SPB15’s through the back seat…


  • Member
    buLLetProoF58 on #1057

    Hi Simon or John, I’m planning on mounting 2 x SPB15’s in my M5 with the magnets facing the seats. I do not have folding seats, but plan to keep the armrest down with some grill cloth over it.

    Questions are:
    1. How much sheet metal cutting do I need to do to allot enough air to escape into the cabin? The seats themselves are just the usual foam and leather…not sure how much air will pass through that. Is it important where the holes are? Should there be circles cut out centered where the magnets will be?

    2. SInce the 15’s barely fit back there, the wood that I’m mounting them to will be pretty much holes and not much wood. Given that’s the case, should I reenforce with metal/layer some birch instead of MDF/or other things of that nature?

    3. Lastly, any advantage to running stereo? Obviously not for imaging, but for phase issues? Thanks!!!


    Member
    simon5 on #10422

    1. It depends on your crossover frequency. If it’s low enough, only the armrest down could be enough. It will act a bit like a 4th order bandpass box.
    2. You need to find a way to isolate the front wave from the back wave, so I hope there won’t be much holes… yes birch plywood is a good idea, much more stronger than MDF.
    3. Again, this will depend on your crossover frequency, if it’s low enough, there is absolutely no advantage in running stereo.


    Member
    buLLetProoF58 on #10423

    Thanks for the quick response Simon. I have 4 x 6.5″ midbass in my front doors, so will likely xover subs at 60hz…80hz at the absolute highest. There will definitely be no holes to let front and back waves interfere. Will make it tight and caulk.

    I guess the only thing I’m having trouble getting a handle on is question 1. Restricting the airflow to the cabin will act like a high pass? Is that right? And does the cutout of the sheet metal, where the path of air can flow, need to be right in line with the cone (i.e. circles cut out), or does it just matter how much air can escape, period (like multiple little holes punched or triangles or rectangles or what have you). Example: If I JUST cut out a rectangle where the armrest is, and I have the subs faced the other way than mentioned before (magnets facing trunk), and the cone is right up against some sheet metal (recessed with enough room for excursion of course), will that effect it differently that if it’s flipped with the magnet up against the sheet metal and cone into trunk? (all with xover @ 60Hz). And if I cut more metal out, there’s obviously still the foam of the seats blocking the path, should I assume like 10%/20% of breathability through the foam? Kind of like an AP enclosure…ish?

    IF there is a such thing, can you give a concrete number of the minimum number of square inches of sheet metal I should cut away for the subs to breath the way they are designed. (Though I know you’ll be tempted to say, “as much as possible) :mrgreen: 😉 And if I don’t cut enough, then what happens, I just create a high pass filter essentially by not letting it suck enough air? Or would it get muddy…though…can <60 even sound muddy? ❓

    Lastly, would stereo at high crossover settings be advantageous for imaging or other reasons?

    Many thanks,
    Ari


    Member
    simon5 on #10424

    At high crossover settings, or with a low order crossover (1st or 2nd order), running stereo is a good advantage for imaging yes. But, you are still in a car, so you can’t expect miracles, they are behind you so the imaging is still subpar, compared to the situation if they could be in front of you.

    You probably saw a 4th order bandpass box before, if you cut out a rectangle where the armrest is, it’s quite a big port area, unless your armrest is tiny. The mechanical highpass filter will be quite high in frequency if you ask me. That mechanical highpass filter could be a good thing, by filtering and diminishing midrange leakage from the back of the car, to let your midbass do their work alone. You could even go smaller by using proper software like WinISD 0.7.0 or latest UniBox, but it’s a bit hard to calculate precisely in a car. That could be a way to mechanically increase the slope of your highpass filter, if you manage to cut “just” enough of metal to match the crossover point of the mechanical filter with the electric filter. If you want to go that way, you better start small, because it’s easy to cut more, and hard to re-weld that after… 😆

    If your electrical filter is good enough, I wouldn’t worry much about it on the other hand, could be too much hassle for negligible gain, so just cut enough of it.

    You can calculate the port area of that armrest, then compare with woofer area. The area will be closer than it appears, so woofers should see negligible compression effects… not to mention leakage thru the seats and to the outside.

    If you are into SPL on the other hand, you want that midbass, so in that case, it could be a good idea to punch thru the seats, and bolt your woofers there… 8)

    Seriously, if I understand correctly, with the armrest down, the subwoofers still don’t have a direct access to the front cabin ? It seems like you still need to cut sheetmetal ? I hope it won’t affect rigidity of your car or anything…
    I would start by cutting that part, because I think it will have the less impact, and I think it will be enough, if they have direct access after that. If not, you can cut more after…

    The shape of holes is not really important under 60 Hz because the wavelength is really long, about 5 meters at 60 Hz, so you are mostly into pressure mode…

    You could say that playing thru the foam seats could be a bit like AP enclosure yes. Again, hard to calculate precisely, because it will be “hybrid”… a bit like a sealed enclosure, a bit like a 4th order bandpass, a bit like AP enclosure, then a bit like Dipole enclosure due to leakage to outside world…

    If you put the cones right against sheetmetal, you need to calculate the excursion, but after that is calculated, I would allow an extra couple of inches at least, so air can flow free. If it’s too tight, it will flow much better when the woofer is moving one way than the other way, assymetric loading on the woofer…


    Member
    buLLetProoF58 on #10425

    Incredibly informative. Thanks Simon!! Will keep you updated in the coming weeks.

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