linear and extended frequency range with a high level of tonality allowing recordings to sound their best. The system will not be a "boom car"; rather, it will be a place I can enjoy music as it was recorded. I don’t plan to deviate from the stock speaker positions, as I want the ability to easily reverse the work at the time of sale, and do it in a manner that fully restores the system back to the condition it was in when purchased.
A little about me, my experience, just so you know where I’m coming from in this conversation. I’m in my early 50s and currently work as a technology manager for a large electronics retailer. I was an electronics (Sonar) technician in the US Navy on active duty for six years and retired from the military a few years back after completing 20 years in the Air National Guard (US Air Force) in the telecommunications field. My undergraduate degree is in computer science, and I’ve been a hobbyist in audio and electronics since high school. My field of specialty in my employment is in home audio/theater, video, cabling, lighting, control/automation and networking. I hold various certifications from control-system companies as well as Lutron Radio Ra2 and HomeWorks lighting control system design/programming certifications, as well as CEDIA (the trade body that covers home-technology trades folks). While I don’t currently teach, I’ve held certifications from the State of Washington Department of Labor & Industries for continuing-education courses for licensed electricians. Oh, and I've installed 100+ car audio systems over the past three decades--simple to highly complex--some of which have participated in (and won) IASCA car audio sound-quality competitions. That should serve as a bit of a basis for knowing a bit about my background and experience.
Phase 1: Goal “More, better (tonality) bass response for minimal work”. This phase of the project is simply to see if I can get additional, better bass response from the system without adding amplification or otherwise altering the sound system. I reasoned that if the BOSE amplifier could power the small BOSE “vented” enclosure with a single 5” woofer, that same amplifier could power an aftermarket subwoofer, provided that sub had the same impedance as the BOSE speaker.
To test my hypothesis, I purchased a Kicker 43TCWRT82 pre-loaded sub-woofer enclosure that has an 8” dual-voicecoil speaker and passive radiator. The enclosure has a larger woofer, a larger enclosure than the BOSE, so I reasoned that the speaker would have a higher sensitivity than the BOSE speaker so it would provide more output with the same amount of power the amplifier delivered to the BOSE speaker. The Kicker sub has an average impedance of 2-ohms. I knew the Leaf's BOSE door speakers are 2-ohm models, so I reasoned that the BOSE sub-woofer would be as well (I was not correct), and that all I needed to do was unbolt the BOSE sub, then connect the Kicker sub-woofer to the speaker cable coming from the amp and I’d quickly and easily have more output and better tonality.
I began the work by unbolting the BOSE sub enclosure from the car and proceeded to remove the speaker from the lightweight fiberglass-plastic enclosure so I could disconnect the speaker’s wiring harness from the BOSE speaker. I didn’t wish to cut or splice into the car’s harness, so my only option was to disconnect the wiring from the 5” BOSE sub, then use that as the basis for the plug-in harness into the car, with the other end going to the new Kicker sub box.
Bottom of the BOSE enclosure. 5\" woofer driver seen fires down into the car's hatch floor area. BOSE woofer removed from enclosure. \"0.5 OHM\" label seen on magnet, indicating the average impedance of the driver.
The BOSE enclosure came away from the car easily and the speaker also came out of the enclosure easily, but that’s when I learned the BOSE sub was not the same impedance as the car’s door speakers. Once I read the markings on the BOSE sub’s magnet, I knew my assumption was wrong. Rather than it being a 2-ohm speaker, it’s actually a 0.5-ohm speaker. I immediately knew that if I connected the Kicker sub enclosure to the stock amplifier, that the amp would deliver just 25% of its power into a 2-ohm load. Put another way, the Kicker sub is 4x the “resistance” of the BOSE sub. Curious as to how it would sound, I connected the Kicker to the amp and gave it a listen. It had almost zero usable output. The Nissan/BOSE amplifier simply was expecting a lower impedance, so it was pushing very little power into the Kicker.
Resolution: I knew the Kicker sub-woofer was a dual voicecoil speaker with two 1-ohm coils wired in series, producing a 2-ohm load. The fact that the enclosure used their dual 1-ohm coil speaker was a stroke of luck for me, actually, as it meant I could rewire the voice coils in parallel, creating a 1/2-ohm load--effectively the same "load" as the stock BOSE sub. I quickly removed the speaker from the enclosure, changed the wiring, re-installed the sub into the box, connected the amp and jumped into the car for a listen.
