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Maybe I missed it, but I dont see anything here about a SS filter. With only 2 8in subs for your sub-stage you don't have nearly enough displacement for anything below 40hz with any sort of authority.
So you are probably distorting them to death and running them hard into xmax when you touch your gain knob.
You have no sort of air spring on the cone. So it wants to move like crazy. Unless you limit that, it's going to sound rough at high volumes if there is any sub 40hz content in the music you are listening to.
If you want to run IB and not make an enclosure for them, your gonna have to fab something up for larger drivers with more displacement imo.
 

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Think of it in terms of displacement alone. You have four 6.5in mids, each of them has a cone area of 132cm each. Then you have two 8 in subs with a cone area of roughly 216cm each. You have 18% more cone area with your mids than you do with your subs. So it's not a surprise your mids are giving you nearly what you want and the subs are underwhelming.
132 x 4 = 528cm vs 216 x 2 = 432cm

That doesn't take into account linear travel, but even if your subs have 2x the xmax of your mids, that's still only 60% more air moved at xmax.
With have the same SPL level at one octave lower, you need to move 4x the amount of air.
So if you wanted to be able to match the spl capability of your mids running at 80hz @ xmax w/ 528cm, you would need to be displacing 2112cm. Drop that down to 30hz and you need 4224cm displaced. That also doesn't take into account the rise in the lower octave that is typically wanted.
Moral of the story, if you want to keep up with the 4 6.5in mids, you need to displace much more air than two 8in subs will provide when running in IB without a SS filter in place.
 

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So you either need to be more realistic about the amount of bass that 2 typical 8in subs are capable of, or you need drivers that have massive amounts of travel and a high cone area. Something like 2 JL w7 8in.
 

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Maybe you are right when you are talking about just the total air displacement, considering just spl, but one larger speaker with more cone area than two smaller speakers with drop lower. For example, two 8” subs displace 50.24 square inches of cone area whereas a single 15” sub has 47.1” of cone area but we all know that the 15” is going to catch a lower note much easier. This isn’t about just overall spl. I want good sound over a wider range of frequencies. I know two 8’s can do what I want, I just haven’t found the right driver yet. I’ve only tried two anyways. The bad thing about this is I can’t change the volume of the trunk (think changing box design or size) to change the sound, so I’m having to try different drivers. I keep seeing you guys keep harping on bigger subs, bigger subs....I get it. But it’s not an option. I don’t have the room in the deck for it. Or the depth. I wish I did because this would be much easier.
Yes and no. It all has to do with displacement in the lowest octaves.
Will a larger sub tend to play lower? Yes. But is it ALWAYS the truthful? No.
But that is because of 2 main things. Displacement potential, and driver resonance. Larger drivers tend to have a lower resonant frequency due to the extra mass of the larger cone, spider, coil, and surround.

There are 15 in midranges used for prosound that have a high FS, low excursion, and can't play below 100hz for anything. There are also 6in drivers like the exodus anarchy with a FS of 40hz and the 5in dayton epique with a fs of 40.

At the end of the day, in a low order enclosure (like IB and sealed), displacement is all that matters for the last octaves.
 

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I will say this one last time before I give up.
If you want to have more low end than your 4 6in speakers will provide, 2 8in subs will NOT give you what you are looking for in IB. If you want response below 40hz at a reasonable SPL, you plain and simple need more cone area or you need to go to a higher order enclosure like ported or a bandpass.
If you are fine with +45hz, than a subsonic filter at 40hz and more power MAY give you what you are after.
 

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Well I didn’t go back and look who posted, I was just in a hurry to prove someone wrong. In which it backfired on me. Lol
Just to be clear, I don't mean to be rude. I just mean to save you a lot of time and trouble of replacing subs a bunch of times and still not getting your desired result.

If you are stuck with 8in subs in IB as the only configuration that will work, look into jl w7 subs. They have an 8in that moves a ton of air for its size.
JL Audio 8W7AE-3

It has 245cm of cone area with 19mm xmax vs the typical ~215 of an 8in sub w/ ~10mm xmax. That is 931cm3 peak to peak. Compared to a typical 8in of around 431cm3 peak to peak (taken from a 8in focal flax Evo w/ 216cm cone area and 10mm xmax). So each of those JLs will move twice the amount of air as a typical 8in sub. And you are approaching what a good 10in sub will put out (and a typical 10in sub will have much more xmax than most 8in subs)
 

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Based on your pictures, it looked like some shallow 10s would fit if you are okay with getting out the plasma cutter. Which it seems like you are
 

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Here's a few threads with some discussion of subwoofer compression/restricted airflow.
Yeah, air is tremendously compressible. If anything, the issue would be less with the restriction behind the driver, more with the fairly large portion of unsupported sheet metal behind the driver that can resonate.
 

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The issue is going to be depth. With the factory adapters, there's a depth of about 2 5/8" before making contact with the fuel tank. Some folks add a 1" spacer on top of the factory mounting adapters, to get 3 5/8" mounting depth; but in this case, KFXGUY can just print his adapters 1" taller ... although with 10" drivers, interference with the rear window now comes into play.

