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Sealed HT18 - An Engineer's Take

12601 Views 53 Replies 16 Participants Last post by  Talley, 
K
8
This will be my first home subwoofer enclosure build. I did one MDF build in college some 10 or 15 years ago, but it was very basic.

Here is some basic information regarding my build

Driver: Stereo Integrity HT-18
Enclosure Type: Sealed
Enclosure Volume: 5.0 cubic feet
Number: Two independent boxes
Usage: General use. Music, Movies, etc.

I started with the desire to replace my current subwoofer, which was a JBL 12" 350W sub. The amp died and I wanted to start a project so here I am. Once I decided on the driver, type, and volume I started designing the enclosure. I'm a mechanical engineer, so I decided to use 3D modeling and FEA tools.

The two boxes will sit next to each couch in the room. I will build these boxes to appear as end tables. The sub will be downfiring, about 5" off ground.

Overall box view. I haven't modeled in the details such as the cosmetic parts. I plan to add strips down each corner to give the appearance of having 4 table legs. Should look similar to a mission style end table.
Box by Kevin, on Flickr

This shows the bracing inside. The main cabinet will be made from baltic birtch, with a 3/4" and 1/2" sheet glued together, for a total thickness of 1.25". The baffle will be made from two 3/4" sheets for a total of 1.5" thickness. The braces will be the 3/4" and 1/2", for 1.25" thick bracing.
Box2 by Kevin, on Flickr
Box3 by Kevin, on Flickr

I laid out the flat panels on on 4x8 sheets, to see how much plywood I needed. Here is that layout.
BoxFlatLayout by Kevin, on Flickr

Then onto a modal analysis, to find the natural frequencies of the enclosure. This is showing the final design, which was somewhere around the 12th iteration. I was trying to keep overall weight low, and stiffness high. The requirement was to not have any natural frequencies below 500Hz.

This picture shows the mesh used. I modeled the plywood using baltic birtch properties, and I also modeled in a ring of aluminum where the driver flange will interface. I used a bonded contact where the box and driver flange meet.
BoxMesh by Kevin, on Flickr

The first mode. 495 Hz. This is somewhat a torsion mode of the box.
BoxMode495 by Kevin, on Flickr

Second Mode. 625 Hz. Baffle dominated mode.
BoxMode625 by Kevin, on Flickr

Third Mode. 658 Hz.
BoxMode658 by Kevin, on Flickr

At this point I have purchased all the wood to make the boxes. The main is Baltic birch plywood. On top of the box I will also be attaching a 1" thick hard maple top. Hoping to start fabrication this weekend. Later tonight I'll post my WinISD modeling as well.
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chrapladm
Looks good so far. Interested to see how your aluminum ring works out. I was looking at doing the same thing for my next subwoofer build.

And since your not using your subwoofer up past 120hz I think you will be just fine for having a box that is inert and not causing more" noise," in the subwoofers BW.:T
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K
The aluminum ring was only for the modal modeling. It is there to simulate the stiffness the speaker basket adds. It will not be there in the real thing. Now that you mention it, I need to add a mass in the model to represent the driver mass.
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K
4
WinISD Modeling.

Transfer Function
TransferFunction by Kevin, on Flickr

SPL
SPL by Kevin, on Flickr

Cone Excursion
ConeExcursion by Kevin, on Flickr

Amplifier Apparent Load
AmplifierAparentLoad by Kevin, on Flickr
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dgmartin
Good to see things are happening! That will be solid coffee table for sure. Your bracing looks pretty efficient. The driver flange gasket interface will take care to damp the remaining baffle modes.

I was a bit concerned about the down-firing configuration give the very compliant suspension but the static deflection turns out to be around 1mm, which is not too bad considering available Xmax.

Besides, have you received some input on your amps?
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K
At this point I'm considering the behinger nu6000 both in the DSP and non DSP models. Not sure if I'd be better off with the DSP amp, or to go with a miniDSP for my DSP needs.
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Mike Edwards
Kevin Doe said:
At this point I'm considering the behinger nu6000 both in the DSP and non DSP models. Not sure if I'd be better off with the DSP amp, or to go with a miniDSP for my DSP needs.
Click to expand...
Both options are great, but I'd choose the balanced minidsp if you go that route
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K
Thanks Mike.

I ended up purchasing the NU6000 w/o the DSP built in. I plan to use a MiniDSP 2x4. I have read and read, and I still don't understand which plug in would be recommended.

Currently my AVR only has one sub ouput (unbalanced). I will run that into the MiniDSP, and use two of the ouputs ( have two subs). Each output will go to a separate input channel on the amp. In the future I will be upgrading my AVR to something more modern, which may have more than one sub output. In that case, I'd like my MiniDSP plugin to be compatible with two inputs, two ouputs.

I seem to find recommendations for the 4 way advanced, the 2x4 Advanced, and maybe others. Even looking at the data sheets, I'm not sure which plugin is right for me. Can someone help me with this.
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Mike Edwards
Kevin Doe said:
Thanks Mike.

