I have started this thread to detail by effort to provide a first class bottom end to my 2011.5 XC60's Premium Audio System. While it took several weekends to complete this project it is not a difficult one and I hope others here give it a try by following these instructions. I will also offer support and welcome comments and questions in this thread.
To start at the beginning in order to provide context, I admit I am a longtime audiophile and desire to have my daily commuting environment able to satisfy my wide ranging musical tastes. When originally evaluating the XC60, I was please to see the partnership with DynAudio as I was quite familiar and impressed by their home speakers. However, when I actually went to test drive one, I learned that the system had been changed mid-year and the dealer was not sure whether DynAudio was still involved or if not who replaced them. I also learned to my dismay that a subwoofer was not included as one of the 12 speakers in the Premium option though it was an additional option. Further I found out that this option was not available in the U.S. and in Europe was the equivalent of $1200. Even though I was disappointed overall the ne Premium system appeared capable and the car was on the top of my list for other reasons. so I went ahead and ordered it.
After taking delivery and loading up my music and setting everything flat I was appalled by how emphasized the bass was. Even my wife commented on it. Fortunately the Premium system has a 5-band equalizer and by reducing the 60Hz seven steps and boosting the 200Hz by 2 steps, I got the sound reasonable but unfortunately there was no quality low bass in the 30-60Hz region. I will say with if this was the flat setting, most people would have little to complain about. Unfortunately, when it comes to music reproduction, I am not most people and thus began my journey to design a custom subwoofer for this deserving car.
For those who want to see what they will get if they complete the project, here is a picture of the completed installation with the cargo floor removed (It does lie flat when installed):
Completed Subwoofer Installation
Since this was not going to be my first sub design, I knew from experience to put down my requirements first. Without a clear target one can forever be "tinkering" instead of enjoying. My requirements for the design and installation were the following:
1. Extend seamlessly the existing system's response to the mid-30Hz range at low distortion and comparable dynamic capability.
2. Stay comfortably within the XC60's alternator/battery capabilities
3. Lose no significant cargo space.
4. Make no irreversible changes or modifications to the car.
5. Have the driver(s) protected from physical damage through normal use.
6. Be within my capabilities to implement.
For starters, I researched as much online information as I could regarding DynAudio's OEM subwoofer design. While I never have actually seen one, the pictures, diagrams and specs, I have found indicated that it was a vented bandpass design using 2 6.5" drivers side-by-side in an enclosure that ported forward into the rear seats. From the its size and knowing DynAudio's drivers I knew that while better than simply using the doors, simply reproducing that design was not going to give me the low end extension I was looking for. However, the existence of this option, did provide me with a candidate mounting location that would not interfere with cargo and was fully protected. I did consider the more "in fashion" approaches of building a custom fiberglass enclosure in either the spare tire wheel or on the side. I dismissed both due to the power needed to overcome the small enclosures and the requisite low-end signal boost to overcome its acoustical rolloff and the fact that it would be very difficult to physically protect the driver without interference. I also did not have significant experience working with fiberglass and did not want to farm it out due to cost. Bottom line, find a way to use the existing subwoofer space.
One reason, DynAudio undoubtedly decided to use the vented bandpass design was that it provides bass gain where a sealed/vented design of the same size would have a bass roll-off. By placing the drivers face-to-face and wiring them out of phase in an isobaric configuration, I knew that I would get the extended low end of a box twice the size. Additionally I would get lower distortion due to harmonic cancellation as the drivers are working out of phase. The only problem then was to find drivers that were shallow enough to mount face-to-face in an enclosure that at best would have 5" of internal height. Fortunately I found a 6.5" sub from Earthquake Sound (SWS-6.5) that has only 1.75" of depth but has an inch of cone travel. Entering in its parameters into WinISD revealed that I could easily achieve my desired low end response with a 1 cu. ft. enclosure as long as the vent could be tuned to ~50 Hz which meant it had to be approximately 8" long by 2" in diameter. As one needs to have at least 1x Vent diameter in rear space, this meant the enclosure needed to be at least 10" plus the thickness of the rear panel deep. Best of all, these drivers are sold as a pair for less than $100.
My next task was to figure out how to design the enclosure in such a way that it would be air-tight where it needs to be and still be able to have access to the drivers if I needed to replace one. Also since the tuning of this design is dependent on the volumes of the respective sealed and vented areas as well as the vent length I needed a tuning capability. By mounting the drivers and internal dividers on the front panel, and by making the panel initially a bit longer than required, I was able to tune it by sliding the panel thus changing the ratio of the two internal areas and also have a way to gain service access to the drivers. Below are pictures of the two pieces.
Front of Top Plate resting on Enclosure
Rear of Top Plate resting on Enclosure
The construction diagrams for the enclosure:
To construct the enclosure, I chose to used MDF for its stability and ability to hold an edge. The sides and interior are made from 3/4" MDF in order to have sufficient width to accept #10 x 1 1/2 wood screws edgewise. All joints should be glued and screwed every 3 to 4" Ensure that 1/8" holes are predrilled and that screws are placed so that orthogonal ones do not run into each other. Additionally the interior and front panel seams should be caulked to ensure an airtight enclosure. An adhesive-backed gasket should be applied to all of the edges that contact the front panel and interior pieces. This will take the place of the glue and caulk you will not be able to use. As this MDF is thick relative to the size of the pieces tolerances should held as precise as possible which means that the thickness of the saw blade must be taken into account on all cuts.
The trickiest part is cutting the circles for the drivers and the vent. I used a circle jig and my router. A heavy duty jig saw can also be used if you have a steady hand. Neither circle needs to be pretty as they both will have their edges covered. Also the driver cutout does not have to be precisely centered. IMPORTANT, the drivers will need to be bolted together to either side of the MDF. There is not much clearance between the opening and the screw holes. Make sure you have enough distance to support the screws and flange.
I chose a Precision Port 2" double flange part because it is not only well made and adjustable, but using flanges on both ends reduces the overall length required to reach the desired frequency. I chose to gasket and glue to to the front panel but it can be screwed as instead.
This design requires that the drivers are connected in parallel but out of phase. What this means is that the positive (+) terminal of the sealed driver is connected to the Negative (-) terminal of the vented driver which is then connected to the Positive (red) terminal on the enclosure. The other negative wires are connected in the same way but in a N-P-N configuration. To ensure you don't get this wrong and damage the drivers, make sure that when you bolt them together they are aligned to allow the wiring not to have to cross. To mount the drivers first run a circle of the gasket on each side of the board around the hole. I decided to use the plastic ring since the drivers' rim could use the additional support. The 4 #10x32 machine screws should be threaded through a sandwich of flange, ring, gasket, wood, gasket, ring, and flange. I used #10 locking nuts to ensure the connection will remain tight.
There are several approaches one can take to the speaker terminals. I could have simply used a pigtail arrangement, but I wanted to be able to remove the speaker without having to remove the wire-run to the amplifier. I chose the sheathed banana style as it allows a quality soldered connection and there is no chance of the connectors shorting together or falling out. The internal wiring as seen in the picture is very short and can be 12 - 16ga. I recommend all connections be soldered. As the driver terminals are small and vertical clearance is tight I recommend using a continuous run for each wire and for the vented driver that needs to support two wires, strip away a section of insulation and insert the terminal through the center of the strands and solder on both sides. Below is a picture of the wiring from the top.
Subwoofer Wiring - Top View
Note that you need to drill a hole for the wires from the the bottom driver and that the hole is sealed with caulking as are the terminals. Also I recommend that you apply heat shrink to connections as it will prevent shorting and provide a more stable connection.
More to come (including a complete parts list) ...