todddchi

iTrader: (3)

Hi all,

I have a JBSlim sub box in my Z28 vert. The way the box is set up, the woofers fire into the trunk cavity, with the ports firing into the cabin area. In terms of it being a standard ported enclosure, each of 4 8" subs (RF stage 1's in my case) have about 0.18 cubic feet of space, with port tuning at about 80Hz.

I know this is not ideal, but it is the best obtainable with my current arrangement. So please don't just criticize the setup, because I am asking for ways to work with it.

As it is, the box sounds fine, but it doesn't hit like it should, and obviously the tuning is off due to the small enclosure volume and limited room for proper porting. I don't have any enclosure simulation software, but what I am thinking about doing is just flipping the woofers around so the cone is firing into the box volume and then out of the port.

What I think this would end up being is essentially a bandpass enclosure with all 4 8" subs sharing the trunk volume as an infinite baffle enclosure. Of course, now the woofers would be hanging out and over the trunk area, but I think that is a sacrifice I can live with if it ends up sounding a lot better.

Can anyone out there comment on how this might work? It's a bit of a hassle tearing all those woofers out and dealing with the wiring again. I'm willing to give it a shot but I'd like it to be based upon some logical justification.

Each woofer would be front firing into a 0.18 cuft space with a port about 5" long and about 3" cross section. The trunk would be fairly isolated from the cabin area but definitely not perfectly sealed.

I have the 4 subs wired to 1 ohm, driven by the 400rms class D channel of a Memphis Baby amp. I know that more power with equalization would help, but I'm not willing to upgrade the electrical system to run any more juice.

wickedwarlock

iTrader: (4)

seems to me the subs need more airspace. You said it's about .18 per sub, correct? You would probably have more bass from a pair of 10s using that box.

todddchi

iTrader: (3)

Thanks for the reply, I know the subs should have more airspace, but the packaging environment doesn't allow for it. The box sits right up on the shelf of the trunk, which in convertibles is a very limited space. Subs in a vert is a whole different story than vs. coupes. If it was a coupe I'd have a hefty 10 in a stealbox and be done with it. Verts are not so simple.

Because of this limitation I'm wondering if I'd be better off flipping the woofers around and going bandpass/infinite baffle. But I don't have the software or books to know if the theory is even sound...

99blancoSS

iTrader: (115)

You might need to switch them to something else. I had a friend who was doing a project with 8" in a STi, I'll ask him what he did and what he used and post back.
You know, we've kind of talked about power already in another thread of mine. That Memphis might be a little overated as well and could account for your lack of punch. 4 8"s should punch and produce some good bass if there is enough power behind them.

todddchi

iTrader: (3)

Well, it's not that they don't hit at all, they hit pretty well. But the response of the enclosure itself is obviously not tuned very well at all - that's what I am trying to get around. I want to make the most of what I got. Imho I should be able to get depth and volume similar to a pair of 10"s, and I'm pretty short of that down low. I do realize that the lack of hatch reienforcement is HUGE, but even correcting for that I don't think I'm quite where I should be.

I'm not claiming to be an expert but I have a pretty good ear for response. The current setup is very peaky around 100Hz (math tuning says 80-90Hz) and the response below that is pretty weak. So it's actually great for rock music but R&B/rap is really missing alot of the deep bass. It's right on that edge were much of the synthesized content is very weak, but drums and so forth hit really hard.

I'm all about making the most of what you've got so I'm looking for some ways to improve it, and was thinking this infinte baffle/bandpass arrangement might work.

Far as I know Memphis amps are very honestly rated, if not even a little conservative. It's definitely pushing the subs with an honest 100rms each.

wickedwarlock

iTrader: (4)

since you can't get the 10s there, try using just a pair 8s. I think the air space isn't enough, that's just me. You have enough power and you will get a little more low out of them too. That would be enough for .36cf per side, plenty for any good sub to work in.

