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Discussion Starter #1
** Disclaimer - do this at your own risk, I am simply sharing my testing experiences **

Hi, hopefully some of this info might be useful to others contemplating the install of a sub under one of the front seats in a 2014 Mazda 3 - non Bose car.

I have attached pictures that show:
  • REW Sweep overlay of dB:Hz before and after
  • Firewall 8 Gauge power cable punch through point
  • Battery connection and fuse location I used
  • Sub installed under the seat
  • Pyle (yes questionable quality but it's ok) dB meter pic's of max achieved dB and at what frequency before and after (using old skool IASCA 360Hz to 10Hz audio)
  • Rear speaker amp splice near factory Mazda amp unit I did

What you'll need:
  • Kicker 11HS8 (or sometimes referred to as just HS8 ) Hideaway Subwoofer
  • 8 Gauge + and - power cable. The HS8 comes with enough wire, but I think it's rubbish and opted for something better. Installing the thicker cable now also easily allows me to upgrade at a later stage to a bigger amp. I used a SuperCheapAuto SCA Power Wiring Kit - 8 Gauge kit, the quality of the wire in my opinion is better than the equivalent stinger kit I have; better feel, better soldering stick, tougher insulation. See photos for packaging of exact product.
  • 2 pairs of speaker wire, I actually used 4 Core Pink Speaker cable to run from the factory Mazda amp to the HS8 unit. The multicore cables keeps things relatively neat and much easier when routing through the car.
  • Standard Socket set to remove battery and nut holding trim panel near factory Mazda amp.
  • Etorx set for the seat bolts
  • 80mm black heat shrink tube

I chose the Kicker 11HS8 unit because it doesn't need a remote in connection for auto turn on and off, it can just do signal sense. I also got it on special at JBHifi for $300 AU.

Quite simply, here is what I did:
  • Remove the battery altogether, this is the only way you will fit a cordless drill into the location to drill a small hole through the firewall. The location I have used avoids cables etc. on the other side of the firewall - perfect. Use a rubber grommet as shown also - you're asking for problems down the track otherwise. I also had a vacuum running to suck up the metal debris when drilling the hole.
  • Remove the FBCM up a little and drill two small holes in the side of the plastic that the FBCM slides into (not in the FBCM itself!). I used small zip ties to secure the + power fuse block. You can see in one of my photos where mine is located, I hope this gives you the idea of what I am trying to say here.
  • Remove the front passenger seat (in Aussie, cars are right hand drive, so I am installing the HS8 under the front passenger seat - on the left hand side). It would fit under the front drivers seat, in fact there is probably slightly more room but I didn't want vibrations on my ass directly.
  • I ran the power cable down the left side of the car under the trims and then under the carpet to where the air vent is under the seat. You will see in the photos where my cables pop out.
  • I cut the air vent back to the chassis and replaced it with large 80mm heat shrink tube - this is flexible, easy to heat shrink attach to the existing air duct and manoeuvre it around the HS8 power/speaker cables. I still get airflow to the back passenger then.
  • Earth the - power cable near the vent also, I made sure I sanded down to nice metal before fixing the - power cable with a short metal roofing screw. It is very secure.
  • I ran the multicore cable from the HS8, along the centre console, around under the dash and then across to the Mazda factory amp unit.
  • I spliced into the factory loom carefully, soldered connections and used insulation tape as required on each wire. And then wrapped the loom back up for a factory finish (photo shows splicing and soldering but not end result - you get the point)
  • Install the amp; I used the supplied straps and simple cut small niks into the floor carpet and ran the straps around to secure it as shown in the photos.
  • I was pretty well done. I made some HS8 amp adjustments and put the seat back over it. I re-connected the big white plug (very important) before re-connecting the battery. (Otherwise you will get DTC's because it can't see the seat airbag modules etc. in the seat.)
  • I tuned the system as I saw fit and it was good.

Notes:
  • Solder all your joins! I use 60% Tin and 40% Lead solder still.
  • Use insulation tape as required, pull it nice and tight around wiring etc. so that it takes well.
  • Make sure you disconnect the car battery! At the least, the - terminal.
  • For my REW tests I use a decent calibrated microphone - not cheap stuff!

Before the install, I got a max SPL of 111.1dB at 170Hz. After the install I got a max SPL of 118.0dB at 180Hz. These are the two photos with the Pyle dB meter and the CMU shown. Here are two videos of the complete audio sweeps also:

At the end of the day, it's nothing special, it's just to add some additional bass to the factory sound system and I am quite satisfied with it. It doesn't compare to my previously owned Alpine, Sony, Pioneer and Orion gear of 10, 12 and 15" subs etc., but it doesn't take up boot space either. :)

I'm happy to answer any questions where I can help, or run different tests if someone thinks it would be useful.

Cheers!
 

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Discussion Starter #2
Minor update: I have added Anderson connectors to the +/-12 supplies and also XT60 connectors to the line level wires. So now I can chuck in a 12" sub into the boot when I want; just disconnect from under the seat where this 8" kicker sub is and throw the big sub box wiring wire over the backseat to connect up. I only do this when I use the car and going somewhere where I wont require the use of the boot.
I can tell you that it is ridiculously loud - this is a ported box also. I haven't put my dB meter on it yet but will post a video when I do.
 

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Excellent report!! Taking the time to document this work, and include a qualitative analysis, that's a gift to us!

So.... this is an interesting sub. On the frequency response graph, I see the original response is flatter and more gradual than the new response, where the new response has 30 - 80 Hz gain and 80 - 110 Hz loss. To imagine what that sounds like on your average pop/rock recording, it's like adding a bit more bass drum thump and taking away a bit of the bass guitar.

So, how would you describe what music sounds like after adding the 8" Kicker under the seat? Also, since you mentioned an update mod that would allow the future addition of a 12" sub in the boot, is this an indication that you are not satisfied with the 8" under the seat? I ask, because I am, naturally, skeptical of these very small box systems, and I wonder if a product like that 8" Kicker could ever really be worth the effort. I've spent a lot of time building my enclosures, and I'm just tired of doing it and taking up space in my cars, and so I would love it if a product like that could actually provide enough extra punch without compromising a gradual and fairly flat response.
 

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This is a mild resurrection... but in response to gruuvin: Note that OP didn't remove any existing components, he added *more* bass. I would speculate that the sharp dip at 80ish Hz is due to destructive interference or something along those lines. The rest of the dip in the 80-110 range is rather marginal (I think you could chalk it up to measuring error at that point).

To the OP: Just purchased a 2017 M3 without Bose and I think I'm going to follow in your footsteps with this mod. I may even do two to see if that mitigates the potential aforementioned standing wave issue and adds a little more punch. If so I'll try to remember to report back. Thanks for the write up!
 

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Thanks for the response, erplanes, and good point about destructive interference possibly being the sole cause of the loss.
Also, if you go with a couple of these, let us know how you like it!
 
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