Hard 8

I am hoping (!) that the eBay seller came through and finally shipped my JL audio subs to me. I paid for them weeks ago. They are supposed to get here today. They are a pair of 6.5 inch JL audio W0 subwoofers. I intend to replace the parcel shelf speakers with them. They require a cabinet of some sort (the JL website has the recommended cabinet volume for them).

Anyway, here's my half-baked idea for the cabinets: Instead of building MDF enclosures, I thought I might use large PVC pipes, like sewer pipes; maybe six inches or seven inches in diameter. The sub faces would attach to a panel that I would bolt tightly to the underside of the parcel shelf, with rubber gaskets or foam weatherstripping to form a seal between the panel and the shelf.

Below ("behind") the subs, hanging from the panel's underside, the pipes would drop vertically for four inches or so, then they would connect, using huge T connectors (also PVC pipe material) to a horizontal section of pipe, with end caps. The total volume of the pipe would be whatever JL recommends on its site. I would glue the sections together, pack the pipes inside with sound damping material, and coat the outside with some kind of rubberized material, like car undercoating.

Any unwanted resonances would largely wind up inside the trunk, not where I would hear them. If necessary (what do you guys think?) I could put each sub into a separate enclosure, using this same approach. Or they could both be in one space. I don't know. I just think the enclosures would be easy to build, relatively good looking, and have nicely rounded sides.

I also bought a decent Alpine two-channel amp to power the subs, and I intend to upgrade the front door speakers to components but just power them off the head unit amp (if that sounds lousy I may add amps for them, too).

Does this idea sound sensible, or is there some reason (resonance or sizes or shapes or something) why plastic piping is a stupid idea for this purpose?

Thanks.

Sputnik

First, plastic itself isn't a bad material. The issue is whether the walls will be thick or strong enough to resist flexing. But in a tube, the walls don't have to be as thick as a cube. Second, optimally you want to avoid parallel surfaces in an enclosure, to eliminate unwanted resonances inside the enclosure, and a tube is advantageous for that. According to JL audio, the recommended sealed enclosure is 0.15 to 0.20 cu. ft. This means that a pipe with 7" internal diameter would need to be approximately 7" to 9 1/2" long (just to give an idea).

The tube does not need to have sound deadening applied to the outside, nor should there be any "unwanted resonances" in the trunk (they wouldn't be restricted to the trunk anyway, they would get through). If the tube is stiff enough to be effective, there shouldn't be any sound "leaking" through the walls, just like they wouldn't in a stiff box made of MDF. If the tube is not stiff enough, and sound does leak through, then the walls flex too much, and are not going to work for this enclosure.

It would be wise to keep each sub in it's own separate enclosure in this application, and a "T" connector is not a good idea. First and primarily, it's not really necessary. If you are talking about a 7" inside diameter pipe, and the largest 0.20 cu ft. enclosure, a simple tube would only be 9 1/2" long. A "T" connector for that diameter of pipe would be longer than that.

The second reason is a little more complicated. Regardless of whether an enclosure is sealed, vented, or a bandpass type, the air inside the enclosure supports the sub, helping to stabilize and control the sub's movement. If that air acts consistently, the resulting sound is smooth, solid, and loud, and the sub will last longer. But if that air acts erratically, the resulting sound will be weak and fuzzy, and the sub will be fighting the air instead of working with it, which will wear it out. It's very similar to having bad shocks on a car. Among other things, weak and/or parallel walls have an effect on how consistent that air is, along with how symetrical the enclosure shape is. When the air inside that enclosure compresses and decompresses, the airflow of the enclosure affects how smoothly it compresses and decompresses. If you put a straight "T" shape in an enclosure, the airflow wouldn't be too smooth, and the sound will suffer. If you put something like a curved elbow for example (that would smooth airflow), that should work. Of course, how much of an effect the airflow would have depends on how sharp and severe something like that is, and you can't always make a "perfect" shape. A vehicle's interior will require compromise. But it's something to keep in mind when designing an enclosure, so that you can make the best sounding enclosure you can.

---jps

Hard 8

Whoa! Seriously valuable information! Thanks, Sputnik. You just saved me a lot of work.

I'm gathering from your comments that my idea would work if (1) the piping has thick walls, and (2) I just use a separate straight or gently curved 7" - 9.5" long by 7" ID pipe (assuming such a thing exists) below each sub, with a cap on the end.

That could work for me very well, as long as it doesn't block the trunk mouth too much for luggage purposes. Does that approach sound viable to you? Is sealed better, or ported? If ported, what size, shape, etc.? (I want fairly tight, clear bass.) Thanks again!

FastRX8

I don't think there would be any problems with your idea.
FYI - Pepboys sells a plastic speaker housing. They're sort of what you described.
I've seen them for 6x9's and 5.25" round speakers.

Good Luck -

- Cesar -

Hard 8

Excellent news, Cesar. I'll check 'em out. You guys rock. Thanks.

Hard 8

Also, where does the Bose amp (not part of my base package) under the parcel shelf draw its power? It sure would be nice if there's an untapped socket back there, although I understand there isn't.

And I wonder where I should buy a power cable for my amp (10 gauge?) and a fuse block. I was thinking a stereo installation place. I was also thinking I might mount the fuse block next to the battery. Is that a good idea?

Thanks again.

Sputnik

The plastic speaker housings that you'll find at Pep Boys will not be appropriate for enclosures. They are designed for different uses.

Yes. But with #2, the thing to remember is that even if such a part exists, you have to carefully calculate the volume on a curved piece, and get it just right (the smaller the enclosure, the more accurate you need to be). Also, even a sharply bent elbow might add more volume than you need, and it won't save much depth or anything. I would do the math and see how much of a difference there would be between a curved enclosure, and a basic tube.

If you are keeping the Bose amp, you should not use that power feed (but should run a second one). That power feed is really barely enough to support the Bose amp, and won't handle an additional amp well.

When running a power line from the battery, you should have a fuse within one foot of the battery. That is very important. Then if you are going to have multiple amps, it is more practical to have a fuse or distribution block near the amps themselves. The length of the cable and the amount of power to be drawn determines the thickness. And with the length of a run for a trunk mounted amp in an RX8, you need to start with 8 ga., and if you might expand in the future, it's ALOT easier and cheaper to run 4 ga. now, then to have to do it all over again.

Whether one is better than the other depends on your application. In your situation, I would suggest sealed. It will be much easier to design and build (especially if you are doing a tube enclosure), and it will have a broader frequency range. For a ported enclosure, the port length and diameter is in the sub's manual (and I would NOT put bends in a port, as they are VERY sensitive to air flow). A ported enclosure is a trade-off. In a certain range of frequencies, the output from the port combines with the output from the front of the sub to produce significantly louder sound than a sealed sub at the same power level. But outside of that frequency range, the port basically acts like a hole in the enclosure, and the sound is significantly softer than a sealed sub at the same power level (well, it's not an on/off affair, it rolls off, but you get the idea). So, if you need a certain range of frequencies (like most hip-hop music, or a have a system designed for that frequency range), or want to be the loudest on the block, then ported would be better (bandpass would be best in some of those cases). But if you need to cover some range, or go lower, then sealed is better.

---jps