99blancoSS

iTrader: (115)

Ah, got a message back. He didnt end up installing in the STi. Wasnt cost effective for the owner and there really wasnt enough room under the seat. Did you already have the 8's? Just wondering why you went with them instead of 2 12's or a 15 in the trunk. Not being critical, just curious. How much trunk space do you have to work with? I have another friend with bandpass box experience that I will see tonight. I'll have him take a look at this thread and see what he suggests. But the main goal that I understand here is to get more bass out of what you already have. (which is 4 8"s in the trunk).

MikeFbody

iTrader: (2)

by flipping your subs around, it's not going to be a free-air band pass like you mentioned. It may change the sound of the sub, but only because you are giving it a little more airspace in the box by haivng the basket facing outside. Aside from the airspace change, a sub sounds no different mounted basket in or out of a box, provided any phase issues are addressed.

I would seal up the ports temporarily and see if that improves it. Small sealed boxes are bad enough, small ported boxes are even worse. Ported boxes like to be big. I would just cover the ports with some 3/4" wood and listen to that.

mike

wickedwarlock

iTrader: (4)

This is 100% true, you need about double the air space if your going to port them effiectively.

TA guy

iTrader: (1)

By turning your woofers around you will still have the same type of enclosure, the difference will be that the drivers see a larger front chamber (from .18cuft to ~.23cuft) and you will have a slightly lower tuning due to the larger enclosure. The observable result is that you will gain a db or two on the low end and lose the same on the high end of the passband.

If you want to model what the theoretical responce will be, download WinISD Pro (it's free, and pretty good) or Unibox, which is an excel spreadsheet, also pretty good. When I was designing my setup I modeled similar sudo-bandpass setups like Johnny B's with a number of different drivers and found them to be lacking compared to more traditional ported and sealed alignments. Than said, if you post the T/S parameters of your drivers I can model them too, and see if i can see any other option you may have.

wickedwarlock

iTrader: (4)

Just buy a stealth box. It will fit in the vert passenger or driver side. Hell, get one of each.

http://www.subthump.com/acamarostealth.htm

todddchi

iTrader: (3)

Thanks Mike, that makes sense. The problem I have if I seal the box is that the sound makes it into the cabin from the ports. The box is essentially acting like the standard divider that is there in convertibles - woofers fire rear into the trunk - ports fire forward into the cabin / through the top when the top is down. So you see I'm a little stuck. If I seal up the ports completely now I essentially have just a sealed enclosure firing directly into the trunk.

I'm using 8" drivers because that is what you can fit in a box that will sit on the shelf area in convertible trunks. Not enough room for bigger. I stayed away from using brute force and plopping a 12" in the trunk because then it just fires up into the trunk lid and rattles it all to help. I also needed to keep what little trunk space I have to work with, so the shelf pass-through box is very attractive.

Mike, do you think the best approach would be to have 4 small sealed volumes, with the woofers firing forward into the cabin?

I can't do that with the current box but I might try to build something else up in the future. I'm working with JBSlim's box, I can't post a pic right now but it's fairly popular.

TA guy

iTrader: (1)

With the box he is using, he is only listening to the port, the other side of the woofer is sealed by the trunk, this is definitely not a standard bandpass, but I believe this is closer to a bandpass than a standard ported box, regardless of how he orients the woofers.

99blancoSS

iTrader: (115)

Are you saying that now they all share the same air space?

Here a pic of his box I think

MikeFbody

iTrader: (2)

I would try a box with only 2 8's, giving them more airspace each then what you have now.

If it were me, I would build a box in the bottom of the trunk well.

Actually, if it really were up to me, I'd ditch the back seat and build a box there similar to my box in my Firehawk coupe.

wickedwarlock... I'm pretty sure a driver side stealth will not work in a vert, as the top hydralics are in the driver side cubby.

Pass side will work, if you are willing to give up the spare. Which I would do in a heartbeat.

todddchi

iTrader: (3)

Yes that is the correct box, but each woofer has it's own working enclosure.

If I drop down to 2 8" subs, which I did consider, there are a couple problems. First off it's a one way street with the box, I'd have to hack it up. Second, the Memphis amp is only good for 200 watts rms into a 2 ohm load, which is all I could do with my current drivers. So I'd still only have 100rms to each woofer. The question becomes if two with .4 ft volume would sound better than four with .2ft volume, feeding the same power to each scenario.