Woofer sitting in the same area as the stock BOSE enclosure. It has not been secured to the vehicle at this point, but will be shortly. Finished assembly dressed with fabric wiring harness tape for a neat appearance Connection at Kicker enclosure. The 2-conductor cable wires are each terminated with wire ferrules and the jacket is covered with a 1\" length of shrink tubing for a neat appearance
The level of the bass was certainly higher by a few dBs, and the tonal balance was better, but it was well under my expectations. I'd say the improvement was, perhaps, 20%. Some songs are more “fun”, while others offered less improvement. I knew the OEM amplifier was very low power, so this made some sense. I've heard Kicker sub-woofers in custom-made enclosures as well as ones in their pre-made systems (like this one) in quite a number of car systems over the years and know that with the right signal and amplification, they can be exceptional performers, so the speaker I bought for the Leaf is not the weak link.
OEM systems have a variety of signal processing related to the volume level to prevent speaker damage (dealers hate replacing speakers under warranty), and they also limit the frequency range sent to speakers to tailor the sound for a target customer, so these two elements combined can severely limit performance when the OEM system is blended into aftermarket equipment. Based on what I heard, I knew some funky stuff was at play in Leaf's audio system. I can say with confidence that if I had paid a retailer for the speaker and the installation work, I’d be underwhelmed with the investment. Is it more fun? Louder? Yes. Better tonality in the bass? Yes. But....the system as it stands is far from what’s possible with the Kicker sub-woofer enclosure, and, far from what’s possible in the car generally.
In the next post, I’ll share some measurements taken from the BOSE amplifier showing exactly what the amplifier is doing with respect to equalization, power and volume limiting.
In this entry, we’re going to take a close look at the signals coming from the BOSE amplifier to understand 1.) why the aftermarket Kicker sub-woofer is underperforming. 2.) if the speaker-level signal from the amplifier is a good connection for an aftermarket add-on subwoofer amplifier. 3.) if there’s any volume limiting on the sub-woofer signal. 4.) what frequency range and equalization is applied to the front door speakers.
I'm using an AudioControl DM-RTA signal tester connected to the speaker wires. The audio signal fed into the amplifier is 20Hz-to-20kHz pink noise coming from my iPhone X, connected via USB, playing the audio file from a test CD streamed through Tidal. While the DM-RTA has the ability to measure acoustic frequency response, I am not performing those tests at this point in the project.
We’ll begin with the signal coming from the BOSE amplifier at the sub-woofer. BOSE amplifier signal at sub-woofer
The ideal signal is one that’s flat from 20Hz up to a low-pass frequency, usually in the 80 to 100Hz range, or where the front speakers begin to come in. As you can see, rather than a flat response, the signal from the BOSE amplifier is anything but flat. Rather, it’s got a very strong peak centered at 55Hz, with little energy above or below this peak. My guess here is this boost corresponds to the tuning frequency of the BOSE sub-woofer enclosure, and it also happens to be the area in music that corresponds to the “punch” in bass and drum sounds. The car’s small cabin naturally boosts frequencies beginning around 60Hz (a phenomenon referred to as “cabin gain”), so even though the amplifier is sending little signal below 55Hz, the BOSE sub’s interaction with the car’s interior naturally boosts up the lower octaves. I didn’t test the acoustic response in the car using a microphone, so I can’t say how it would have measured.
As I ramped the volume, I discovered the signal to the sub-woofer stopped increasing at around the 60% range on the in-dash unit’s volume control. This is not uncommon at all, as it ensures the woofer is not ever damaged at high listening levels. The amplifier’s powering the sub-woofer likely runs out of power at this volume level, too. There’s simply just no more “gas in the tank” so to speak.
I also measured the voltage of the BOSE amplifier and it showed a maximum voltage of 5.6v RMS sent to the sub-woofer corresponding to 65 watts into a 0.5-ohm load, which also accounts for why the Kicker sub-woofer doesn't have much, well.....kick.
