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theksmith

moar bass! (inexpensive 12" shallow subwoofer in a Jeep JK Unlimited)

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Posted (edited)

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i've posted before about how satisfied i am with the OEM premium Alpine sound system (ever since i installed an aftermarket head unit anyway). that factory upgraded system consists of a small amplifier, Alpine component speakers front & rear, and a small oval subwoofer in the cargo area floor.

 

my only complaint has been the little sub can't hit extremely low frequencies. there are several songs i like that are just missing certain bass notes - it was time to fix that!

 

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project goals

 

  1. improve very low bass response. specifically be able to hear the lowest couple of notes in Skrillex's Summit, C2C's F.U.Y.A., and Kendrick Lamar's Not Like Us
  2. not reduce visibility in my rearview mirror.
  3. not interfere with overland trips (i.e. not block my sleep platform or take away any essential storage spaces).
  4. not break the bank (stay under $500 grand total).

 

i was hoping to be able to leave the new sub in the rig all the time by placing it on my Front Runner cargo shelf. however, as long as it was easily removable before a trip, then that would still meet goal #3.

 

also, i wasn't trying to shake the windows, this was all about hearing the full range at reasonable volumes.

 

 

initial tests

 

i first tried a few slim 10" active enclosures, including a Sound Stream Labs LOPRO10 and an Alphasonik AS100A. they only made more bass volume, but didn't add the lower frequencies i was looking for.

 

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however, these early tests were promising in terms of the cargo shelf working as a mounting location - it didn't seem to have any major rattles even at higher volumes. this is likely because i used to carry my Hi-Lift jack on there and had added additional bracketry to wedge the shelf securely against the roll bar 

on both sides so it wouldn't sway with all that extra weight.

 

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after the disappointing performance of those 10" active subs, i decided that i needed to go up to a 12" sub with a separate amplifier to get any real volume below 40hz.

 

 

sub/enclosure

 

i've had good results with Pioneer shallow subs in previous vehicles so i purchased their 12" TS-A3000LS4. in terms of excursion, it can't compete with a JL Audio or Alpine shallow, but it's also 1/5th the price of those and still has better specs than any of the similarly priced "budget" slim subs (DS18, Rockville, etc.).

 

after a little searching, i found this tiny QPower SHALLOW112 sealed enclosure that seemed like it might fit on the shelf and not block my view.

 

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based on conventional wisdom, i did add a small amount of Poly-Fill because the enclosure is slightly smaller that what Pioneer's minimum spec calls for. 

 

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i actually tried the vented version of that same box first, but there was substantial port noise at higher volumes and i just really prefer the "tight" sound of a sealed box.

 

my first test were done just ratchet-strapping the box to the shelf, but once i was satisfied this was going to work out, i made mounting ears out of 1/2" plywood strips from my scrap pile. then i was able to use M8 bolts in the t-slots of the Front Runner shelf to attach to those.

 

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i M8 tie-down rings in place of regular nuts so that the whole contraption is easily removable without tools. plus i now have additional places to attach bungee cords.

 

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here's a shot from my perspective looking in the rearview mirror - it worked out perfectly to not block any more visibility than the spare already does.

 

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amp

 

space is at a premium in my rig, i've filled nearly every nook and cranny with something already. in fact, if it weren't for my air compressor being mounted under the passenger seat, i would have tried the Alpine JK under-seat 10" subwoofer first.

 

so, i needed a really small amp if i hoped to find somewhere to stick it! it also had to be capable of 400W RMS into 4 Ohms in order to drive the Pioneer 12".

 

i ended up with a tiny Sound Stream Labs RSM1.4000D class D monoblock. it's rated for 600W @ 4 ohms so that gave it plenty of headroom. in my experience these Pioneers can handle a bit more than they are rated at for a short period. therefore i wasn't too concerned about blowing the sub (plus the 600W rating is probably exaggerated a bit).

 

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thanks to the Sound Stream's miniscule package (9" x 3.75" x 1.75"), i was able to easily fit it beside the Bestop drawer (thanks @scottL) under my driver seat. 

 

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wiring

 

i already had 4 gauge power wire running to the compressor under the passenger seat, and had it fused at 100A. i  tee'd off that to run power to the amp, and used a factory ground lug under the driver seat for the negative. the Sound Stream's terminals fit the 4AWG easily. the amp is rated at 90A max, and the compressor draws less than 80A - so as long as i don't run the amp at high volume while also airing up, then sharing the same wire and fuse should be fine.


a 17ft set of stereo twisted-pair RCA's were run from my Pioneer headunit's sub output to the amp, along with an 18 AWG amp turn-on lead.

 

i used 12 gauge speaker wire, which might be overkill even for this moderately long run. the amp's terminals fit the 12AWG just fine. i had to solder the bare ends of the wire and then flatten them with pliers to get them into the sub enclosure's spring terminals though i sheathed the speaker wire with some PET braided sleeve and ran it inside the front-most t-slot channel of the cargo shelf.

 

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tip: i recommend using only "pure copper", often sold as OFC (oxygen-free-copper), or "tinned copper" wiring in a vehicle. copper-clad aluminum (CCA) is cheaper but has less current handling capacity for any given gauge, and will break more easily if flexed over and over again.

 

i took apart the SSL amp's remote level adjuster and wrapped the internals in heat-shrink so i could mount it with a more factory look...

 

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this OEM switch module to the left of the steering wheel was mostly empty space, so it was easy to modify it to include the remote level adjuster.

 

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dialing it in

 

after some initial tweaking of all the settings, here are some RTA (real time analysis) graphs as the system sits. i may still do a little more tweaking, but I'm pretty happy with it as-is.

 

note these are with EQ set to personal taste (i.e. not trying to hit a flat curve). i used the free REW application and a Dayton Audio IMM-6 calibrated mic with a TRRS extension cable. measurements were taken in the 1/12 octave RTA mode with psychoacoustic smoothing applied. the "Y" axis is uncalibrated SPL.

 

here is the "before" (blue line) compared to the new sub set at 10% on the remote level knob (green). the system previously dropped off fast below 40hz. the new sub hits 26hz at that same SPL and then falls off slower than before.

 

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the above graph was with the sub's polarity reversed, as it seemed louder that way when i first installed it. however, after getting all the other settings dialed in, i tried flipping it back to straight polarity and that actually made a big difference at lower levels (below in purple). it widened the new peak SPL range while also improving the performance all the way to the bottom.

 

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here's the "before" along with the new sub at 10%, 30% and 100% levels on the remote knob. this is with the gain on the amp and the output level on the head unit set a little conservative as to not likely clip or over-power the sub even if where to crank up the master volume a bit more.

 

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conclusion

 

since i already had some of the wire and miscellaneous other materials, this project cost me just under $350. the solution met all my stated goals and i'm very pleased with the overall sound!

 

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Edited by theksmith
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I afraid your efforts would be wasted on my mostly dead ears.

However, as always I admire your clean installation and attention to detail.  Your ideas for mounting the control switch are next level

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On 7/11/2024 at 6:28 AM, jgaz said:

I afraid your efforts would be wasted on my mostly dead ears.

However, as always I admire your clean installation and attention to detail.  Your ideas for mounting the control switch are next level

Same here on being lost on these old deaf ears Jim...   & I agree...   Sweet install as always...  ;)

 

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