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Car Forum / Driving, Maintenance, Tuning / Car Audio / October 2006Sub in the rear shelf, with a twist
I want to conserve as much luggage space as possible when installing my sub, so I have been thinking about this solution: Install the driver in the rear shelf, just like the 6x9s, but in addition build a (sealed) box that is hanging under the shelf into the boot. The box would have a short "neck" the same diameter (or a little larger) as the driver to reach through the shelf, and be relatively wide and flat to invade the boo as little as possible. Is there any reason this is a bad idea? Would the "neck" cause any problem, like start acting like a bass reflex vent or something? Will the wide and flat box sound much different from an "ideal" box? Will the sub opening inside the car sound worse than if installed in the boot? I'd guess it'd sound better?  SignatureRoRo I believe there are a few pitfalls to avoid when doing something like this. First, in a manual I read at some point it stated that when putting a sub in a box, it should have at LEAST three inches of clearance in every direction from the bottom of the sub (inside the box) to operate nominally. So it sounds like your hanging box has to be taller than you might want it to be... [I wish I could remember where I read this, because I'm sure to get flamed for saying I read it somewhere and not being able to cite it... I've used this rule on several installs, though, and feel that the logic is very sound (no pun intended)] Second, if you're planning on making this a ported box, I would recommend porting into the cab, the same direction that the sub is firing. Otherwise you're porting into a different space and are essentially making a very large, very strange bandpass box. If you're making this a sealed box, then nevermind. Third, GO FOR IT! I think that this is a kick-a.s idea!!! My only reservation about it, personally, would be that everyone is going to see your sub... ~The Lull > I want to conserve as much luggage space as possible when installing > my sub, so I have been thinking about this solution: [quoted text clipped - 14 lines] > Will the sub opening inside the car sound worse than if installed in > the boot? I'd guess it'd sound better? He could use a shallow mount sub such as the one from Pioneer or Elemental Designs. And in the case of the ED sub, a rep from the company on their forum says it only needs .5" of clearance between sub and box and could potentially be as low as .25", but is not ideal. So something like that... very shallow mount, very shallow enclosure, might not be bad at all. Then just make a decent grill covering or redo the stock one and good to go. Brandonb > I believe there are a few pitfalls to avoid when doing something like > this. [quoted text clipped - 38 lines] >>Will the sub opening inside the car sound worse than if installed in >>the boot? I'd guess it'd sound better? >First, in a manual I read at some point it stated that when putting a >sub in a box, it should have at LEAST three inches of clearance in >every direction from the bottom of the sub (inside the box) to operate >nominally. Does that mean sideways all around the driver, or just around the magnet area? The driver will be placed inside a tunnel which is an inch or two deep, so the magnet will be in quite free air inside the box. >So it sounds like your hanging box has to be taller than >you might want it to be... I don't intend to make it extremely shallow, so clearance below the driver will not be an issue. I haven't done any maths yet, but I expect a few inches of clearance. >making this a sealed box, then nevermind. Yes, I am planning on a sealed box. >Third, GO FOR IT! I think that this is a kick-a.s idea!!! My only >reservation about it, personally, would be that everyone is going to >see your sub... Yes, I have thought of that as well. I'll be putting black carpet on the shelf and a black grill over the sub, so it shouldn't look too bad. It's only a ten-inch sub, and it is no fancy brand, so theft should not be too much to worry about, either. Thanks to everyone who replied. I will start building the box as soon as the shelf itself is done. SignatureRoRo that would be fine to do. If your box is going to be sealed, it doesn' really matter what shape the box is in. The front wave is separate from the rear wave (which gets trapped in the box), and that's all tha really counts. Don't really have to worry about standing waves due t odd corners because they won't be going anywhere anyways As for the rearward clearance, Lull aluded to it but wasn't quit complete. If your subwoofer has a vented pole piece (i.e.- a hole i the back of the motor), you should allow one length of the diameter o that hole to the back of the box for the air to escape and spread. s if your hole is 2" wide, leave 2" between the motor and box. If it' too close you'll likely get mechanical noise. If there is no vente pole piece, however, there is no need to leave any room behind th motor structure -- KU40 > that would be fine to do. If your box is going to be sealed, it doesn't > really matter what shape the box is in. The front wave is separated > from the rear wave (which gets trapped in the box), and that's all that > really counts. Don't really have to worry about standing waves due to > odd corners because they won't be going anywhere anyways. You don't have to worry about standing waves in a nominal sub enclosure, period. See the section entitled "Enclosure Shape" here: http://mobile.jlaudio.com/support_pages.php?page_id=164 > As for the rearward clearance, Lull aluded to it but wasn't quite > complete. If your subwoofer has a vented pole piece (i.e.