Here are what the factory mounts look like for the stock 6.5" subs ... very similar to what he has printed for his 8". The picture is a bit deceiving, as the rear of the mounts are a lot closer to the rear window then it seems in the pic.

View attachment 310932

Then there's this cover which goes over the top (I added CCF as a decoupler to the bottom of mine)

View attachment 310933

And here's what it's like when put back together

View attachment 310934
Shallow mount subs are very compact
 

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3 in depth on this dayton.
3in deep on this Pioneer
Much more expensive Rockford, but also 3in deep and 50% more xmax

I would suggest the top 2 because of the more narrow motor structure that would be easier to fit in a smaller opening without having to cut the entire deck.
 

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3d printed 10 inch sub box. Regular size but.....
Think this....2 6inch subs....t-line enclosure. Hanging from the rear deck. Subs and port firing through the deck. 2 subs on one side port on the other.
Problem solved man.
**** infinite baffle
I absolutely love IB. It's just not nearly enough cone area.
Ib needs cone area. If you don't have cone area, it atleast needs a SS filter where the knee of a sealed box rolloff would be. But you don't get the small amount of box boost at the knee like you would in an actual enclosure.
I would take an IB over sealed config 9/10 times though if I have room for the cone area. You can't reasonably get any soft of output down at 18hz any other way. (Reasonably meaning without a massive 8cf box lol)
 

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Dude, if you're willing to buy and return a ton of 8in drivers just to see if one way do something magical that another one won't (which isn't going to happen, sorry man), why not just buy a couple shallow 10s and test for fitment? Then you will atleast know if they will fit or not.

I dont think you realize why there are some people getting frustrated with this. You're asking for help and then disregarding any of the help given. I would imagine that's why most people have left the thread.

Cut some holes for larger drivers, or build a baffle and reverse mount them firing through the stock speaker holes which will have more than enough breathing room.
 

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I haven't looked into your car, but if you actually have a trunk, this is a silly exercise that we are doing.
Build a legit box, with a legit sub, screw your amp to the box (the ONLY time I will reccomend this), and connect the amps power on a large quick disconnect like this :
Amazon.com: Driver Recovery 2-4 Gauge Battery Quick Connect / Disconnect (2 Pack) 175 Amps Wire Harness Plug for Winch or Trailer : Automotive

You now have your cake, and you can eat it too.

This is an even better option than any of the others since you are dropping even more weight than you would by keeping 2 8in speakers in IB (no speakers are lighter than 2 speakers)
 

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Ok. I just got home from work, I’ll run out there in a few and make a 10” cardboard circle to show you guys why. I measured the other night. It’s just not doable for the amount of cutting I’m willing to do to the car. I’ve said it in my other thread, I’m not going for max spl, earth shaking bass. I just want it louder and clearer than the stock stereo in my my 2019 Silverado. It’s louder right now but the bass isn’t as wide a range as my trucks stock stereo and I don’t even have the premium system. I really don’t think I’m asking for much. It’s just so far the drivers I’ve used just aren’t the right thing. I’m hoping the cerwin Vegas sound better than the no name subs I have now. They are “loud” enough, they just don’t have any kind of decent range to them. They really only seem to sound good around 40hz. No lower, no higher. Bare with me. I’m just looking for that perfect driver, which may or may not exist.
A higher quality sub of the same size isn't going to do what you want. Higher quality subs have less distortion. This makes them seem quieter than a cheaper sub at the same SPL level.
They are all going to sound peaky at 40hz due to them not playing with any appreciable spl below that and cabin gain doing its thing. The best thing you can do is pull the 12db or whatever it is out of the response via parametric EQ try and level out the response.
Feel free to keep chasing your tail even with everyone telling you otherwise.
Use your trunk and make a removable box, or invert mount some 10s. Those are your only options if you want anything more below 40hz.
 

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Alright. I sense some of you are getting agitated with me. Well I get it, you think I’m not listening. Truth is, if 10’s would fit, I’d do it. But I’m stuckwith 8’s. So that’s what I have to work with. None of you guys are here looking at this with me. I have pretty good fab capabilities, but I don’t want to cut my car up like that. I took some pics so you can see.
First off, this is why I don’t want a box and/or use the spare tire well. It’s being used. I have tools, air pump, cleaning supplies, extra parts, clamps, spark plug wires etc

View attachment 311222
This is my ultra small trunk. Helmet in the pic for scale.
View attachment 311223

Now here’s a 10” cardboard circle.
Please notice the curves it runs into and runs into bracing for the seat. It almost hits the back glass. I won’t have room to move the woofer up to space it. Trust me, it’s not going to fit. 8” is tight enough.

View attachment 311225
View attachment 311226
It looks like inverted 10s would fit fine from those pictures. And you don't have to use your spare well. You can use your trunks corner, it should be easy since you have fab skills.
I have a toyota 86, super small car with a trunk the same size. I have a 1.5cf box with a gb12 stuffed in the corner with disconnects to allow me to get it out to gain access to my spare. It's possible.

Automotive parking light Car Automotive lighting Hood Motor vehicle


edit
Upon second look, your trunk is smaller than mine, maybe 70% the size. Regardless, plenty of room to fit a 12 if you wanted to
 
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