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Old 28th February 2014, 01:14 AM   #1
T3mpest is offline T3mpest  United States
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Default Midbass/subbass Plenum?

I had a thread a while back about how best to use a B&C21sw152-4.. long story short for those who followed the thread, I did build PWK's enclosure, it did perform as it was indicated in the thread it would, output was bad above 80hz, but I digress..

Also in that thread someone suggested the best use of these woofers would be in the midbass, despite the fact this is going in a car, I'm inclined to give it a shot

I plan on running 2 drivers, off seperate amps in seperate matching enclosures. I want to run them between 30-300hz as midbass/subbass. They will be taking up the entire space where my rear seats are, from ceiling to floor.. approx 55inches widex44inches tall and 23inches deep.

I want to do ported enclosures, with the ports facing forwards in the center of the car, basically firing into the space between my seats. The issue, is how to orient the woofers? If I put the woofers facing forward, my left and right midbass wouldn't be very far apart. Especially the right midbass as the center of that cone would be directly behind me..

Here is where I have a solution, but dont' know how to calculate the effects. If I face each driver to the left and right respectively, firing them at my doors, they can be placed much further apart. However, since this needs to be full walled off and I dont' want it to rattle and buzz.. I will be putting another wall of the enclosure in front of my doors, effectively making the speaker fire into a small duct..

My question is how wide does this duct need to to minimize the ducts effects in the midbass region and still allow the lower tones the cone would be producing to find a way out. (port tuning on the rear would be lowish, 30hz or so so 60hz and up would all come from the cone, that has to fire through a duct) Also how much duct area do I need? I've been told hornresp can model this, but I downloaded it and after a few hours I still dont' know what I'm doing I've found some guides, but again, I know someone who can run through the program can give me a better idea.

I assume the duct needs to be as short as possible (width of the driver more or less since it mounts sideways) to push the 1/4 wave resonance up.. Height of the duct I have no idea, again it can be as small as the driver in this dimension as well. So 22in tall by 22deep would me minimum.. Width I just want to give it enough surface area allow all the bass to get out unimpeded.. I have read 1/2 sd is generally acceptable.. Since I would be pusing higher frequencies through this side, I don't know how that changes things as usually 1/2 SD is what I've heard for IB type setups doing low frequencies as the lower limit of surface area for the duct exit.. Hopefully if it behaves as a bandpass, it will be outside the intended region..

Finally, I am aware that placing up to 300hz behind me MAY be an issue. However I know as rooms get smaller, the schroeder frequency moves up, I figure if I wall of the car, it should be small enough to get me into the 200hz or higher region without localizing. Even my fullsized car could do up to 120hz and even my large suv is good up to 80hz.. Once I cut the length of the car down to maybe 1/5th of that, it should jump up a lot.

I know I haven't given enough info to model anything concrete.. looking for more general input and someone willing to mess with it to see if it's even feasible. I could always fire the woofers forward and be done with it, but I think sideways firing is better, IF the duct doesn't ruin acoustics...


Thanks in advance for anyone who reads this or gives any input.

Last edited by T3mpest; 28th February 2014 at 02:51 PM.
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Old 28th February 2014, 05:02 AM   #2
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You are describing a band pass sub, an SBB6 as they are referred to. Not designed for much of anything beyond sub duty, so forget the 300Hz as the top end for them. Are you removing the seat and installing a baffle wall or building a box that will wall off the trunk? If the latter the drivers can be facing each other slot loaded in the center with the midbasses on either side in their own isolated enclosures. I normally run the rears a little hot on the top end of the bass range/ low midrange as door speakers eg DOORs make the worst enclosures possible for bass reproduction.
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Old 28th February 2014, 01:58 PM   #3
T3mpest is offline T3mpest  United States
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Quote:
Originally Posted by Greebster View Post
You are describing a band pass sub, an SBB6 as they are referred to. Not designed for much of anything beyond sub duty, so forget the 300Hz as the top end for them. Are you removing the seat and installing a baffle wall or building a box that will wall off the trunk? If the latter the drivers can be facing each other slot loaded in the center with the midbasses on either side in their own isolated enclosures. I normally run the rears a little hot on the top end of the bass range/ low midrange as door speakers eg DOORs make the worst enclosures possible for bass reproduction.
This is in an SUV so I will be removing my second row seating and putting the box right there. I will still have my cargo area, but no rear seats and the front and back of the cabin will be completely seperated by the box. While it's technically a 6th order, it has a very small loading for the front chamber as the port and the front loading chamber are one in the same in this alignment.. I just dont' know how high that will push the resonace points.
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Old 28th February 2014, 02:25 PM   #4
GM is offline GM  United States
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FYI, ducts are 1/2 WL resonators: Resonances of open air columns

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Old 28th February 2014, 02:48 PM   #5
T3mpest is offline T3mpest  United States
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Quote:
Originally Posted by GM View Post
FYI, ducts are 1/2 WL resonators: Resonances of open air columns

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In my case the duct would only be open on one end, does that change things? The rear side has to be closed, the side the leads to the front of the car would be open.
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Old 28th February 2014, 02:51 PM   #6
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??? Please make a sketch............

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Old 28th February 2014, 02:52 PM   #7
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First SBB6 sub I designed was 25 years ago and utilized 4/10" woofers in a dual opposing isobaric configuration. Sub duty worked fine, anything higher was a mess. At these higher frequencies and their associated shorter wavelengths the small front enclosure and port induce resonances, phase shifts and standing waves that just kill the response. For midbass/low midrange duties is always best to either be direct radiator or horn. The latter is far larger, more difficult to design and nearly impossible to achieve for auto use.
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Old 28th February 2014, 02:56 PM   #8
Djim is offline Djim  Netherlands
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Quote:
Originally Posted by T3mpest View Post
In my case the duct would only be open on one end, does that change things? The rear side has to be closed, the side the leads to the front of the car would be open.
Hi T3mpest,
A duct usually functions like a chamber with a port (half wavelength resonator).
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Old 28th February 2014, 03:32 PM   #9
T3mpest is offline T3mpest  United States
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Hopefully this pic works, if not I'll have to reply when I get home. Anyways the blue represents where the speaker cone is on the baffle, the motor would go inside the box, with the cone firing into that short "port". I'm trying to minimize the 6th order effect on an aligment like this, but I'm not sure how to push most of the gain out of the bandpass of the woofer, or if that's even possible. It could be as small as a baffle would need to be for the speaker.. Nor do I know how wide that "port" needs to be..

Nothing in this pic is to scale, but it shows the basic arrangement I'm shooting for.. Like I said i could fire both the woofer and port forward to avoid using a duct like this all together, but if I do that the left and right moves much closer to the center of the car, the right midbass would be directly behind my seat.
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Last edited by T3mpest; 28th February 2014 at 03:40 PM.
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Old 28th February 2014, 04:41 PM   #10
Djim is offline Djim  Netherlands
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Hi T3mpest,

If you turn the inside enclosures around you can make a Push-Pull configuration which helps to suppress distortion (even harmonics). You need to make the 'plenum' as small as possible if you want to cross high. There are many posts about PPSL here on DIY forum if you need more info.

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