Suitability for connection to an aftermarket amplifier: it’s my view that unless you’re willing to use a DSP processor, or an amplifier with a DSP stage at the signal input to correct the non-flat signal on the sub-woofer amp channel, it’s not ideally suited for this purpose. Also, the volume-limiting feature is an issue that will limit performance at levels above about the 60% point in the volume range. The best point in the car’s signal path to pick up a signal for an aftermarket sub-woofer is at the input to the BOSE amp (thus bypassing the amp altogether). If a person were to use a sub with a remote level control, this could be compensated for, as you could adjust that control if you wanted more bass when listening above 60%. I haven’t measured the signal coming from the in-dash CarPlay head unit, but historically, in cars with an OEM amplifier, the signal coming is the in-dash unit is full-range, flat, with no volume-limiting, as all of these functions are performed inside the amplifier.
Here’s a look at the signal at the (R) front door speaker. This was a bit of a surprise to me. I expected to see a signal that has been high-passed (no low bass, as that’s handled by the sub), but I expected the signal to be reasonably flat out to the highest frequencies, or around 20kHz. As you can see, the signal is high-passed (low-bass is removed), but it’s also equalized, likely to remove peaks in the speakers response in the 315Hz and 1.3kHz ranges, and the higher frequencies are rolled off beginning around 5kHz at about a 9db/octave slope. The door speaker doesn’t have tweeters, and is mechanically incapable of playing the range a tweeter would generally handle—around 4kHz and above—but I was surprised the signal coming from the amp didn’t even contain much signal in the higher two octaves (5kHz to 10kHz and 10kHz to 20kHz).
BOSE amplifier signal at front door speaker. Note the very uneven response, with two peaks, two cuts and quick drop-off above 5kHz.
What does this mean for adding aftermarket speakers to the BOSE amplifier? I think it’s clear that because the amp is removing the higher range, any aftermarket speaker will have its performance limited from a frequency range standpoint. It will sound far, far better from a tonal standpoint, but the speaker will be handicapped from what it is capable of. These measurements demonstrate that the only way around this is to bypass the BOSE amplifier entirely. The good news, though, is music—especially streamed, compressed music—has limited signal content about 12kHz. Also, even though human hearing extends out to 20kHz, most adults—especially male adults—have less-than-ideal hearing, so their ability to hear much above 15kHz is limited. I include myself in this group, I should add.
Before we step away from this, while setting up to take measurements, I listened to the car's system with the front door speakers disconnected, playing just the rear door speakers. BOSE is definitely low-passing these speakers, scrubbing a lot more of the mid and higher frequencies from them. They are effectively "fill" speakers, likely playing between 80Hz and 1kHz. That is, they do not get the same frequency window as the identical 6.5" front door speakers, and they are also set to a lower level than the fronts when the fader is set to the mid point. This means simply replacing them with aftermarket speakers will lead to significant disappointment and offer a very limited return for the purchase. I'd avoid doing that at all costs if retaining the BOSE amplifier.
Okay….that’s all for this posting. On the next post, I’ll share images of the signal sent to the a-pillar tweeters. The wires for the tweeters, I believe, route back to the amplifier and are wired in parallel with the door speakers, driven by the same amp channel. As a result, if this is true, they will also receive a signal that declines beginning at 5kHz. That could definitely account for the lack of “air” heard in the signal...but we'll see what the meter shows. When I listen to music with high end, there's just not much “there” there, so to speak. I suspect that's the BOSE system tuning, where they tailor the sound in a way that is preferable to the typical customer. BOSE has designed speakers in this way for a long, long time. It's not that they can't make a better-sounding speaker. They certainly can. Rather, their research into customer preferences drives how they tune sound systems. It's the BOSE "house sound", so to speak. Next time we'll also begin the work of disassembling the front door panels and replacing the 6.5" BOSE speakers with Infinity Kappa co-axials.
After additional reading, I’ve discovered that the front BOSE speaker system (A-pillar 1” tweeter and door 6.5” woofer) are actually actively-powered by the BOSE amplifier, with dedicated amplification and crossover filtering for each of the four speakers. This explains completely why the tweeter works, while the door woofer rolls off at 5kHz. If they were wired together and the amplifier filtered the high-frequency signal, the tweeter would have very little signal to play. I didn’t expect the LEAF’s audio system to have a stock 7-channel amplifier and be this sophisticated. Go figure.
