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Dr_EM 22nd August 2009 11:00 AM

Bass enclosure for 3 way
Well, the time is approaching that I'll need to think about the bass enclosures for my 3 way project. You can see the top section thread here:

I have some pretty clear ideas on it myself, but I'm sure there are refinements that can be made.

The driver is Hi-Vi M8a and the enclosure will be sealed. With a final volume ~36L, this gives a Q of 0.62, so slightly on the large side. I believe this gives a more relaxed and potentially more dynamic sound than Q ~0.7 or more. Sealed was chosen for future integration with a subwoofer system. It should also work best in this small room up until that point. Does sealed usually need a small hole drilled to account for atmospheric pressure changes and the like? What would be a good size if required?

The woofer plays to 820Hz, 24db crossover. I want the sound to be very neutral, non-resonant; a pretty standard goal here. I plan an enclosure with 18mm ply and 9mm MDF laminated together as this should help minimise panel resonances. I also with to use acoustic foam inside the enclosure as well as a lot of wadding material:

This is a thick foam, and as far as I can figure, only a thick high density foam will have much influence over the lower frequencies. Are there any potential problems with using such a thick foam though? Will it disturb the frequency response? There would be foam left over to go elsewhere in the cabinet too, not shown. I'd perhaps consider some thinner foam for the sides around the woofer?

Here are my CAD drawings:

I have also included "spokes" between the side panels, alongside the typical holed braces. These will be drilled into the 18mm ply and glued into place. I'm not certain they'd do much though, they only tie the walls together, mabye an "anchor" point is needed? At the bottom are some drilled solid wood battons for strength, I may use more of these throughout.

The front baffle will have the top half of ~30mm hardwood backed to 9mm MDF and the lower half of 9mm MDF with 18mm ply and 12mm MDF again (or similar).

I basically want as much rigidity as I can get with these 27mm walls and without loosing too much internal volume! Any tips or tricks welcome!

PeteMcK 24th August 2009 12:47 AM

Looks good to me, overkill with dowels can't do any harm.
I'd consider adding some damping material at the bottom to reduce any standing waves...

CLS 24th August 2009 02:04 AM

The bass section plays up to 820Hz, that's actually a lot of midrange!

Strong cabinet walls etc do help a lot, but I always feel (from my own works and others I've listened to) the shape of the box plays an even bigger role -- especially in the mid to midbass range.

From the design shown above, it looks quite some effort to build (already). So, why not put some 'unparallel walls' elements into the design? I think unparallel walls help a lot on suppressing the box sound. Not less than the structure and proportions.

Also, that will blend nicer to the mid-high section by some degree of coherence of the forms. :)

ps. I don't mean mimic the shape of mid-high section. You got a lot of options in making unparallel wall: slant front or back, tapered sides (front to rear), or even tapered downward to a tip at the bottom...

Dr_EM 24th August 2009 10:54 AM

Thanks for the replies!

I intend to fill basically all of the enclosure with BAF wadding type material (mabye lamswool near the driver rear if worth the extra expense). This should actually increase the effective volume too, which is good. I can put the remaining thick foam at the bottom if that's a good location for it.

I had thought about sloping walls. I think it'd be simplest to add them inside the box form, realising this does effectively "waste" volume but it'll be easier for me to do. I could do a few different things here. I could add a gentle tapering back wall profile (sloping down to a smaller area at the bottom), which I think is the best as it effectively acts like a closed end, tapering TL, too short for bass but enough to "suck out" some midrange once heavily stuffed.

Alternatively, I can add a sort of triangle form into the bottom section, almost creating two short TLs. I'd ensure they had non-equal ratios. If I really needed the volume I could even drill holes into the bottom of this triangle insert to reclaim internal volume without much negative effect?

As another possibility, simply adding profiled blocks of wood inside to break up the parallel sides. This is easy to do, but I understand they have to be quite large to have much effect. I actually have some suitable ones which are offcuts, big trinagles of pine with wavy edges. Again, I may be able to drill out a lot of the material to reclaim volume?

CLS 25th August 2009 01:29 AM

Ah, yes.

A rectangular box is much easier to build than an irregular one. Stuffing with closed-cell type of foam materials should work very well in forming "unparallel inner walls". Many kinds of packaging materials would do. And they are very easy to be moved in and out for adjusting volume if needed.

Andersonix 25th August 2009 05:09 AM

I would leave out the 'spokes' as they look like trouble to mount, may ring like musical instrument strings, and can only act as, um, spokes, ie only really have strength in tension and not in compression.

I'm sure you've taken care to space the internal bracing to avoid creating wall section spans of equal dimensions?

I also wouldn't bother gently sloping the walls, as the needed differences in distances are too large to implement without eating up the volume. But perhaps a 'reflector' oriented at 45º right behind the driver in the upper corner could be useful?

sandyhooker 25th August 2009 05:47 AM

:)what sort of "packaging materials " are you referring to?Most common foam is unsuitable so I've heard.'

Dr_EM 25th August 2009 11:34 AM

Thanks for the input again!

This seems like a good solution, but I'm not certain if there are "side effects". Adding in a sloping board inside, but not joining to the bottom of the cabinet, hence retaining nearly the same internal volume. It seems it would form a 1-0 tapering closed end TL of quite short length, but will the box overall still act just like a sealed? Impedance variations shouldn't be a big concern since the driver is actively powered.

Also adds further bracing to the box. The remaining thick foam can be placed at the end of the line or on the side walls (which are still parallel); I still think it's wise to have most of it directly behind the driver though. The cabinet would still be heavily filled with acoustic wadding too.

Any thoughts on this?

I will be spacing the braces unequally. Is there a technique using prime numbers here too?

Dr_EM 29th August 2009 12:52 PM

Any thoughts on that design? I do have the MJK worksheets, but am not skilled enough to accurately model something like this :(

Andersonix 29th August 2009 08:56 PM

Like you said, that internal wall creates a pipe, so don't do that unless you want a TL. What I was suggesting earlier with the 'reflector' was just a smallish piece (that ends maybe as far down as the center of the driver?) in the upper rear corner (where you have the foam) to help stop sound from bouncing straight back out through the cone. This piece would also have a bracing effect.

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