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Discussion Starter #21
Frequency response of a bandpass worries me too, I'm more looking for SQ so I may need to look into my best possible IB option.
 

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- IB, which are loaded through a snorkel, could change how the come off the front of them.
They will certainly have some "trapped" air to push against.
  • Having the snorkel soming off of a chamber could make it possible to model it?
  • Are you worried about the band pass on the low frequency? Or the higher end?
 

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Discussion Starter #23
I'd be more concerned with low end. And I assumed the more peaky response of bandpass would make it more difficult to blend with the mid woofers.

I'm trying to picture the snorkel loaded IB? Just a manifold with the proper length vents to reach the stock location? Or would it be the bandpass with small front/large rear chamber so that it models like IB, as you described?
 

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Picture this:
275932


  • The box on the left is the trunk.
  • The box on the right is a manifold, and the port is the snorkel... so it can poke out of the right hand box.
  • The port determine the tuning freq as well as the upper cutoff frequency.
  • The low frequencies are below the port tuning freq, and they just unload out of the port with no blockage what so ever.
  • if the port is large in diameter and short, then the tuning freq will be so high, that the gain will not matter much if the signal is limited to not play us that high in the first place. Or if you want it to choke out higher frequencies, then tune it to 80 or 100-Hz.
And the box on the left is just the trunk...

The only think the box on the right does, it to allow one to seal off the IB.
 

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Discussion Starter #25
OK, now I see. I should be able to model this as a bandpass in winisd, correct? Using my trunk volume as the rear box?

By the way, any advice on the design of the "snorkel" itself? I found where someone 3d printed one for the ls400 but I don't have access to that big of a printer. Will bent 4" pvc be my best bet?
 

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I would ask @Focused4door ... but I think it would be an insert with a hole in it that fits the existing holes.
Then port tube would be sealed into the insert.
So sort of like a blow through in a pickup truck bed?
 

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Discussion Starter #27
Yep I can picture it! So as long as my desired tuning allows the port to be long enough to reach the box, I should be fine... I think that if I come down at the correct angle I can maybe just run a straight 4" port with no bend?

Seems almost like I will need to base my driver selection around this... ie I can't run a 24inch port from a .5 cube manifold and reach the stock hole, with my current Ultimax 10. A quick model of a JL 15 calls for a 2 cube box that will make port placement easier.
 

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I think you will likely be going towards a 6 or 8" port.
(It depends on the manifold to deck distance, and tuning freq... which is probably ~40-70 Hz?)

I am not saying that any of what I mentioned is overly factually... it is just ideas to get someone else to opine on it.

...
... A quick model of a JL 15 calls for a 2 cube box that will make port placement easier.
Yeah JL calls for the box on the box being 2 cubic feet (55litres)
That is the box on the left, and in you case it is 1000 litres.

I think that you will need to model it as a bandpass, or an IB?
Just any old sub may not be optimal in this situation.

Maybe you could model it as a T-line if the snorkel is long?
 

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A bandpass box doesn't have to be boomy one note box. You have to understand the knobs you have to work with and tweak. You can have wider bandwidth at expense of gain, or vice versa.

I come from the 80's, we successfully built bandpass boxes prior to computers and modeling software being readily available. We didn't even have access to Microsoft Excel. There was no REW, if you had a 31 band audiocontrol spectrum analyzer you were in heaven but that was not really sufficient.

To build a bandpass, all you had to do was build a sealed box, measure impedance and build a ported enclosure tuned to that the frequency of the impedance peak. You could do that with an oscilloscope and pencil and paper.

If you didn't have an oscilloscope, you could calculate the frequency of the sealed box, and tune the ported section to that frequency.

People chased gain back then because amplifiers were no where near the power they are today, and the power classes encouraged high gain to get spl over adding more power. Bandpass of 6 dB gain was a common way to rip surrounds, tear or fold cones, and also gave bandpass boxes the reputation of boomy one note boxes. Subwoofers are much more durable now than they were back then.

Today, you can go into WinISD and have it do the work to calculate a 0 dB gain, 0 dB ripple box. go try this.

After you do that, make a second one and start tweaking the knobs to see how the gain and bandwidth works.