The work began by disassembling the doors. This was probably the easiest and simplest door disassembly I’ve done—possibly ever. The panels are secured with just two screws, with the remainder of the panel attached with press-fit pins. One screw sits behind the door release latch panel, with the other under the lock and window control panels. I used a pry tool to remove the panel to gain access to the screws. And from there, a solid tug beginning at the lower section of the panel was all it took. The panel was removed from the door in under a minute.
- Infinity Kappa 62ix coaxial 6.5” speakers
- Metra 82-7400 speaker brackets
- Metra 72-7401 wiring harnesses
- Stinger Road Kill RKFR6 Fast Ring kit
- Tesa 51608 19mm fabric wiring harness tape
Using a pry tool to lift the control panel. A slight lift and the unit unclips from the door. Pull the panel backwards to remove it from the keyed channels
I’d read that some folks said the speaker brackets they purchased, that were designed for Nissan vehicles, weren’t a perfect fit for the LEAF, and I worried that may be the case for me, too. I wondered why this could be the case, because it’s not like Nissan has an incentive to design the LEAF any different than their other vehicles. We’ll see if that’s the case as soon as we remove the BOSE speaker and compare the hole patterns and spacing. The speaker came away from the door easily using a 10mm socket wrench. Simple. I disconnected the wiring harness and quickly checked that the Metra wiring harness plugged into the LEAFs wiring. It did. I’d never heard about others having problems with the BOSE system using a proprietary harness compared with the non-BOSE LEAFs. I was glad the harness fit and clicked into place easily.
When I placed the Metra speaker bracket to the door, I quickly saw it wasn’t a direct fit to the door’s speaker hole pattern. It was close, but not identical. See photos detailing this. To see if I would simply remove some of the plastic from the Metra bracket to get the needed spacing, I overlaid the bracket on top of the BOSE speaker and saw I could make it work if I removed about 5mm of plastic—deepening the cutout in the tabs. The ideal tool for this is the Dremel with a spiral-cutting bitl. Once I’d deepened the groove, the bracket fit the door’s hole pattern, centered on the speaker opening.
Bracket doesn't align with OEM speaker mounting holes Deepening the mounting channel using a Dremel tool Test fit. The Metra bracket now fits the door's mounting holes.
There’s a video on YouTube detailing a speaker installation where the car’s owner ran into the same issue and chose to drill out the metal to support one or two screws, rather than adapting the bracket. That was likely because he didn’t have access to a tool to do this work. One of the goals of this project was to make no modifications to the car’s structure, so modifying the bracket was the only option if I was to keep that goal. Metra is the largest company making these kinds of parts, so I wonder if a different bracket exists that’s a direct fit, or if this is a problem everyone’s facing. I know the most ideal scenario is not to have to modify the bracket, because not everyone has the tools for this work. After all, these parts are designed to be “direct fit” parts. Hmm…
On to the speaker harnesses. They fit properly to the Nissan wiring, and that’s great, but Metra didn’t include enough wire to reach to the speaker’s terminals inside the door. As a result I needed to double the harnesses length, adding approximately 6” of 18ga wires. Because the harness is on the front side of the speaker and the connection to the speaker is on the back side, I used the Dremmel tool to cut a notch in the speaker’s bracket as a pathway for the wiring to transition into the door to the speaker. See photo for detail. To protect the speaker harnesses wiring, I wrapped it in Tesa fabric. I added extra tape around the area where the harness passed under the speaker bracket as filler, to ensure a proper seal.
Metra speaker harnesses extended, terminated with ferrules Tesa wiring harness tape wrapped around harness.
The Infinty Kappa speaker uses insulated screw terminals for connections rather than the terminals that are commonplace with most speakers. This is a really smart and thoughtful design choice on the part of Harman. There’s no chance of breaking off the tabs, or for shorting against the car’s metal. This impressed me greatly, and it also allowed me to terminate the Metra harness using wire ferrules for a clean connection.
Back of speaker showing harness attachment to speaker
Before installing the speaker, I cleaned the metal behind the speaker with a spray kitchen degreaser to get a clean surface in advance of attaching the Fast Ring foam “plug” that is designed to absorb the back wave coming from the speaker. About a decade ago I used the Cascade Audio Deflex panels in a Honda Accord and they appreciably improved the door speaker’s sound by reducing some coloration that came from the back wave of the speaker reflecting off the door and coming back through the speaker. The foam plug that’s part of the Fast Ring kit works on the same principle—and far, far less costly than the Deflex panel.