Is there a possiblity of setting up some sort of push/pull arrangement and coupling two woofers working in the combined .4ft space?

I have read in many different places that conventional trunk boxes were not a good idea in convertibles, because you have to deal with what happens when the top is down. I tried dumping a 1cuft 12" sealed box I have in my Trailblazer just to prove it out, and it seemed to be correct. Sounded fine with the top up but drop the top and all you got was trunk lid rattling. So I went with JB's approach, which still manages to put some bass through when the top is down.

Wouldn't a stealth box behave in the same fashion as a small sealed box just put in the trunk? The only way I could see it being different would be if you could route a port up somehow snaking back into the passenger compartment, but that is also a big adjustment.

So far it sounds like the best solution would be to set the box up to package 2 8" subs, but that has me hacking the box up, and ideally investing in a different pair of subs to get down to 1 ohm loading. That's a pretty big hit in the pocket book, but does that sound about right? I could modify the box easy enough, and deal with having to sell of the stage 1 subs. It's just a lot of hassle so I want to have some confidence in the path.

Keep in mind that I also want to keep my trunk space, and not have the decklid rattling all ghetto. So the stealthbox might also be a good option but I just don't see how that would be different then any other enclosure dropped in the trunk. Do they port up through the spare tire area or something?

TA guy

iTrader: (1)

Assuming you have Rockford P18S4 8" subs, and assuming a port that is 3" x 2" x 5" I modeled your existing box, it looks like you run out of excursion by 40-45hz at 50-80 watts per driver, and you have a huge, narrow, peak (+5 db or so) at about 110 hz. So as you turn up the volume the low bass dosen't get any louder while the midbass keeps comming on strong as it is never excursion limited. Does that sound about right? Reversing your woofers will help very little, Isobaric loading will change the behavior of the box, but you will only be seeing a different set of problems. The best bet with those woofers IMHO is a rear seat delete ported box. If that isn't acceptable, and you don't want to build another box, you could try running 3 subs sealed and opening up the cavity of the 4th enclosure to allow the sound to pass from your trunk to the cabin. you don't want to point a sealed box straight into the cabin as there is some gain to be had by pointing them into the trunk with an adequate vent to the cabin, I discovered this by experimentation.

todddchi

iTrader: (3)

This is pretty much spot on. Is there any tinkering with the current hardware and packaging that can fix this tuning? Or perhaps a different sub? I'd be much more willing to work with different subs than deal with hacking and gambling on a bunch of fabrication work.

The subs were only $30 each so I didn't pass them up. I've never really had anything quite this complicated to deal with, and figured they'd be overkill for my wants. But if they need to go, they can.

"Assuming you have Rockford P18S4 8" subs, and assuming a port that is 3" x 2" x 5" I modeled your existing box, it looks like you run out of excursion by 40-45hz at 50-80 watts per driver, and you have a huge, narrow, peak (+5 db or so) at about 110 hz. "


Also,

I've been kicking around the idea of picking up a spare rear seat to work with. I was thinking something along the lines of having the woofers facing rearwards (such that the magnets were in the front of the seat) and working porting to fire out of the center. It's a little beyond what I'm comfortable with but it seems like it could be pretty promising, and still have usable back seats for emergencies, though understandably the seatback wouldn't be very comfortable. I'm not sure I want to put that much energy into it though. So much work for something tied to just this car.

Matt281

lol

todddchi

iTrader: (3)

I tried sealing up the ports this afternoon and giving it a listen. The 100Hz peak was gone, but in essence, so was everything else. I tried letting the box lean rearward a bit so there was plenty of room to get sound up into the cabin, but there just isn't enough to work with in the trunk to get the sound back up front. Convertibles are a lot different than coupes, if I had that big chunk of hatch glass to work with I'm sure it'd be different.

The fact of the matter is the box sounds much, much louder with the ports open, with the ports blocked, you can barely tell it is there, even with the top up.

So I think there is more going on here than can be explained with a normal box design approach.