2018 XC60 Subwoofer Project.
First of all, I give massive credit to mark-sf for his initial excellent article on constructing the subwoofer for the earlier XC60, the guidance on amplifier and speaker selection, wiring and installation (https://forums.swedespeed.com/showth...to-the-Low-End). I was really keen to go the same route as Mark, wanting a stealth installation and not needing massive bass like years gone by. All of the success criteria that he laid down appealed to me and after reading the article many times, I decided to have a go and use the same recipe.
I soon found out that the box dimensions for the earlier XC60 were not quite suited to the later model, without having to cut a hole in the cargo tray floor and I did not want to do that. The maximum depth of box I could fit was 5.5” and still keep the cargo tray sitting in the correct position. My model is fitted with air suspension and so there are a couple of aluminium reservoir cylinders on the sides of the boot, but there was still extra width and depth in the boot to enable me to increase the box volume to make up for the reduction of thickness.
Using the WinISD speaker program, I played with the internal volumes and tried different duct lengths and diameters. I could not buy the Precision Port duct without importing from the USA at great cost and various alternatives from e-bay turned out to be too small, then I realised I could 3D print my own duct through a friend. I made the rear (sealed) internal volume 8.40 litres versus 10.0 litres front chamber volume. This required the centre of the internal baffle to be installed 1” towards the rear chamber from the box centre. Using 3D CAD software, I designed the duct to the dimensions I needed, with a 1” flare inside and out. This gave a tuning frequency of 49.25Hz, a duct diameter of 53mm and length 233mm. With this non-standard diameter, I was able to reduce the maximum air velocity to below 29m/s, based on 30 being a maximum recommended I had seen elsewhere. The length also ensured that I had at least 2 duct diameters clear inside the box at the end of the duct. My friend Rod 3D printed this in two pieces with a joint. It’s fitted into the front of the box with screws. Final external box dimensions of my box are 26”x13”x5.5” and the build is otherwise the same as Mark’s.
I’ve included the screenshots of the speaker parameters in WinISD for this SWS6.5X, as initially I had some issues entering in the correct units from the manufacturer’s box, which were throwing the curves right out. Also the performance curves of my subwoofer using the software.
The Earthquake SWS6.5X speakers can just be made to fit in the box of this depth, but the ¾” MDF is not sufficient to separate the dust cones of the speakers when bolted face to face. Bolted to the wood, the cones could be heard scratching each other. I got my friend Rod to 3D print 3mm solid rubber gaskets, which gave just a little lift off the wood to stop contact.
The speaker magnet bolts fouled the insides of the box and so after marking the locations, I routed some grooves to recess the bolt heads. These speakers are a tight fit! I added 2mm thick neoprene tape in the locations behind the magnet to reduce chance of contact vibration in this area. I cut in a speaker terminal box to the front to connect the internal wiring to, instead of banana sockets.
The installation of the box required the sawing off of two studs on the boot floor, centre and RHS. A batten of timber approx 1.5” x 1” high was bolted to two existing brackets on the floor, to support the rear of the sub box. I initially fixed this to the box itself with two shelf dowel pins into the boot floor brackets, but due to vibration issues, then modified it to fix the batten solidly to the boot floor instead. I drilled through the batten into the box, and used four shelf dowel pins to provide a secure location of the box to the batten and still allow removal by lifting off as required.
The front of the box is secured with a steel L-bracket that I fabricated to pickup two threaded bolts on the seat cross member. I’ve yet to get any nuts to secure this as they are a weird thread, but the weight is adequate to hold it down with some neoprene tape to reduce vibration. The box is lifted slightly off the floor to avoid transfer of vibration from wood to metal that I discovered during initial fitment, as there are some bumps in the floor that create uneven contact otherwise (just spotted this point in one of Mark’s later posts!).
Using the Rockford Fosgate PBR300X1 amplifier recommended by Mark, I screwed this to the RHS end of the subwoofer box, with a plate of aluminium as a token heatsink, and it sits neatly under the floor without any extra fuss and keeps the wiring short.
Power is connected via 10GA wire (as recommended by Rockford Fosgate) nearby to the battery (in the boot of this later model), via a spare fuse slot in the battery terminal housing – I had to take the battery terminals apart completely to bolt in an extra midi fuse and run this to the amp, but it was worth it, as it keeps it looking stock. The earth wire connects locally to the boot floor. I did not need a remote cable, using the amp sense to switch on.
High level speaker connections were taken from the connector on the factory amp under the driver’s seat and these cables run under the carpet from the air duct under the seat towards the driver’s door, as I was able to push the cables through a void under the seat by removing the trim panel below the driver’s door and lifting the carpet. From here, it’s a simple route to the boot behind the trim panels. Splicing into the connector block wires was a little fiddly, because I didn’t want to cut the wires and I couldn’t remove the pins, which I would have liked to have done to enable me to heatshrink the connections. I bared the wires then soldered and wrapped with insulation tape, not ideal, but adequate. Wiring colours are different on the later model, but the same pin numbers as noted on Mark’s instructions. I found further photos and help on the later amp wiring on www.xc90audio.blogspot.com.
Front left connected to Brown (+ve) and Yellow/Grey (-ve). Front right connected to Purple/Brown (+ve) and Yellow (-ve).
My boot undertray and polystyrene insert were carefully sawcut to make space for the speaker box. Once I have finished testing and the box does not need to come apart again, I intend to carpet the box to match the boot floor.
So far I am very impressed with the low bass that the subwoofer produces, although using spotify I think the losses in the mp3 music format become apparent, especially with the narrow band of the bandpass design. Hence, I have since saved 320kb/s mp3 files to a memory stick, which is much better quality and seems to overcome this issue of missing data at the low end. Maybe Spotify on the highest quality will be as good. In terms of settings, I am running the amp crossover at 150Hz, the gain at 5. Factory amp equaliser settings are 100Hz 2 below centre, 300Hz at centre, still undergoing testing. Volume is a good level and doesn’t put so much bass through the doors. What I will say is that the peak of the bandpass box is more evident and perhaps the rolloff above could be a bit higher. As shown on my WinISD graphs, the sub peaks between 30-80Hz. The low end sounds good, so impressed for the size of speaker! Any way, testing still continues and now I am wrestling with the tags on my media library to try to get compilations to show as albums, not individual songs!
mark-sf – massive thanks for the original article and assistance!
WinISD program for allowing me to calculate speaker box volumes and duct dimensions.
www.Xc90audio.blogspot.com for an interesting audio upgrade article, but which mostly helped understand the wiring colours and pins from the factory amp.