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Old 6th August 2004, 02:56 PM   #1
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Default Multiple subs- need advice (car audio)

I took the plunge and bought 6 15" car stereo subs. These will probably be made to a W frame dipole after I buy 2 more when the car stereo scene gets old and I can afford to buy 2 more 15s for a total of 8 woofs.

So in the meantime I need some recommendations for the car, I've never done anything with this many woofs at once. I wasnt about to ask in the car stereo forums either because 90% of the poeple dont know what their talking about.


Soooo... anyway, the car is a C5 corvette coupe. The rear window is "bubbly" and it almost to serve as a scoop to hornload the bass, except that the horn is going the wrong way (it isnt hyperbolic, it would be 1/2 parabolic).

For now, this system is going to have SPL take priority over SQ. Im not going for "absolute" loudness, so I dont need one 18 db peak at 45 Hz, however I am willing to sacrifice some deep bass for decibel.

I also plan to do a ported enclosure, each woofer will be 2.0-2.5 cubic foot according to my rough estimates for that additional SPL, but I will also use port plugs (PVC test pipe fittings) to seal the boxes incase I want to do 10 Hz vibrations.

Heres my issues though: I've done single 2 subs before in a box and when I had them in the middle of the car with the top off, there was very noticeable cancellation around the x'over frequency. In my current installation, I have a single 15 at the extreme rear of the car and there is no/very little cancellation at the x'over frequency, so this tells me that the car sub is sensative to positioning. My x'over freq is 90 Hz right now, Linkwitz/Riley configuration. Perhaps it has to do with the length of my car? It is almost 15 feet in length exactly and that translates to 75 Hz or so for a 15 foot wavelength of sound.

Also, yesterday I had some cardboard boxes infront of my sub that sits in the rear of the car and I noticed that there was a significant reduction in bass. I so I have a feeling with 6 15s I may have some trouble with cancellation.

I dont want any cancellation. All the woofers will be within a foot of each other. I'm hoping that they will all "mutually couple". Where do you think I should put the ports? Should I try to consolidate the ports to all exit out the middle of my rig (that would look kinda cool), or just put the ports all right next to their respective woofers? But would this cause any type of undesireable cancellation?

Advice please.... thanks
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Old 6th August 2004, 06:13 PM   #2
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First off, good luck fitting 6 15's with 2-2.5cu.ft a piece plus displacement for ports in your C5. Also, thats an awfully small enclosure size for a 15" sub in a ported box meant for high spl. I think your trying to put too many subs in your car and its not necessary. I have a 1990 CRX with 1 15" Rockford fosgate Power HX2 in a 3.5cu.ft box with 2 15" custom built Passive Radiators and I take home a first place trophy every Wednesday at a local DB drag event with 144db on the new termlab sensors. If I were in your situation, since you already have the subs, I would isobaric them so you have 3 pairs that will work in half the enclosure volume and handle twice the power of just 3 15's. This way a 2-3cu.ft box each will work perfectly and you will only need a total enclosure volume of 6-9cu.ft which should leave plenty of room for ports. I would also go with a minimum of 50sq.in's of port for each isobaric pair.
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Old 6th August 2004, 07:02 PM   #3
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Hi, I think you underestimate the size of a C5. It has 25 cubic foot of storage space in the hatch. Even at 3.0 cubes, there still would be a little bit of room left, but of course thats assuming I use fiberglass and have a non-removeable enclosure thats molded to the car. The woofers I have now arent over the top 30 lb style woofers either. I do have one Kove Amageddon Z15D which I think is awesome, but even at close out when parts express had them at $300 a piece, I'm not spending $1800 on drivers alone. Oh well, ill try several different enclosures over the next few months or so, each time I build something I learn something new (or break something haha)...
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Old 6th August 2004, 08:24 PM   #4
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I know its not much of an option for you cuz it would require so many of them, but I'm am now thoroughly hooked on Passive Radiators for car installs. Its amazing how much space you can save not needing a huge traditional port and they are much more flexible as far as tuning.

When I built my PR's I put a bolt through the back so it would stick out the front and made custom weights that thread on, so when I'm at a DB drag even I just take them off. With the weights on its tuned at 30hz and with them off its at 52hz.

I also like the fact there arent any unwanted port resonances and turbulence. My 15" driver is mounted on the bottom of the box and can only be seen through a Plexiglas window on the top. Its great, at first glance all you can see it the 2 15" PR's with their Plexiglas centers and big lead weights, people don't know what they are, they just stand there and scratch their heads.
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Old 6th August 2004, 08:31 PM   #5
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Oh, I also wanted to make a suggestion about your port placement. I have a lot of experience with installs in cars with hatches and the best results I have gotten were from 1 rectangle port at the very back of the car under the bottom of the window. Just remember to keep the rectangle port under a 9:1 ratio.
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Old 7th August 2004, 06:14 AM   #6
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Thanks for that reply- I also thought about doing a slot loaded port, infact I may modify my current box for one of those as early as tomorow. :-)

As for the passive radiators, that is something I've never tried and I'm willing to try it. Dont PR's have to be larger then the active cone? I'll probably try to do that in a home stereo setup.... where the box can be the size of a wall.

One more thing- the rear woofers I will definately try the slot load to the bottom of the window. However the front row of 3 woofers I may not be able to do, because i plan on having them 2 seperate boxes for easy removal. Not sure where to aim/position the ports for the front woofers.
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Old 7th August 2004, 06:53 AM   #7
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The rule of thumb for PR's is, they either need twice the Sd with the same xmax, or twice the xmax and the same Sd as the driver. This is all dependent on the power input though. With my setup I have twice the Sd with four times the Xmax because of the amount of power im using(about 3,000 watts rms to one 15" sub).
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