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I have been doing car audio work for ten years now, almost all of it in BMWs. Both of the other suggestions posted to your question are at odds with what are to me central principles of dealing with car audio in general, and BMWs in particular.
I first dealt with this subwoofer problem in an E-34 535-- it has a steel back seat and package shelf that is a lot like a 6er. The thing is, you have the same choice as car manufactuerers do when they engineer their cars: you can engineer your stereo system with an eye towards RESULTS, as most car companies build their cars, and to hell with how you get there. Or you can do it the way that BMW does, where only ELEGANT results count. In other words, you have a problem-- now do you want to 'fix' the problem, or engineer it out of the system altogether? There is a little fact I know about the development of the E-36 325 motor that illustrates the point: The intake plenum (which is made of plastic) made a horrible buzzing noise at one particular
engine RPM. What was happening was, the flat portions of the six intake pipes (which are shaped like a rounded rectangle
inside) were sympathetically resonating with the engine. It made an 85dB noise, definitely noticeable inside the car (most cars have 80-90dB of road noise at 60mph, and that's cars that are 'pretty quiet' to most people). The first 'fix' they came up with was a plastic plate that would cover the entire plenum, which was backed with molded sound-deadening foam to completely surround the six pipes on the plenum. This reduced the noise to where it COULDN'T be heard anymore... but the noise was still being made. Now, at this point, Ford would've started counting the beans to figure out how much the foam and plastic would cost; and how much it would impact production. BMW told the engineers, "not good enough". The engineers remembered that while FLAT surfaces do resonate, ROUNDED ones cannot. So they remolded the plenum with a groove running down the exterior of each pipe-- now it was (and is) shaped like a figure-eight or a heart on the outside-- all continuous curves. No more flat surfaces, no more noise. Note that the INSIDE of the pipes is exactly the same as it was-- those grooves you see on the outside are only to deal with this problem, and they are not replicated inside the thing (it looks like it, from the
outside). So rather than cover up a problem to where it wouldn't be noticed anymore, they REMOVED the problem itself, with superior engineering. In my experience, if you want your BMW to keep being "like a BMW", and especially if you want your stereo to be an EXTENSION of that ideal, not a dilution of it, then you need to stick to the principles that made the damn things so great in the first place.
First of all, let me say something about unibody car construction: That sheetmetal is the FRAME of your car. Don't cut huge holes in it and expect the car to be the same as it was-- it won't be. You are directly attacking what makes the car solid in the first place, and; since the suject here is AUDIO, it is germane that this stuctural integrity is also your best friend against NOISE.
What good are your subwoofers to you if you just introduced 10dB more of road noise? Which is mostly bass, by the way. Of course, if tearing out the sound deadening that BMW put there, then adding your own to kill the rattles you just created sounds reasonable, I suppose chopping the hell out of your sheetmetal is no biggie. But like I said, this is not a question of "works/won't work" it's a matter of philosophy about how it gets done. Rather than try to let everything in from the trunk, then get rid of what we don't want; why not bring JUST the part you want (the bass) into the car and leave the rest out there where it belongs? Remember that every dB in road noise you get rid of is a de facto dB rise in the appearant loudness of your system.
It takes a DOUBLING of amplifier power to achieve the same 1dB gain-- welcome to the logrithmic nature of sound, my
friends.
There is only one solution to this problem that appeals to me-- and it's the one I'm currently using in my 6. What you need to do is build a so-called fifth-order bandpass enclosure for your subs and port the sound DIRECTLY to the interior. A fifth-order box is one in which the subwoofer has a SEALED back cabinet, and a PORTED front cabinet. Really all it is is a conventional sealed enclosure with a big filter on it. The port into the front cabinet allows certian frequencies to pass through it, while keeping
all others out-- hence "bandpass," it "passes" one "band" of frequencies. The beauty of this for you and me is that we can ENTIRELY ENCLOSE the subwoofer, isolating it from the trunk, then use the ports to bring the sound directly into the car.
It's also a relatively easy type of enclosure to build. Many of the new subs are designed for a really small sealed enclosure-- so this is especially ideal. I'm using two 10"s that call for less than half-a-cubic-foot apiece. Envision a rectangle... now put lines at 1/4 and 3/4 of it's length. The lines are the sub plates. The big square in the middle is the common ported section (both woofers face this way). The smaller boxes on each end are the sealed sections. Two three-inch ports nine inches long are called for, so I
cut two three-inch holes in the center section of the package shelf (take everything off of it, and you will see how perfect this is-- a big flat right in the middle, between two HUGE depressions for those storage compartments. Personally, I'm making an overlay for the shelf out of MDF (which you should build your enclosure out of, incidentally), so the ports will go through that, too. By placing sound deadening between the MDF and the metal, and on both sides of the back seat sheetmetal, and putting sound deadening in the trunk where it's never been before, I think I can actually bring the bass in with even LESS noise than the car had stock! (BTW, I'm talking about using that felt-like padding, not a real heavy product like Dynamat).
If you want more details about how to do all this, get in touch with me. It's really not hard, and I honestly believe that it's much better than cutting 10" or larger holes in your shelf, or cutting everything possible out just to let in the bass (along with everything
else). There are other places to put the ports (in the 'armrest hole' between the seats is another good choice)-- but the principle is the important thing: Cut as little as you can get away with... structural integrity is paramount... let in only what you want while attacking what you don't at the same time. I chose the middle of the rear deck in part becuse the metal is so thick there, and
wider than three inches. So the metal will not rattle from being cut (big expanses of thin sheetmetal will), and the structural impact is minimized. Get in touch with me if you want more about all this... (I'm running out of steam for typing)
Andy
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