Stinger Fast Ring \"Plug\" attached to door skin. Foam prevents speaker's rear wave from reflecting off the metal panel, coloring the speaker's sound
Once the plug, speaker and bracket were mounted to the car’s door, I turned to installing the Fast Ring foam sealer rings. They’re designed to be trimmed if needed, but the LEAF’s door bump-out (into the car) is fairly deep, and a quick test fit showed that the rings didn’t need any modification. After everything was mounted, and a quick listening test conducted to verify the door speakers were playing, I re-attached the door panels in the reverse order they came apart, tested the windows and locks and that was it for the speaker installation work and it was time to give the system a listen, which I’m certain most readers are curious to hear about.
Completed assembly. Infinity 62ix co-axial speaker and Stinger \"Fast Ring\" foam acoustic sealing rings.
I use the Tidal streaming music service, and have a number of playlists I use for testing audio systems. Tidal streams at 44.1kHz/16-bit, which is the same as CDs, and is far, far better than most competing music services from a sound quality standpoint. As the LEAF lacks a CD player, listening to Tidal via CarPlay (USB) offers the highest fidelity signal into the car’s system. My immediate impression was the system was far, far smoother in the midrange, with plenty of “body” and far, far more able to resolve details in the audio mix. Gone was the mid-range emphasis so common to the BOSE tuning. Infinity states the Kappa speakers have a 95dB sensitive which is a very high figure. I’d question this number because even though they are 2.5-ohm speakers, and are essentially the same impedance as the BOSE speakers, they were quieter—probably by about 3dB. To get the same volume I needed to get a bit farther into the radio's volume control.
The very good news here is the BOSE amplifier doesn’t clip (distort), so going farther into the volume range on the radio is fine. One big, unexpected bonus was the sensitivity of the Infinity speakers is a better match to the BOSE A-pillar tweeters. They are now fairly well matched in level, and I can now really hear them adding “ambiance” across the dashboard and window, lifting the sound up off the door. You really notice their contribution to the sound by covering them with your hand. If you do this, the sound falls to the bottom of the door immediately. Remove your hand from the tweeter, and it almost magically lifts the sound back up to the windshield. With the BOSE speakers in the doors, the A-pillar tweeters levels were set in the ampliifer by BOSE to be barely audible, with the result being the door speakers dominated the listening experience.
I intend to replace the cheapie plastic-dome BOSE tweeters with a pair of 1” silk-dome tweeters fairly soon. While the BOSE tweeters are now more audible, and making a more usable contribution to the sound, they’re really not that good—at least from a “what’s possible” standpoint sonically. I hope to get my test equipment connected into the signal coming from the amplifier to see where the high-pass filter is set and if there’s any equalization applied to the signal. If there isn’t, I may be able to simply drop in a set of tweeters and have a more open, smooth, transparent sound.
Let’s have a few words about mid-bass. The new speakers have an incredible amount of it—more than my tastes care for on some music. It’s really a bit overdone. Some of this is the better speaker, some of it is the foam rings that ensure every bit of pressure makes it into the car’s cabin vs leaking it behind the door panel, causing energy loss and unwanted vibration….but the 5dB boost of equalization center between 50Hz and 100Hz built into the amplifier is the most significant contributor to this sound. It’s so much of a problem for my ears that I’ve set the bass level to “-1” to tame things down, when I used to set the bass to "+3" with the stock system. If you like pop music and a “fat” sound, leave the bass control a “0”, but for me it sounds quite unnatural at that setting.
All of this testing and speaker work leads me to the firm conclusion that I can’t keep the BOSE amplifier in the system long-term. It’ll do for the time being, but to get the most from the speakers, it has to come out and the in-dash CarPlay unit will connect the unprocessed signal directly to the new amplifier. That’s the best overall option. I know that I'm not the typical listener, though, so please take my words here with a grain of salt, so to speak, and decide for yourself. If I didn't have experience doing this work, was on a very tight budget, leased the vehicle, or had to hire out the work, I'd be quite content with the system as it stands right now. It's "fun". But, I can't say I entirely fit into all of those categories as you can tell.
