Slim sub install in UK spec Civic Prestige (Hatchback)

[Squonky]

I wanted a bit more bass in my Civic, but didn't want the boot/trunk space filled up with a huge sub enclosure.

After much searching, I settled on the TS-WX70DA powered sub from Pioneer. Reasons for choosing this over all the others:

• I might yet buy the space-saver spare tyre kit (UK spec Civics only come with the tyre inflation gunk/compressor combo).
• Very slim - only 97mm (3 7/8 ") thick.
• Only 850mm wide - fits perfectly between the rear wheel arches.
• Class D amp - only needs a 10A (~140W) power connection to get 100W RMS. You can get 180W from the rear cigarette lighter socket! In contrast, the Pioneer TS-WX610A spare tyre sub needs a 'proper' dedicated power supply running from the battery.
• All black - doesn't stick out like a sore thumb.
• 2 x 6.5" drivers and a folded horn reflex enclosure - ought to give out some decent accentuating bass, but not enough to blow the windows out or rattle the license plate.

With reference to my photo collage -

1. This is what the finished install looks like. I can still put a folding commuter bicycle (Tern p7i) in the luggage compartment. I bought a 4-pack of black nylon luggage straps from Amazon and used 3 of them to secure the sub to the rear seat latches and the child seat anchors that are in the seat backs. I wedged a couple of pieces of scrap high-density foam block beneath the sub, to stop it moving and to leave a small air gap for the sub’s air vent.
2. First job for hiding power wiring under the trunk carpet was to take the luggage compartment floor out. This is held in by two screws and…
3. …a couple of plastic push pins.
4. The plastic trim pieces either side of the rear seat squab can simply be pulled upwards and outwards. This is a little tricky, and it feels like you’re going to break something but it just pops out. (Note, photo shows the RH side of the car - which is where I hid my rear dashcam power supply wire).
5. IIRC, that plastic trim piece is only secured by one green plastic push clip - which likes to stay stuck in the car chassis. You can see one just peeking out from under the rear seat squab. I cable-tied my speaker-level signal wires from the factory amp to the existing wiring harness. I also managed to route them between the seatbelt and the B-pillar by pulling a section of the rubber door seal trims off and prying the grey plastic panel that hides the seatbelt away from the chassis just enough to push the wire through.
6. I didn't want to cut any wires or even use splice connectors, for fear of voiding the manufacturer's warranty so I made a special wiring harness up with 0.5mm brass pins at one end. These slot firmly into the back of the factory amp’s speaker output connector.
7. Bit of heat-shrink on the brass pins to make it neat and short-proof.
8. Location of the factory amp, in the passenger side front footwell.
9. Close-up of the big connector coming out of that amp. I connected my signal wires to the Rear speaker outputs, by pushing the brass pins up the tiny gap between an existing connector pin and the plastic connector shell - right in the corners of the square pin holes. (Sorry - no photo). I cable-tied my signal wire bundle (two lengths of twisted pair screened 22AWG cable) to the existing amp wiring harness, to stop the pins pulling out. The cable screens are connected at one end only to the amp’s power ground input. I chose the rear speakers because I always have the fader on the head unit set slightly to the rear of the car. This means that the signal to the sub won’t go up/down if I tweak the fader within the rear zone. If I have people in the back seat and I want to push the sound all the way to the front so as not to annoy them, this has the bonus effect of killing the signal to the sub.
10. Convenient earthing point for the sub, behind the vent panel at the back of the luggage compartment. Pull the plastic trim panel straight up to remove it. There are no clips; just plastic protrusions that fit into holes in the chassis.
11. My 8AWG earth wire to the sub. Yes, it’s red - I bought 3m of red wire from Amazon and didn’t want to pay double to get some black wire too. The electrons don’t care what colour the insulation is. You can also see the inline cigarette lighter socket that I slung underneath the factory wiring harness. This is for my rear dash cam.
12. Big fat cigarette lighter plug with internal 10A fuse. I managed to shoehorn a piece of 8AWG power wire in there. 8AWG is overkill, but it looks cool and you might as well get rid of as much series resistance in the power supply as possible. Pioneer’s supplied wiring harness was about 3m long and only 18 or 20AWG.
13. My bundle of signal and power wires, hidden behind the folding rear seat on the LH side of the car. I spliced my 8WG power wires to the PIoneer supplied wiring harness. I chopped about 2.5m off both their +ve and -ve power wires and soldered straight to my 8AWG wires. Then heat-shrinked the joins to make it neat and safe. You can see the 8AWG power wire (red) tucked behind the carpet.

All of this can be quickly ripped out without trace when required. For a less-destructive de-install, the sub can be removed in seconds and the wiring harness tucked behind the rear seat squab.

How does it sound? Pretty good but it required quite a bit of tweaking to get it set up right. I had to add an attenuator (stereo potentiometer - 2.2k if I recall) in between the output from the car's built-in amp and the speaker-level input to the Pioneer amp. Else it was just way too much bass - even with that sub's remote bass control all the way down. Now it's usable.

Here's a frequency response plot that I made, using my MacBook, my Scarlett 2i2 USB audio interface and the free REW software with a white noise WAV file, generated by the same software:

As you can see, the bass rolls off at quite a rate - ~15dB/Octave, with the -3dB point at 40Hz. To make sure this wasn't my USB audio interface adding 'colouration' to the signal, I did a loopback test on that box (the red curve). Pretty flat from 10Hz to 14kHz or so.

There's also an annoying 3dB boost at 70Hz - probably to compensate for the small door speakers and poor enclosure. I contemplated buying one of these 'bass restorer' boxes, but couldn't justify the cost. It sounds pretty good like I have it.

View attachment 142097