OK, I need to build a sub, and I have one ugly space constraint... (1 Viewer)

[Nick Rubenstein]

Appreciate any help you guys can offer a DIY newbie. I've looked through a number of web sites and asked a couple of questions on another forum.
Basically, my problem is this:
The subwoofer cannot be more than 13" tall, including grill, spikes/feet, whatever. Furthermore, it has to generate a pretty substantial amount of bass, otherwise there's no point in adding it onto my Mirage OM-7s. Originally, I had wanted to purchase the Mirage BPS-400i, but... Well, at 22" high, it's just a bit too much.
After looking around at various sites and reading through here for a bit, I've come up with something that I would very much like to have you guys poke holes in. (I haven't understood much, but maybe enough to start roughing out a plan...)
For starters, I intend to use the Adire Tempest in an upfiring configuration. Simple seems like a good idea, particularly on a first time project, so I should like to go with a sealed box.
Internal dimensions are 10.5" by up to 4ft by up to 4ft.
Where I start running into trouble is when I try to calculate out the exact dimensions. I've got LspCAD, but unsurprisingly, I'm pretty lost in it... I've been looking at diysubwoofers.org and the math there seems to suggest a box net volume of about 143 litres. Less than one half of my maximum.
My problem is really that I simply don't understand why I would go for one volume over another. Going with my maximum dimensions and allowing for quite a bit of internal supports, I get 344 litres, minus the volume the tempest takes up...
I intend to build the box out of 3/4" MDF, using 2"x2" wood of some sort (recommendations on something strong, but not too expensive?) to build the support frame. I understand that in addition to the outer frame, I will need to put in reinforcements throughout the box to keep the MDF from resonating. My thoughts were that approximately 1ft between these supports would be good, but with the supports moved a couple inches one way or the other to to keep them from generating standing waves or something.
That's about all I know...
Probably not entirely coherently presented, but I'm doing my best.
Thanks for any help you guys can provide,
Nick

 

[John Parris]

The volume of the box and the amount of power your sub recieves via the amp will both affect the frequency response of said sub... I don't have a capable program, but I'm sure someone could project a response curve of what you have planned and you can see if you like it. Keep in mind a sealed sub won't do worlds of good for HT use (more for music) because the curve starts to bottom out way above 20Hz in a sealed box. It'll add a lot of midbass punch to your OM-7s, but you'll be hard pressed to get a lot of really low bass performance out of it. It all depends on what you'll be using it for. If it's more for HT, you might want to think about going the vented route: I've built several sealed enclosures and vented doesnt seem to be MUCH more complicated... just different.

 

[Pete Mazz]

Is this being built into a platform, or something? That's not a lot of height for a Tempest, especially trying to downfire it. Can you use 12s frontfiring?

How much power?
Budget?
Room size?
What levels are you trying to reach? And how low do you want to go?

Pete

 

[Nick Rubenstein]

The height restriction comes from my projector screen. Thanks to a slope on one side of the wall, I was unable to raise the screen as high as I wanted to.

I'd like to use the PE 250W amp.
I'd like to keep it under $400, if possible.
The room is 13'x17' plus a couple of rather large alcoves. (one alcove is 3x10, the other is 3x6).

I'd like to go as low as possible... And now I'm feeling doubts as far as a sealed design goes.

I don't really see any way to fit 12s in a front firing position... the absolute maximum gross height I can manage is 13," the exact height of the reflective material on the screen. I was planning on an upfiring config to give the driver a bit more breathing room, unless there are compelling reasons not to.

 

[ChrisAttebery]

If it were me I would use one or more Shiva's in a front firing, vented EBS configuration. Your mains probably already go down into the 30-40Hz range so I'd think that you're really wanting to add to the low end of the system. The Vented EBS designs go into the teens and have a flat response.

This is basically what I built. My box is 44", wide 20" deep, and 14" tall. I am using 2 3" flared ports. I used the volume and port dimensions off of Adire's site and combined that with my space/shape limitations.

You should stick to proven designs unless you are willing to do alot of work. I know that Adire has reference designs that you can download. The only critical dimentions are the volume, stuffing (if any) and port length/diameter. As long as you meet those parameters you can make the box any shape you want to.

Good Luck,



Chris

 

[Dustin B]

I don't see any reason why he can't do a upfiring design. The people who have done so with dual driver sonotubes have had no problems. A single Tempest will go lower than a pair of Shiva's and come close to their output ability. Plus it will be cheaper.