When the BOSE amplifier comes out, I plan to replace it with an AudioControl D-6.1200 DSP, a 6-channel amplifier with line and speaker-level inputs, and a full DSP engine. It offers everything needed from a power and channel-count standpoint, and it's a "single box" solution, which is important in a car will little spare room to hide electronics. The rear door speakers will be disconnected, leaving the final speaker complement to be the A-pillar tweeters, 6.5" door speakers and subwoofer. I'll stay with the Kicker pre-made subwoofer for the time being, but will eventually construct a custom enclosure holding multiple small woofers (either 8" or the newer 6.5" subs). I won't know the exact mix until I run air volume calculations. Either way, the custom enclosure will essentially fit into the footprint of the BOSE sub-woofer enclosure and be no higher than the rear seats when folded. The enclosure will have a 2" deep "nook" on the back side for the amplifier and system cabling and will be secured to the car at the BOSE sub-woofer's 10mm bolt threaded attachment points. When it's time for the LEAF to be sold--in four or five years--I can return the car to its factory system fairly easily and then explore selling the enclosure and sub-woofers to another LEAF owner. I suspect there will be someone interested.
Next time, I’ll have screen shots showing the amplifier signal at the tweeters and likely new tweeters as well as well as listening impression of the system with them. I may also run my first acoustic tests, showing the car's frequency response at the driver's seat. For now, though, it's my hope that this entry can/will provide all of the information many LEAF owners need to tackle what is likely their simplest option for improved sound—just replacing the door speakers.
I made a few trips in the LEAF this afternoon and had time with the sound system. I have to say that I'm really, really enjoying what I'm hearing. The system is now appreciably more detailed and open. There's "air" in the sound, where you can hear between notes, and music is more punchy, dynamic. The other aspect of the sound that stands out is the effect of the A-pillar tweeters working with the door speakers, where they're matched evenly in level. This is especially noticeable when playing music with a fair amount of higher tones like jazz, or live music, where the recoding contains the sound of the audience or the hall sound when the singer speaks to the audience. When this happens, you hear the "room" and the door + A-pillar tweeters create the effect of the sound playing across the dashboard and windshield. Wow!
If you own a LEAF and only change the door speakers using the model (Infinity Kappa 62ix), you will get the same outcome. That's a lot of improvement for $150 speakers and $50 in installation accessories.
I connected the DM-RTA tester to the speaker wires at the A-pillar tweeter and the results were certainly interesting. The tweeter's high-pass filter is set (in the BOSE amplifier) at around 6kHz. That part I expected. The surprising discovery is BOSE placed a low-pass filter into the speaker's signal as well, limiting the high range to just above 12kHz (the 6dB down point). I'd have expected the signal to the tweeter to run all the way up to 20kHz, but that was not the case. My guess as to why this was done is it limits the band where noise shows up with poor signal sources (compressed, streaming audio) and, possibly, also the amplifier's noise floor. I don't hear any "hiss" in the system, but that could be a function of the filtering. Nissan LEAF A-pillar tweeter signal from BOSE amplifier
The next phase of work on the car will involve replacing the BOSE tweeters. I've ordered a set of tweeters from a 2006 Sentra from salvage dealer on eBay and plan to remove the speaker's connector harnesses from them. I'd have done this from the BOSE tweeter, but it's a single, molded part, where the non-BOSE tweeters have a "pigtail" harness that's easily removed from the tweeter itself. This will allow me to use aftermarket tweeters, assemble the wiring on the workbench, then and plug them into the Nissan tweeter harness at the A-pillar without altering the car's wiring. Later, when I sell the car, I can easily reconnect the BOSE tweeters. The Sentra tweeters were $25--more than I'd have wanted to pay effectively for a set of wiring harnesses, but given that the aftermarket interface industry (Metra) doesn't make them, that's not too big of a deal. I expect them to arrive in a week or so. I purchased a pair of JL Audio 1" silk dome tweeters, which will replace the BOSE models, and I expect them to be appreciably more transparent, smooth, open-sounding by comparison. Since I'm still using the BOSE amplifier, the new tweeters will see the same filtering, but the difference should be notable.