[kaigoss69]

Since I have gone through many different stages of subwoofer upgrades, I feel I can provide some subjective and objective information on the topic and hopefully will be able to help guide some of the newbies in the right direction and save at least some people from having to go through a learning curve like I did (which can be quite expensive ).

But first, a little history: I originally started with SWS-8's, which quickly left me yearning for more. Not my cup of tea - not enough authority down low, too much distortion, not enough punch, and sucky midbass response.

Then I threw in the trunk a couple of standard sub enclosures. The first one a dual sealed box with 12" Infinity Perfects driven by a 700W amp. I used to have this enclosure in a Honda Accord many many moons ago, and it slammed H-A-R-D. In this trunk however, all it did was rattle the hell out of the trunk panels, with very little bass penetrating into the cabin (I didn't have fold-down seats or a ski-pass hole at the time). Then I tried a single JL 12W7 in an "HO wedge", on the same 700W and it was a complete disaster. Same problem, not loud enough and everything was rattling like crazy. I tried every possible position and orientation of the subs in both cases, and was not able to find a good solution.

I came to the conclusion that the trunk is pretty much a bass trap. I also knew that throwing larger, more powerful drivers and amps at it would not solve the problem, as the rattles would just grow proportionally with the decibels. I was after reasonably loud bass, with good definition and impact, and the ability to blend seamlessly with the front stage. I wasn't going to get that with a sub in the trunk, or so I thought.

But then I started reading on the subject of corner loading and I was intrigued by the physics behind the phenomenon, which basically gives you a free "boost" of the subwoofer's output, through the funneling effect on the sound waves of near panel reflections in the direction of the cabin. OK, that sounded quite complicated LOL but just imagine the corner of the trunk being like the corner of a room, and when you play a sound into the corner, the boundary walls will act like a funnel of sorts and the resulting loudness of the combined sound waves being reflected back into the room will be louder than if there were no walls at all. Anywho, I then happened to stumble upon a used "VP Electricity" enclosure with a built-in JL 10W6v2, and I grabbed it right away since the price was right (and I was desperate LOL).

I have to say this corner enclosure was a very pleasing set-up, as it provided almost all of the characteristics I was looking for. Also, it looked damn good and did not take away much trunk space. What it did very well was its ability to completely blend with the front stage, and almost disappear so that you could barely hear where the bass notes were coming from. This of course was aided by the fact that corner loading is very efficient, meaning you don't need to move much air in order to get the desired effect, which in turn cuts down on rattles and vibrations in the rear of the vehicle, thereby not revealing the location of the sub. As a comparison, when I had the other standard enclosures in the trunk, I had to move a lot more air for the same loudness, and the rattles were always a dead give-away for those flapping, power hungry monsters in the trunk. But as much as I liked the sound quality of the corner enclosure, it did leave me yearning for more volume from time to time. In the end, a single 10" woofer can only get so loud, even with the corner loading boost. I have to say though that most people would probably be satisfied with the achievable volume levels, but I was not content with having too little or even some distorted bass playing during those special back to high school crank it till the ears bleed moments, which still come over me from time to time, I have to admit .

