Enclosure having a center wall with only a relatively small opening - issue?

[geolemon]

I'm trying to take advantage of an opportunity that I have in my car - the fact that this is a car is inconsequential other than it somewhat forces me into a scenario where my enclosure will end up with a center wall.

In the rear deck of my car, there was an opening for a factory 8" subwoofer in the steel rear deck structure.

Rather than have a large box swallowing up my trunk, I originally thought of making a small fiberglass enclosure to fire a 10" subwoofer off the rear glass, and extend a port out the factory hole into the trunk.

However, in the interests of making something more practical and less noticeable, I'm now looking to suspend two eights, and two flat 10" passive radiators just above that rear deck - keeping it hopefully just a few inches above the rear deck, so it doesn't block any view or look obvious.

This will only afford about 1 cu.ft. however (approx. 12" deep, 48" wide, and suspending the 8's about 5" up, the PR's about 3" up).

So, I thought I might be able to make an equally shallow enclosure under the rear deck, to add some volume without impacting trunk space or pass-through much. However, in this scenario, the two chambers of the enclosure (which need to function as ONE chamber) are joined to one another by an opening in the sheet metal for a single 8" sub.

What are your thoughts on how this might impact the performance of the enclosure, if I attempted to create a larger enclosure by adding a lower chamber coupled by a roughly 7" hole?

I'm torn as to whether I believe the opening would work, since I've only got two driven 8" cones and the hole is at least relatively large (and with no real thickness) compared to that - or whether the opening will have some limiting effect, as there are really two 8" and two 10" cones in motion, and in comparison the hole is pretty small.

Curious to gather thoughts on this idea before I start.
Thanks!

[weltersys]

Originally Posted by geolemon

What are your thoughts on how this might impact the performance of the enclosure, if I attempted to create a larger enclosure by adding a lower chamber coupled by a roughly 7" hole?
Thanks!

The hole will create a sort of internal bandpass effect, from the area of the cabinet and the hole size, sounds like that effect would be above the "sub" passband.

You could use a port calculator and enter the actual sizes to see where the resonance would occur.

The resonance probably would hardly be audible using passive radiators.

[geolemon]

Part of me is thinking this effect essentially won't happen because the port length is about as close to 0" as possible - thickness of the sheet metal of the back deck. Even if it existed, it would be such a high frequency as to be out-of-band, especially given the large diameter and short length... right?

Part of me is thinking that this is still a concern in some way though, in the same way you are thinking... not as much frequency-dependent as more fundamentally that this is not going to uniformly resonate in both chambers, given four cones and a 7" opening.

Thoughts?

[weltersys]

Originally Posted by geolemon

Part of me is thinking this effect essentially won't happen because the port length is about as close to 0" as possible - thickness of the sheet metal of the back deck. Even if it existed, it would be such a high frequency as to be out-of-band, especially given the large diameter and short length... right?

Part of me is thinking that this is still a concern in some way though, in the same way you are thinking... not as much frequency-dependent as more fundamentally that this is not going to uniformly resonate in both chambers, given four cones and a 7" opening.

Thoughts?

A port is a port whether it is 1/16", 3/4", or 12" long.
Deeper ports tune lower. Your internal port might tune the cavity to 500 Hz, give or take an octave or two :^0.
If the crossover is an octave or more below the resonance, who cares.

You have not given enough information for a simulation (not that I'd go to the effort of doing one if you did, LOL) but I think any interior port resonance will be a small concern compared to the rattling tin in the trunk and all the other goofy peaks and nulls that are a car environment.

Art

[geolemon]

Art - thanks for the response. I've got s few (over 20, now) years of professional experience in the mobile space, including some time engineering product for Better Audio and Pro Tech - I'm confident that I've got the "car" aspect covered.
I made a small effort to disclaimer that in the first post, because I wanted to focus this discussion on the enclosure, and not on "car vs. home" or any other details that I've got under control.

However, I've never really used passive radiators, so this will be a new and fun experiment for me. And it really isn't something you see much in the mobile world, so it'll be fun to share with others to open minds a bit. IF it works.

The concern is, executing this enclosure means committing to it to a certain degree... it takes a significant effort to create a structure such as this correctly, upholster it to not be conspicuous (much less look stock), with grilles that both function and disguise, and I'm going to need to cut and modify the factory rear deck panel to accommodate this enclosure. I need to feel confident committing to it. I don't jump without planning.
This discussion is hopefully going to stay in the theoretical realm - it's more to establish the feasibility of a "plan B".

I actually have done quite a bit of modeling, and even with the 1 cu.ft. scenario, this system models up with a nice -6dB or so slope from about 90hz down to 30hz... cabin gain will help flatten this. It's an area where being in a car helps.

But in the event that I find my system wildly inefficient, PR's overexcurting and requiring too much mass for decent LF extension, or other potential issues, I'm hoping that I may have an option in adding a small under-deck box to (hopefully) essentially/effectively increase the size of the enclosure. This is mostly for "Plan B" planning.

Plan A really is to just do the thin over-deck enclosure, that will only net out about 1 cu.ft., flattened across the whole rear deck.
...I just don't want to be purely crossing my fingers that it'll work out - I need to explore if this option to extend the box is feasible.

In terms of a port, I'm confident that I'll be many octaves below any effective tuning frequency - even if just 1 cu.ft, 7" diameter hole (estimate), and 1/4" (generous).

I suppose one thing that I could do is model up a dual-chamber vented box with two chambers of the size I believe I'll end up with, use those port dimensions, and see what tuning frequency that yields for the chamber-coupling port.
But more importantly - how that response plot compares to a simple single-chamber vented box. With luck, at least from 80hz/down, the answer is "no different".

Since we're fully operating below where a port would support a driver (the tuning frequency MUST be octaves higher than even my LP Xover frequency), I want to think this will work.

The fact that I've got 4 cones totalling significant total area is what's causing me concern. Even in this respect however, realistically at one end of the frequency spectrum, I'll have some significant excursion from the 8's and very little from the PR's, and at very low frequencies I'll have some significant excursion from the PR's but very little from the eights. So in theory that helps as well.

Thoughts? I want to be rationalizing, not justifying...
Thanks-

[weltersys]

Originally Posted by geolemon

In terms of a port, I'm confident that I'll be many octaves below any effective tuning frequency - even if just 1 cu.ft, 7" diameter hole (estimate), and 1/4" (generous).

Thoughts? I want to be rationalizing, not justifying...
Thanks-

I think your rationalizations agree with mine.
If the two portions of the cabinet were connected by a thin tube, things could get weird, but a 7" diameter hole in a 1 cubic foot box is big, any port resonance (if there were one) should be way higher than the pass band.

I use a Mackie HRS 120 for my studio's control room sub, it's the only passive radiator system I have ever used, it sounds good and has a -3dB point below 20 Hz in room.

Cut away in peace, good luck with your passive radiator endeavor.

Art

[revboden]

The 7" hole will act as a port, but a 7" X 1/16" port with 1cf volume in the upper chamber will "unload" at 141.12hz, therefore at sub frequencies The coupling between the two chambers should work out fine.

[geolemon]

Wow, that's actually not quite as high as I would have guessed it to be!
But right - at least higher than my LP filter.

Thanks for the help!