- a hole in [quoted text clipped - 4 lines] > pole piece, however, there is no need to leave any room behind the > motor structure. Clearance is *definitely* good and the mo' the mo' betta in high-power situations, but what you've got here is another rule of thumb that can be fudged a bit if need be. See any one of the JL Audio stealthboxes for example. I can think of one in particular (and old design for Mustangs--fit in the tire well) that had a piece of plexi over the pole vent of a 10W5 that was definitely *way* closer than the rule of thumb you cite. And to add heat to injury, it was also designed to house an amplifier! -dan ok so maybe that guideline is a bit conservative. in fact, to simulat what would happen if you cover a pole vent, simply put a woofer flat o the floor and put some power to it. once enough power is applied, mechanical noise is likely to show up. lift one side of the sub of the floor, and basically the moment you lift the edge off the floor the mechanical noise stops. normally not a ton of air moves throug there anyways, but I'd rather be safe than sorry when building a enclosure if you can. if nothing else, just because it's extra time t make a new box if by chance there is noise -- KU40 On Oct 11, 6:04 pm, KU40 <KU40.2fj...@no-mx.forum.carstereos.org> wrote: > ok so maybe that guideline is a bit conservative. in fact, to simulate > what would happen if you cover a pole vent, simply put a woofer flat on > the floor and put some power to it. once enough power is applied, a > mechanical noise is likely to show up. lift one side of the sub off > the floor, and basically the moment you lift the edge off the floor, > the mechanical noise stops. I can't help but snicker at this a little bit. It's not really fair to say "play your speaker in free-air and listen to the noises it makes as it jumps up and down on your floor--that's all because the pole vent is blocked." Whenever you play a subwoofer outside of an enclosure, you're bound to hear noises, plugged vent or not. The voice coil leads bumping up against the spider (or in some cases, up against the cone) makes noise. Sometimes the air rushing around sharp edges on a stamped-steel frame can be perceived as mechanical noise. Some of that noise may be the voice coil former violently slamming up against the backplate. And yes, some might be from air rushing through the permeable material of the spider. Regardless of where the noise comes from, it's pretty much a moot issue because subwoofers are not played in free-air and in the absence of the mids and highs. Of course your average car is going to be making enough noises of its own to mask noises, too, unless you're really into it and have Dynamatted the whole vehicle. In real life, you put a box behind your driver and in that box you typically stuff it with a bunch of polyfill or other fibrous damping material which further absorbs those spurrious clicks, whistles and whirs. Even in iso-loaded configurations (remember those days?) where you have one or more drivers' baskets exposed to the listening environment, you don't hear mechanical noises because the rest of the system does a darn fine job masking that. Even if you put duct tape over the pole vents (NOT recommended, by the way), you'd have to be a bloomin' dog to hear mechanical noises. The only serious concern, really, concerning obstructing the pole vents is that in doing so, you wind up reducing the voice coil's ability to dissipate heat, thus making thermal failures more likely. > there anyways, but I'd rather be safe than sorry when building an > enclosure if you can. if nothing else, just because it's extra time to > make a new box if by chance there is noise. Yes, better safe than sorry, but not because of noise--but because of thermal concerns. -dan I have done many cars the way you want to do yours, both ported an sealed and they sound GREAT! If I owned a car, that is the way I woul do it(I have a truck by the way). I have some pics, but they are t large for this forum, and I must say, I am not very good wit computers, so I don't know how to make them smaller. Sorry for th stupidity -- jowens500 I did not say "play your speaker in free-air and liste to the noises it makes as it jumps up and down on your floor--that' all because the pole vent is blocked. -- KU40 On Oct 11, 11:54 pm, KU40 <KU40.2fj...@no-mx.forum.carstereos.org> wrote: > I did not say "play your speaker in free-air and listen > to the noises it makes as it jumps up and down on your floor--that's > all because the pole vent is blocked." http://dictionary.reference.com/search?q=hyperbole -dan this is not a hyperbole, you took my quote and changed it. My example specifically pointed to a pole vent noise, whereas yours could encompass any noise, from tinsel lead slap to the sub actually bouncing on the surface from the vibrations. A hyperbole might be "cover your pole piece vent and your sub will sound like a jackhammer." SignatureKU40 On Oct 12, 8:20 am, KU40 <KU40.2fk...@no-mx.forum.carstereos.org> wrote: > this is not a hyperbole, you took my quote and changed it. If I were quoting you directly, I would have done so in typical USENET style, just as I have done the line above. I was basically boiling down your post into one line to show how silly the whole thing sounds. > My example specifically pointed to a pole vent noise, whereas yours could > encompass any noise, from tinsel lead slap to the sub actually bouncing > on the surface from the vibrations. The real issue here is that your experiment was flawed. If you lift up one corner of the driver, you change the physical orientation of the driver and thus could have tilted the driver in such a way as to make the lead wires stop slapping the surround. It's a classic case of of the Heisenberg uncertainty principle...there's nothing to say that in the particular scenario you cited, that your intrusion into the system didn't alter the system itself. A better way to examine your thesis would be to suspend the driver in some fashion and put tape over the vent hole and compare that to the same driver with the tape removed. And