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The Birdhouse

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Old 08-12-17, 03:26 PM   #1
Diwill05
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Been dealing with an annoying rattling rear deck in my 97 LS400 resulting in somewhat subdued output when trying to play my tunes, After years of putting up with it I decided to take a leap of faith into something that is relatively old, but not often practiced with small subs or rear decks for that matter. I call it the birdhouse due to it's shape, but it is nothing more than a mere dual opposed manifold to house two 8" subs. The main idea is to reduce, if not eliminate vibrations transferred to the rear deck which should result in higher listening levels without the associated rattles most get when upgrading the rear deck sub. This practice is most often found in home audio where infinite baffle is used and the owner wants plenty of output without rattling the attached wall. Sometimes used in competition vehicles, but some older manufactured designs were produced by Linear Power & Coustic (Bass Pump).

How it works is by having the subs opposing, but still firing in phase, the movement of the cones cancel out each other's vibration. A big plus is the increase of cone area that will surely maximize output capability which will allow you to use less than stellar woofers that normally wouldn't cut it if using only one. The subs fire through the manifold which in turn fires into the cabin while still using the trunk as the enclosure, thus it is still infinite baffle, or trunk baffle rather. There should be no large variations in response and if so it should be out of the pass band of the woofer's main use...

The manifold is built of 1/2" mdf to help save on weight since it will be hanging from the deck along with the subs attached. I suspect it will be bit bulbous though I've built it as small as possible while still allowing the largest opening to the hole in the rear deck. Choice of subs are Cerwin Vega Vega series dual 2 ohm (which can be found rather cheap) that will wire in series/parallel to the 5th channel of a JL XD700/5. By the way, if anyone is looking for options for the rear deck, I found the Vega series to be worth looking at after doing some extensive modeling and testing. The amount of suspension stiffness/control beats out the usually suggested ID8 which I almost hated in the deck. Just remember to reduce power to about half. It may appear a bit big, but once the holes are cut for the subs, you will see that it really isn't that big at all. For now, here's a quick pic of the almost finished "birdhouse" sans woofer holes cut and top plate attached. It is upside down while some filler dries as well as I did some sanding to give a bit better finish.



Last edited by Diwill05; 08-12-17 at 04:29 PM.
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Old 08-12-17, 04:21 PM   #2
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The structure doesn't have to be vacuum seal air tight in itself, though a bit of compulsive behavior applied isn't going to do harm. A good quality wood glue and brad nails from an air gun was used. After cutting the woofer holes, the manifold is extremely light though the top plate isn't attached yet. Thicker wood is unnecessary since you don't need two heavy duty woofers (remember the advantage of doubling cone area), plus the benefit of vibration cancellation. It is purposely built lightweight to keep stress down on the rear deck. The top plate is of 1/2" mdf as well.

Here you can see the traced line for the sub cutout exposes how little the woofer mounts are. Just barely enough to mount the woofers. The magnet will be mounted on the outside. You could place the magnet assembly on the inside, but that will require a larger manifold.

Now as to why the birdhouse shape which I should have included in the first post, anyone driving one knows of the issue.... the bane & blessing (fuel pump replacement ease) of the LS400. The dang gas tank. After careful measuring, the angled sides was required to clear the tank. Though one side only needs to clear the tank, I did both for slight aesthetics. I must include that this is going to require a good amount of trimming of the trunk liner piece that hides the gas tank, but it should only be in the immediate area of the sub. I will carpet the manifold later on to match the OEM carpet as best as possible, but for now, testing will be done without the liner. That way I will know for sure how permanent it will be.






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Old 08-13-17, 06:13 AM   #3
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Top plate is attached. I can't speak for my work since I glue, clamp, and brad nail most of my enclosures, but I highly suggest that the top plate is securely attached since it will be load bearing. The heavier the sub, the more secure you want to make it. The top pic gives a clue to the actual size of the manifold in relation to the tank. Not too bad.








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Old 08-13-17, 08:40 AM   #4
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Decided to paint it so it sort of at least disappears into the trunk. Pics aren't too great and probably will be some of the last posted since I'm trying to finish up before it gets too hot outside, but if you can make out how it sits you will get the gist of what I'm doing.


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Old 08-13-17, 01:12 PM   #5
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The manifold is installed with the Cerwin Vega subs. It took a bit of trimming the underside panel to get it to fit as some of the panel is in the way of the woofers and part of the manifold, but if trimmed right (my skills aren't that great) it looks very natural. You have to get down to see where it's been trimmed, but from the view when normally standing while opening the trunk you don't see anything but the subs and manifold which is tucked up under the deck quite neatly. It was a slight pain getting the manifold bolted to the deck since I was by myself, but I used a small bottle jack and stacks of scrap wood to get it to where I needed it. I then secured it from the top with multiple screws. And yes, the stray piece of weatherstripping in the pic from above the rear deck was tucked back in before finishing. LOL


As for performance? HECK YEAHHHHH!!! Not a hint of rattles with plenty of output even on a meager 300 watts total. Extension is great and of course depends on the subs you choose. The CV's does a fine job of articulation, control, extension, and overall output, Plenty of kick, and.when it gets low you feel it like there's a bigger sub in the trunk. This is for a SQ setup so don't think it's all boom. Guitar thrusts, drum hits, etc.. are clear. Far more output available than a single 8" and no box on the floor. And can I say it again??? NO RATTLES!!! Anyways..... it's done and it's staying!

Hope this helps someone as this can be applied to any vehicle with rear deck subs.






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Old 08-20-17, 07:44 AM   #6
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So it's been a week now since the manifold was installed and so far I've been entirely impressed with the results. Can't say for sure if the Vega subs are fully broken in, but the response is absolutely satisfying! Although I run a Helix DSP in the setup, the EQ on the sub is pretty much flat in the DSP and headunit. I may have a slight cut of -3db around 50-60hz, but I tend to do that in all of my setups regardless of vehicle. The xover is around 120hz with a -6db Butterworth slope to mate up with the stock Nakamichi fronts which will soon be replaced with a set of JBL P560c. After that are a few minor areas (crossover point, level, and some EQ) that will be improved on the subs and mains to get that last bit of sound desired, but we're talking about very fine tuning that most could do without or might not even notice. The level of output could be equated to maybe a set of low to medium power 10" buried in the trunk except there still are no rattles and the sound is more direct with very little to no phase issues. The low end is very impressive (it gets lowww) with very good dynamics and accuracy. I experience no issue with extension and overall smoothness in the response. Kick, punch, and thrust are all present with more than enough to fill in what the small door drivers can't present without struggling. And yes, we're talking about bass you can feel even in the lowest registers.

Last edited by Diwill05; 08-20-17 at 07:57 AM.
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