My thoughts to Nick over on AVS was to get up around 240L or more net volume, upfiring Tempest or Dayton DVC15" with a 6" port tuning in the 16-18hz range. Add a 250W plate amp and he is good to go and under budget.

 

[Rick Guynn]

Do you have any space that you could drop an IB into? Then you could go plenty low with good SPL and not have a space problem at all. The cost would be in the neighborhood of $450 for a dual-tempest setup.

RG

 

[Nick Rubenstein]

IB is NOT an option, sadly. Would love to do it if I could.

 

[Richard Greene]

You seem to have selected a specific subwoofer location in advance which leads to your unusual enclosure size requirements--just how sure are you that a subwoofer will sound good from your primary listening seat when it's at that location? Room acoustics have a larger effect on bass under 100Hz. than the subwoofer brand and model.
Next, if you make one dimension very small (13") the other dimensions will be quite large and you'll need a lot of bracing for a stiff enclosure. A cube shape would be stiffer with fewer braces.
Upfiring will rattle any nearby wall more than front firing will. I've tried my Tempest sub as downfiring, upfiring, and now it's laying on it's side used as a frontfiring tube sub (much less visible that way -- it's a 230 liter sealed tube enclosure). Floors tend to be more solid than walls so downfiring is usually better than upfiring (of course this depends on how close your sub is to a wall and how solid your walls are -- but at high volumes upfiring subs located near walls are often "wall rattlers" and a rattling wall sounds much worse than even 10% harmonic distortion!
If the back of your driver is very close to an enclosure wall, I'd worry about a reflection off the back wall coming back through the cone = coloration -- there's just so much fiberglass insulation you can fit in if there are only a few inches behind the driver ... and stuffing is much more effective in the middle of an enclosure (high velocity zone) than on the walls (high pressure zone) -- that's true of listening room sound absorbers too, if anyone cares.
I think your 13" height spec is a handicap that I wouldn't want.
Parts Express has a 12" driver (295-500 at partsexpress.com) with 18mm xmax for $160 that can be used in a tiny enclosure (less than 2.5 cubic feet ported enclosure. It's a well made car driver with a 25Hz. fs which is just low enough for home use (ported only) ...
well it could actually be used in a one cubic foot sealed enclosure but would need bass boost or EQ for sufficient output under 30hz. in that configuration):
http://www.partsexpress.com/pe/pshow...number=295-500

 

[Nick Rubenstein]

Richard:
Unfortunately, the location of the sub is pretty much determined by the location of everything else in the room. I can move it side to side a bit, but my screen prevents it from getting any taller. I can go 4ft wide by 4ft deep, and ultimately I can do lots of bracing.
My Mirage OM-7s go down to 30Hz, and deliver a reasonable punch well below that.
I certainly have a fair amount of total volume available... The spacing issues are more severe, of course.

 

[Dustin B]

I'll have to disagree with most of your arguments Richard.

We have no problems with these drivers when a base plate is 3-4" from the cone. 13" will provide more space than this from the back of the cone so I really don't think it will be a problem.

I fail to see how front, down or up firing will affect walls differently. Bass wavelengths are so long that if your wall is gonna rattle it won't matter which orientation you use. If the sub creates enough of a pressure variation on the wall it will rattle.

And he will have lots of room to properly brace the enclosure.

 

[Nick Rubenstein]

It seems clear to me that there is no really good way to build the subwoofer. It won't be possible to make a perfect cabinet. However, would it make sense to mount the driver on a diagonally mounted sheet of MDF a la:

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Perhaps a semi-front firing setup would be a bit better than anything else? With the front panel mounted at a 45 degree angle, I would get a panel approximately 18.4" tall.

I'd probably mount it at something closer to a 30 degree angle to give the driver a bit more clearance.

Thoughts?

 

[Nick Rubenstein]

And the only rattles are from the light fixtures, which I will be tightening up. The walls themselves are VERY solid.

 

[Dustin B]

That would work Nick. Will be a little tricky to build. But if you are confident that your construction skills can get the baffel and braces fitting this angle tightly, go for it. Would look neat too.

 

[Nick Rubenstein]

Bah, played with the math and there is really no way to do a forwardish firing driver.
Started constructing the cabinet today... I feel like I'm making a sandbox for the HT.

 

[Dave Poehlman]

I feel like I'm making a sandbox for the HT.

I would love to see this project. Please post some pics if you can.

The beauty of subwoofers are, the enclosure shape isn't really an issue, generally, since the wavelengths you're dealing with are longer than the enclosure. I enjoy seeing creative uses of space for subs.

Good luck, Nick.