just so we're all on the same page, I'm *not* suggesting that a free-standing driver *won't* create any noise from a plugged pole vent. > A hyperbole might be "cover your pole piece vent and your sub will sound like a jackhammer." I think you're missing the point--you're getting too tied up in the semantics. Missing the forest for the trees, you are. The whole "pole vent noise" thing is a non-issue in real-world situations. It's not even worth considering. -dan D.Kreft Wrote: > And just so we're all on the same page, I'm *not* suggesting that a > free-standing driver *won't* create any noise from a plugged pole vent. so maybe my experiment was flawed, but you just stated the point I wa trying to make. if a free standing driver *might* create noise from plugged pole vent, what's the difference between that and in a box wit the back panel covering the pole, effectively plugging it the same wa as the free-standing driver was? If you (Mr. Kreft) had a box with a mounting depth capability equal t that of the actual driver you wanted to use, so that the back plate wa touching or maybe within 1/8", would you use that box or build a ne one? base your answer as if thermal issues were nullified -- KU40 > so maybe my experiment was flawed, but you just stated the point I was > trying to make. if a free standing driver *might* create noise from a > plugged pole vent, what's the difference between that and in a box with > the back panel covering the pole, effectively plugging it the same way > as the free-standing driver was? The difference is simple: audibility. > If you (Mr. Kreft) had a box with a mounting depth capability equal to > that of the actual driver you wanted to use, so that the back plate was > touching or maybe within 1/8", would you use that box or build a new > one? base your answer as if thermal issues were nullified. I smell a trap, but I'll bite anyway... **Completely negating thermal concerns**, and assuming that the walls of the box were sufficiently rigid so as not to vibrate against the backplate, then then I would absolutely not hesitate to build such an enclosure if it's all the space I had to work with....or, as is the case with the JL Audio 18W6 where the recommended enclosure is barely bigger than the cardboard box the driver came in (1.25 cu.ft., sealed), if that's all that was needed. -dan D.Kreft Wrote: > I smell a trap, but I'll bite anyway... > [quoted text clipped - 8 lines] > > -dan Fair enough. SignatureKU40 > Install the driver in the rear shelf, just like the 6x9s, but in > addition build a (sealed) box that is hanging under the shelf into the [quoted text clipped - 3 lines] > > Is there any reason this is a bad idea? I can forsee two difficulties: 1. Getting the walls of your (presumably wooden) enclosure to meet-up perfectly all around the top with an uneven metal surface, and getting that sealed perfectly; and 2. Deck vibration. I don't know what kind of car you have nor how big your subs are, but keep in mind that a rear parcel shelf was not designed to hold substantial weight nor provide a vibration-free platform for subwoofers. In the world of loudspeaker cabinet design, vibration is murder because it not only colors the sound of the subwoofer (highly subjective), but it also creates another source of energy loss--if your enclosures are jiggling, your subs are not acting in the most efficient mode. I'm not saying it can't be done, nor am I saying that it won't sound good--JL Audio has a couple of Stealthboxes (which aren't reeally boxes at all, but rather slick mounting pods for their 8" free-air subwoofer: http://mobile.jlaudio.com/products_stealthbox_make.php?make_id=44&model_id=140) which I hear work quite well--only that you need to be prepared to deal with these potential issues. If your car isn't as solidly-constructed as a BMW, you *may* be in for a rough ride. Also worth considering is how well the rear deck and your enclosures will fare on seriously bumpy roads. The last thing you want is to have the screws rip out of your MDF and have your subs hit the floor of your "boot". -dan > 1. Getting the walls of your (presumably wooden) enclosure to >meet-up perfectly all around the top with an uneven metal surface, and >getting that sealed perfectly; and The plan is to cut a hole in the metal, big enough to allow attaching the box directly to the shelf. I'll still need some way of getting a good seal between the shelf and box, but I have a couple of ideas. > 2. Deck vibration. > >I don't know what kind of car you have nor how big your subs are, but >keep in mind that a rear parcel shelf was not designed to hold >substantial weight nor provide a vibration-free platform for >subwoofers. I am going to chuck out the stock "cardboard" shelf and build a new one from 16mm MDF. >If your car isn't as solidly-constructed >as a BMW, you *may* be in for a rough ride. It's an older Audi, so it should not be too bad. >Also worth considering is how well the rear deck and your enclosures >will fare on seriously bumpy roads. Yes, I have thought about that, too. The driver will rest on top of the rear shelf, so that part of the weight will be plenty supported. The problem is going to be the weight of the box itself. I'd like the box to be removable, so glue is out of the question. I am considering using threaded rod from the top of the shelf all the way through to the bottom of the box. That way, the steel rods could also serve as bracing if nuts and washers are placed at strategic places. Alternatively, the rods could end inside the box and conventional bracing be used. The problem, of course, is going to be looks. Steel nuts visible on the shelf would be seriously un-pretty. Maybe some nice looking caps over the nuts would be acceptable. I'll se what I can find or make. I have a metal lathe, so I have options. Thanks for you comments. SignatureRoRo |
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