We have now come to the final part of this journey. I have combined tuning and evaluation together as the way I approach tuning requires among straight measurements, leaving with the system in various states of tune for my ears to adjust before picking the final settings. This process has not yet been completed but I can report on results to date as well as my overall evaluation.
Let’s start with the evaluation and I will integrate tuning where appropriate, Going back to my original requirements in reverse order:
6. Be within my capabilities to implement – After completing everything none of the tasks were altered, because they were too difficult. While I could have spent more time finishing the enclosure, It is after all hidden and even with the cargo top raised only the top inch is visible. The paint was several coats of Rustoleum Textured Galaxy Metallic that matches the insert very well. While this project took time if you can cut a straight line with a saw and knife and solder wires, it is straight forward.
5. Have the driver(s) protected from physical damage through normal use. As the drivers are entirely enclosed yet accessible and the sub is below the cargo panel, this requirement was obviously met.
4. Make no irreversible changes or modifications to the car. If you have been reading closely, there were three modifications made. Obviously the cargo panel had a strut removed and the molded insert was cut out. However, both of these are replaceable parts should that be necessary in the future. The only irreversible change was the 3/8″ hole in the rear seat mount. I could have avoided this one through use of a bigger corner bracket, but as this hole is aligned with the other mount’s hole, I am not suffering this one modification.
3. Lose no significant cargo space. Technically, I lost some tray space in the insert but I don’t classify that as cargo space. Being able to hide it completely under the cargo panel while using the design and drivers I wanted makes me very happy. We need every square foot of that space for our trips.
2. Stay comfortably within the XC60’s alternator/battery capabilities. This was the requirement I was most worried about as it had unknowns that I had no direct control over. Up front I was concerned because of reports of battery drainage issues, and while I don’t have every option, I have most of the power demanding ones. Only time will tell but given that the LED that displays when the amp switches to its higher voltage mode almost never lights, and when it does it only briefly flashes, I am confident that I am not drawing anywhere near even 20A for sustained periods. Thus there should be no out-of-spec stress placed upon the alternator.
1. Extend seamlessly the existing system’s response to the mid-30Hz range at low distortion and comparable dynamic capability.
This is the portion most of you have been waiting for and the one where I will have the most to say. Note that the requirement contains both objective and subjective requirements. Let’s get the former out of the way first as they are the easiest. Computer modeling of the design predicted a -3 db down low frequency of between 35 and 36Hz. I measured ~36Hz when mounted in place. Now some may view this as too high a cut-off especially when compared to high-end home theater subwoofers. Though, the head unit is capable of playing back DVD movies in DD5.1, I have no desire to watch movies in my car. The .1 LFE channel will produce sound effects below 30 Hz but these are not music. Even in the home, if you have a stereo music system that produces good clean bass down to this range you would be very pleased with the bottom end. I should also mention that while down in level I did hear non-doubled 32Hz frequencies from the system.
The low distortion requirement is a bit tough to measure without more expensive equipment than I have. Fortunately, the ear is not very sensitive to low frequency distortion in a sub with a low pass filter. This design has two inherent advantages in this area. As a 4th Order Bandpass design it has an steep acoustic rolloff on both sides of its bandwidth. Also the isobaric alignment provides distortion cancelation as both drivers are moving in opposite directions relative to the signal. I did a frequency sweep test which would audibly reveal distortion caused by spider/surround issues, doubling, bottoming or artifacts such as air leaks, buzzing panels, etc. None of these were detected in the sub; however, I did find a number of spots in the car that I could get to buzz and rattle. I will take car of these if they do so with music.