I ended up purchasing the NU6000 w/o the DSP built in. I plan to use a MiniDSP 2x4. I have read and read, and I still don't understand which plug in would be recommended.

Currently my AVR only has one sub ouput (unbalanced). I will run that into the MiniDSP, and use two of the ouputs ( have two subs). Each output will go to a separate input channel on the amp. In the future I will be upgrading my AVR to something more modern, which may have more than one sub output. In that case, I'd like my MiniDSP plugin to be compatible with two inputs, two ouputs.

I seem to find recommendations for the 4 way advanced, the 2x4 Advanced, and maybe others. Even looking at the data sheets, I'm not sure which plugin is right for me. Can someone help me with this.
Click to expand...
the 2x4 advanced is more than enough really. that's what 95% of people use for subs..unless you're going to have 4 subs in 4 radically different areas of the house, the 2 way advanced is more than enough
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K
14
Put in some work in the last week or so. I cut most of my boards to rough shape (+1/8" in each direction) on my table saw. The plan was to cut to rough shape, glue 3/4" to 1/2" baltic birch, then cut to final shape.

Most piece i could cut on my table saw, but the top and bottom pieces were too long (33") to cut on my saw. So I made up a jig such that all the pieces ended up the same. Here is the jig I made. Its made to be used with a circular saw.
Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

I glued and also used screws to apply the pressure for gluing. With the screws, I pre-drilled the top piece, and also put on countersinks. I then applied the glue with a roller to ensure even coverage to each piece. Then I screwed from the center to the outsides. I also used blocking to ensure the pieces lined up as closely as possible.

Even got my wife and older son to help!
Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Then I made some templates to cut out the bracing. I made them out of 3/16" hardboard. I rough cut the shape out with about 1/8" material on. After that I used the template and a pattern router bit to clean up to the right shape. The pattern bit uses a bearing to ride along the hardboard template.


Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Then onto adding 1/2" radius to all the edges of the bracing.
Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr


At this point all the pieces are cut out to final shape, and ready for assembly. I made a circle cutting jig for my router and practiced on some scrap pieces last night. I plan to cut out the holes in the baffle this weekend, and hopefully start assembly.

For assembly I'm planning on gluing, clamping, and using a generous amount of pocket screws.
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dgmartin
Mike Edwards said:
the 2x4 advanced is more than enough really. that's what 95% of people use for subs..unless you're going to have 4 subs in 4 radically different areas of the house, the 2 way advanced is more than enough
Click to expand...
+1. This is also what I have... You do not need and AVR with 2 sub outputs as you simply use a "Y" splitter to feed the same signal to the 2 MiniDSP inputs.
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EndersShadow
dgmartin said:
+1. This is also what I have... You do not need and AVR with 2 sub outputs as you simply use a "Y" splitter to feed the same signal to the 2 MiniDSP inputs.
Click to expand...
That depends..... if your AVR has the ability to EQ each sub independently then you lose that by using the y splitter....

Normally only a big deal if your subs are not equidistant from the sweet spot, but that does happen from time to time...

But your correct in that the plugin you need is the 2 way stereo advanced...
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dgmartin
EndersShadow said:
That depends..... if your AVR has the ability to EQ each sub independently then you lose that by using the y splitter....

Normally only a big deal if your subs are not equidistant from the sweet spot, but that does happen from time to time...

But your correct in that the plugin you need is the 2 way stereo advanced...
Click to expand...
Of course! But MiniDSP also have variable delay (although limited).

You're saying X.2 the receivers equalize the subs independently, or does it also asks for a combined measurement in order to optimize power handling etc ?
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Owen Bartley
Kevin, nice work so far, and very cool modelling technique. This should be one of the more effective enclosures in a DIY build to date with that advanced FEA method. I think I remember seeing someone use it once before, but I have no idea who.
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K
Thanks guys.

I started assembly of the cabinets today. Here is my progress so far.

Untitled by Kevin, on Flickr

Assembly using Titebond III and pocket screws.
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theJman
Chamfered edges on the braces? I love seeing someone with my compulsive tendencies build something. :T
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dgmartin
theJman said:
Chamfered edges on the braces? I love seeing someone with my compulsive tendencies build something. :T
Click to expand...
Even the bolt pattern on the braces looks well-planned!
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K
3
Yes, I'm compulsive and overly thorough. I surely can't hide that. The fillets are 1/2" radius on the braces.

Untitled by Kevin, on Flickr


In the next two pics, the baffle isn't attached. Its just sitting there to check the fit. Same goes for the feet, those are simply just sitting up there.

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr
This one I was just trying to get an idea of overall size and look. A lot of work left to do, but making some good progress. Once I get the baffle attached, I'll probably switch over to the other box and assemble that one. Then start the trim work, and finish work. I also need to figure out what size bolts I can use with the HT18, and get my hands on some threaded inserts.
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K
3
1" hardwood dowel. 2" into leg, 1" into baffle. Plus glue. Legs are 5" tall.

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr

Untitled by Kevin, on Flickr
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theJman
OK, now I'm beginning to wonder if you need a 10 step program. :D

The bright side is that thing could probably withstand an earthquake.
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