So then I went on the net again and read some more. Soon enough I came across a crazy sounding concept calle "infinite Baffle" (short IB, aka "free air"). It sounded crazy because back in high school I had a free air setup, and it sounded absolutely terrible . But people who had it swore by it, and the install itself sounded very easy, so I decided to go for it. I went ahead and cut a hole in the back seat to create the "ski pass" opening and decided to build a simple baffle board out of 3/4" plywood and simply use it to mount the woofer against the back seat. The IB concept works by separating the front waves of the woofer cone from the back waves. So in this case the front waves travel through the ski pass into the cabin, whereas the back waves stay in the trunk. Since our trunks are so well sealed, this actually benefits the IB set-up! The first subwoofer I tried was one of those Infinity Perfects I had left over. By that time I was using the JL HD 900 amps for sub duty, so 500W on tap for a woofer only able to handle 400. I was careful - in the beginning - not to give too much juice to the Infinity. However, it quickly became clear that I was absolutely digging the new sound. It was exactly what I was looking for - accurate, musical, tight, chest-thumping loud bass, very addicting. So naturally I soon had one of those special moments, and 500 watts were indeed enough to roast the voice coil of the sub (oh yes, the burnt metal smell coming from the trunk was the proof LOL). At this point I was like a kid in a candy store though, and I found another good deal on what was supposed to be one of the best subs for this particular application, the AE SBP-15. And this brings us to the present moment, where I am still very happy with the AE sub playing IB. It does everything I want it to do, and it does it effortlessly, and efficiently, without producing any annoying rattles. I can warmly recommend this set-up to anyone who is looking for the best sounding subwoofer set-up you can put into a BMW sedan, period. ( I know the "period" statement has lost some credibility lately, but I am definitely not lying! )

Sooooo, with all that in mind (and I apologize for the long-winded introduction), I came up with a small table below that compares the above set-ups I have tried, and rates them in categories that I feel are important, and that I believe others may be looking for. It is only a draft at this point, so I may still tweak it here or there.

[kaigoss69]

While the info in the first post is mostly subjective, here comes the objective part:

Subwoofer Locations:

1.) Under Seats.
The enclosures under the seats can be outfitted with specialized aftermarket subwoofers (Earthquake SWS-8) that are capable of decent low-end output. Don’t expect “earth-shaking” bass, but most people seem to be satisfied with the results from this upgrade. The Earthquakes do well with 100 – 150W per channel. They can be run mono, but a stereo signal will give better separation in the mid-bass range. Speaking of mid-bass, these speakers don’t handle this part of the spectrum particularly well. If you’re planning on upgrading to a trunk sub, you’re better off installing dedicated mid-bass speakers in this location.

Example of SWS-8 Installation:



2.) In Trunk.
The trunk itself is a “sealed” enclosure. Think of it as a large metal box sitting in your living room, and then placing the subwoofer enclosure inside of it. It is going to be difficult to get any appreciable output outside the metal box from any subwoofer set-up inside it because the metal box (trunk) is so well insulated from the factory. Thankfully, a phenomenon called cabin gain helps a great deal in amplifying the output of the subwoofer so that it can still get plenty loud inside the passenger cabin of the car. However, there are side effects. The more powerful the subwoofer set-up, the more the trunk panels will want to vibrate. This results in annoying rattles. These rattles can be heard inside the car, but also outside. There may also be a problem with the gas cap spring mechanism becoming damaged from excessive vibrations of the gas cap door.

In order to allow more sound to enter into the cabin, you can open up the ski-hole if your car is equipped with it, or you can fold down the seats, which may not be very practical however.

With the above in mind, there are three different types of trunk set-ups you can choose from:

a) Inside the storage area below trunk floor (not possible in 335 and M3 models). A 8 or 10” subwoofer and a custom enclosure can be fitted into the storage area. The advantage of this set-up is the usage of dead space, and that it is completely invisible. The disadvantage is that the output of the subwoofer is somewhat muffled by the trunk floor lid.

Example of Subwoofer in storage area:



b) A standard subwoofer enclosure placed on the trunk floor. The enclosure itself can be “off the shelf” or a custom-built solution. The subwoofers can be faced toward the back of the trunk, or toward the cabin (results are usually better with the subs facing towards the rear). The disadvantages are the loss of trunk space, and excessive rattles. The advantage is the simplicity and the capability to produce a lot of “bang for your buck”.

Example of standard subwoofer enclosure in trunk:



c) A corner-loaded enclosure. The sub enclosure is “molded” into one of the trunk rear corners using fiberglass. The advantages of this set-up are efficiency and space savings. It hardly takes up any trunk space and the corner-loading effect “amplifies” the output of the single subwoofer to make it significantly louder than the same subwoofer in a standard enclosure. The other welcome side effects of this set-up are that it produces very little rattles and that the bass very easily blends with the front stage. Most sound quality oriented people prefer this type of set-up.