The seamless integration requirement is both objective and subjective. If the factory system was nominally flat one would usually incorporate both a high and low pass filter to integrate the sub. This was not the case with the XC60 as I was already rolling off the bottom end with the 60Hz EQ control in order to get reasonable mid to upper bass response. This produced an asymmetrical arrangement as changing the “high pass” rolloff (EQ) also affects the signal the sub gets. This can easily produce a “chasing your tail” attempt at optimization. So what I did was to select a number of frequencies that produced fairly uniform sound level in various positions as my markers and then using a combination of the 60Hz EQ, Sub Gain, and Low Pass Cutoff control leveled my markers at about 80 db. I then started listening to my reference CDs for evaluating bass for the final tweaks. I am not finished with this process but currently am pleased with 60EQ: -8; 200EQ: +2; Sub Gain: Just past 1, Sub LPCutoff: 150Hz. If the 150 sounds high, you should remember that there is a built-in acoustical rolloff at 70Hz. Subjectively, at this point the system sounds very well integrated. You do not hear the typical car hump in the 80-120Hz region that causes boominess. For each of my CDs which span genres of Classical, Rock, Jazz, World, New Age, Country, etc, what first strikes you is that you know exactly which type of instrument is creating the bass notes and the bass does not interfere with the audibility of the other instruments or vocals. As there are no cabinet resonances and this is a close to critically damped design there is no bass overhang that would obscure the bass note decay. This allows you to clearly differentiate for example not only the type of drum but whether it is hit with a stick or mallet. Can this system be better integrated or equalized? The answer is yes but what issues I am now hearing are in the upper bass and upper midrange which are only apparent on certain pieces and I expect would take a DSP-based EQ unit to smooth out.
Finally, there is the comparable dynamic capability requirement to report, and this too is both objective and subjective. Obviously, one can measure the peak output at various frequencies. Not being able to observe the drivers in motion I was conservative in running up the volume as I knew from my amp gain setting that I would likely run out of driver excursion before clipping the amp. I ran the system up to 105db with the only strain being to my ears. While obviously not a competitive SPL level, it is one which more than hour of exposure will cause hearing loss. Quite sufficient objectively.
To report on my subjective evaluation of the dynamics I need to first explain some characteristic differences in designs in this area. Dynamics of course does not simply mean how loud but also how soft. Many arrangements score bass lines that are soft and subtle but no less important. These bass lines can give weight to a rhythm or convey a mood. Soft bass reproduction can be a problem for large driver in sealed boxes as to get the driver moving the signal not only needs to overcome the mass of the cone but the damping factor of the enclosure. This is an even greater problem for designs that run the driver below its resonance by mounting it in a very small enclosure as is typically done in car subs. This is because they must work against their suspensions as well. To provide a car analogy, you may have all the suspension travel in the world but if you hit a series of ruts at a speed above the resonance of your suspension, you will feel every one as if you did not have a suspension. These systems typically in my experience exhibit an on/off characteristic much like hitting turboboost. with enough power and piston area, they go very loud but disappear once the bass gets soft. Now one can argue that a car environment is so noisy that one can’t hear soft or subtle anything but the ear is quite discriminating and can filter out a surprising level of background noise.
Now having explained what is not a good design for subtle bass, how does this design and implementation stack up. First, the drivers have a very lightweight aluminum cone. They are driven in push-pull mode which applies double the force to the cones for a given signal level. Therefore instead of using the additional driver to move more air, it is used to lower the system resonance (emulating a bigger box) and provides better acceleration. The net result is an ability to reproduce bass that one may not have known was there. When I was evaluating the XC60 vs. the RC350, I found that given the stock tires the XC60 was quieter on the freeways due to less intrusive road noise. Thus with everything closed and blower on low or off, I can get very good dynamic range and I don’t feel the need to turn up the volume in order to hear the bass.
In summary, the result was better than I expected. I had anticipated that I would probably have to add a DSP-EQ unit for reasonable integration and the impact of having the port firing into the cavity behind the rear seats was unknown. Neither of these proved to be issues. As to what I would do different if I did it again, so far outside of the few changes that I already have incorporated in these instructions, there is nothing that I would change. Quite frankly this is the best quality, totally hidden, car sub I have heard. Now I do believe the overall system could be improved by adding a DSP and 5-channel power amp in addition to this sub and its amplifier, but for my money (even with my champagne audio tastes) this is well down the road of diminishing returns. For a bit over $300 and labor there is not a better audio enhancement that you can make to the XC60.
Hope you found this journey interesting and informative. Please feel free to respond with questions or comments. I would prefer they be posted here instead of by private message, so others can benefit from the exchange. Hope some of you give it a try. Please let us know if you do.