Try this:
1) adjust the ported side box smaller than WinISD calculated, leaving all other things the same
2) adjust the ported side box larger than WinISD calculated, leaving all other things the same
3) lower the sealed side volume, set the ported side tuning to the new WinISD calculated Frc below the sealed box volume. Make the ported side volume the same as the sealed side to start with, you can adjust the ported volume with the knowledge you learned in steps 1) and 2) for the new sealed volume and port tuning

Congratulations, you now are an expert in bandpass boxes, and you didn't have to use a pencil and paper to get there. You can account for cabin gain in your design, same as any other box.

It greatly helps to use a subwoofer that the T/S parameters works well for a bandpass box based on EBP, but most subs that work in a sealed box will work.

Like with any box, need to check power handling/cone excursion. You can still tear surrounds and cones today, and lots of the spl guys chasing 6 or even 8 dB gain do so, although with much more power and higher spl than back in the day. Subs are much more durable now, so on sane power and sane box gain that should not be a concern.

3 dB is considered really safe level of gain. Back in the day people used to simply ratio the ported box volume as a short cut, people still do this but seeing the results in WinISD allows you to do so much better.
 

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Discussion Starter #30
Focused, Thanks so much for all the info! I was really struggling to figure out what box parameters to play with.

I think you will likely be going towards a 6 or 8" port.
(It depends on the manifold to deck distance, and tuning freq... which is probably ~40-70 Hz?)

I am not saying that any of what I mentioned is overly factually... it is just ideas to get someone else to opine on it.



Yeah JL calls for the box on the box being 2 cubic feet (55litres)
That is the box on the left, and in you case it is 1000 litres.

I think that you will need to model it as a bandpass, or an IB?
Just any old sub may not be optimal in this situation.

Maybe you could model it as a T-line if the snorkel is long?
I am modeling it as a bandpass with a left side box as my trunk space. As far as ports, is dual 4" a replacement for single 8"? There's no way I'll be able to bend a 6" or 8" port to curve around my gas tank.
 

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The port doesn't have to be round ... and also the suggestion above didn't involve bending the port ... it involved installing it on an angle.
To answer your question regarding dual 4" to replace an 8" ... the math doesn't work like that. An 8" round port has a cross-sectional area of 50.27 square inches ... you would need four 4" round ports to get that (4" round port has cross-sectional area of 12.57 square inches).
If the cross-sectional area is small the port noise will increase, you want to aim for less than 30m/s air velocity (below 20m/s would be preferable). WinISD has a plot for air velocity (for rear and front chambers)
 

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I just banged out a quick bandpass option using an Audiofrog GB10D4, it would only need a single 4" port with a length of 5" to tune at 35Hz. 106dB plot @ 28Hz ... nice!
 

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Discussion Starter #34
Then I'd have the strange problem of building a box to within 4" of my deck! Haha. Port velocity looked OK on that?
 

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The port air velocity is what drives the port area requirements, to high of an air velocity and you get audible chuffing noises. Bandpass has a shorter port since it is tuned higher in frequency, but needs more port area than bass reflex

Don't forget to check port velocity in WiISD, as well as power handling.

Remember, more port area needs a longer port for the same tuning, so use that to your advantage to be able to reach the rear deck.
Multiple ports also requires longer ports, might be easier to bend a pair of 4" ports to the deck. PVC will bend with heat, it takes some practice to do though, and helps to bend it against a wooden jig versus freehand. Large tubes sometimes takes a little more heat, people often duct tape one end shut then fill with heated sand then bend with heat gun on the outside. Old pot on the grill will heat up pretty quick.

You could always build a port from foam carved to shape, then fiberglass, then carve or melt the foam out.

Bandpass tuning is higher than bass reflex box, depending on your amount of cabin you may be tuning as high as 50 or 60 Hz.

Make sure you have a way to adjust the box tuning. You can do that by shortening the port, or adjusting the box volumes but don't count on being exactly on. Or just tune with the DSP and call it done.

I am a bit OCD and always end up sanding off the port until it is right on.
 

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Velocity on that maxed out at 21m/s @ 26Hz. Going to a pair of 4" doubles the length and drops the velocity below 11m/s
 
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