Example of corner-loaded enclosure:



3. Behind Back Seat (Infinite Baffle).
Here we place a subwoofer inside one of the walls of the metal box we call the trunk. The trunk becomes the enclosure. Using the proper subwoofer driver(s), this type of set-up can result in excellent sound quality, while requiring very little amplifier power, and producing the least amount of rattles. A good IB install will require very little EQ to have a relatively flat response between 20 and 100Hz.

Example of infinite baffle (IB) install:




Integration into Stock Audio System:

This assumes that no other audio upgrades are being done and that the subwoofer and amp are the only changes you are making to the stock system.

1. Base Audio:
You will have to tap into the full-range speaker level outputs of the Head Unit. The best place to do this is at the connection for the underseat woofers. The best method of doing it is to use a Technic harness connector as pictured below. No splicing, no cutting, fully reversible:



The signal can then be fed into an aftermarket amp via the high-level (or speaker level) inputs, if so equipped, or through an external LOC (line output converter), which will then give you a low-level signal via RCA for connection to the amp.

You will then need a switched 12V “remote” or “turn-on” signal for the aftermarket amp(s). If you are using an external LOC, make sure you get one with remote out, so you don’t need to worry about getting the remote signal elsewhere. If you don’t have that option, the most appropriate source is pin 13 at the back of the OEM head unit. You will need to run a wire from there to the amp location in the trunk. As an alternate, people have also used the cigarette lighter wiring, as well as the satellite radio fan wiring (which is conveniently located under the trunk floor, if you have it). As always, any splicing of OEM wiring is at your own risk!

2. HiFi & Logic 7:
The HiFi & L7 amps have dedicated, low-passed subwoofer outputs. The amps are located in the rear left corner of the trunk, under the gray plastic cover. You need to tap into these outputs, and feed them into a LOC (line output converter) which will convert the high level signal into low level signal so your aftermarket amp can use it. Some amps have built-in signal converters, but they rarely are designed for the relatively strong outputs of the HiFi, and especially the L7 amps. The last thing you want is damage your amp so always use external LOCs.

If available, always try to use forum member Technic’s add-on harnesses, which make the connections a snap and everything is fully reversible. He uses only the highest-quality LOCs, with built-in remote level outputs.

Technic HiFi Harness:



Technic Logic 7 Harness:

[nimo]

This should be a sticky. Thank you for the great write up.

Since technic sell harness kits, would you or someone start selling a carpeted infinite baffle (IB) board kits? I think there will be a lot interests. For someone with a ski pass hole already, and has minimal wood working experience and/or does not have saw and router resources, the kit would be an easy solution. It would probably take the seller 30 minutes of there time to cut the wood for the square, the hole, and possibly for a spacer (excursion travel), then carpet everything, and screw in a few mounting brackets. I believe the shipping may cost as much as the material though.

Another idea is to sell a ski hole DIY kit for those of us with no ski pass holes. It would include a stencil template, some pre-cut fabric mesh or grill mesh, and possible a black rubber strip piece ( to cover the cut edge).

[bmw325i]

Very nice writeup. I just wanted to share a few pics of my current setup since its not on your list.

I have 2 Alpine SWR 12d4's in a prefabbed ported box powered by an Audioque 2200d which is 2200 watts at 1 ohm. The box is around 1.8 cubes tuned to around 34-35hz. I made the front baffle and also the panels my amp and fuse holder are mounted to. The front baffle is held in by neodymium magnets.

[kaigoss69]

Thanks BMW325i, that is an interesting solution. The baffle is effectively preventing sound waves from entering the trunk, resulting in higher efficiency and preventing trunk rattles.

You definitely have an SPL oriented set-up with the ported enclosure. Another way to do this would be to use a sealed enclosure in the same way. It would be less loud but have a flatter response curve. Of course, if you took the enclosure completely away and just left the baffle, you would have IB.

[bmw325i]

Originally Posted by kaigoss69

Thanks BMW325i, that is an interesting solution. The baffle is effectively preventing sound waves from entering the trunk, resulting in higher efficiency and preventing trunk rattles.

You definitely have an SPL oriented set-up with the ported enclosure. Another way to do this would be to use a sealed enclosure in the same way. It would be less loud but have a flatter response curve. Of course, if you took the enclosure completely away and just left the baffle, you would have IB.

Yes it works very well. It is an spl setup but it still sounds very good. I wouldn't mind replacing the Type R's with IDmax's since they are more efficient and have better sq. I chose this box because the ports are right behind the ski pass so it works good even with the seats up. I usually build my own boxes, but this one was a perfect fit. The box is 14" tall and 36" wide btw if anyone wants to do something similar.

I never thought of doing something similar with infinite baffle though. With 2 12" Fi IB3's it would actually work pretty good.

[Bicax]

Great guide, sticky!!!

[nick380]

I've done the infinite baffle with help from kaigoss and I gotta say it thumps hard and accurately too! very easy to setup and the most efficient when it comes to bang for buck, because of the trunk amplifying the sound so intensily.

Originally Posted by bmw325i

Yes it works very well. It is an spl setup but it still sounds very good. I wouldn't mind replacing the Type R's with IDmax's since they are more efficient and have better sq. I chose this box because the ports are right behind the ski pass so it works good even with the seats up. I usually build my own boxes, but this one was a perfect fit. The box is 14" tall and 36" wide btw if anyone wants to do something similar.

I never thought of doing something similar with infinite baffle though. With 2 12" Fi IB3's it would actually work pretty good.

[bmw325i]

The only thing is you can't truly have infinite baffle in a car. The trunk acts as a enclosure the cabin also acts as a big enclosure. I'm not saying it doesn't work, it just doesn't work as well as it would in a home theater where you have enough space.

[kaigoss69]

Originally Posted by bmw325i

Yes it works very well. It is an spl setup but it still sounds very good. I wouldn't mind replacing the Type R's with IDmax's since they are more efficient and have better sq. I chose this box because the ports are right behind the ski pass so it works good even with the seats up. I usually build my own boxes, but this one was a perfect fit. The box is 14" tall and 36" wide btw if anyone wants to do something similar.

I never thought of doing something similar with infinite baffle though. With 2 12" Fi IB3's it would actually work pretty good.

From what I've read the FIs are actually not that great for car applications. Acoustic Elegance SBP or IB, anythig Image Dynamics, JL W6 or W7, JBL GTI would be great candidates though.

[kaigoss69]

Originally Posted by bmw325i

The only thing is you can't truly have infinite baffle in a car. The trunk acts as a enclosure the cabin also acts as a big enclosure. I'm not saying it doesn't work, it just doesn't work as well as it would in a home theater where you have enough space.

Yes, it should be called AIB (almost Infinite Baffle)

[bmw325i]

Originally Posted by kaigoss69

From what I've read the FIs are actually not that great for car applications. Acoustic Elegance SBP or IB, anythig Image Dynamics, JL W6 or W7, JBL GTI would be great candidates though.

Good point. The trunk is probably too small for 2 of them. I think the Q would be a better option from Fi, since it has a lower vas.

This thread is already making me want to try something new.

[kaigoss69]

Originally Posted by bmw325i

Good point. The trunk is probably too small for 2 of them. I think the Q would be a better option from Fi, since it has a lower vas.

This thread is already making me want to try something new.

Time to get WinISD out and model your Type R's against the usual contenders!

[bmw325i]

Originally Posted by kaigoss69

Time to get WinISD out and model your Type R's against the usual contenders!

There's a shop a few hours from me that has a termlab. In a couple weeks I'll be driving by there and I can get some actual numbers out if it to compare to winisd. I'll try a few different frequencies.

[bmw325i]

Originally Posted by kaigoss69

Yes, it should be called AIB (almost Infinite Baffle)

Works for me