More Sonosub Questions (1 Viewer)

[SteveCallas]

A while back I started a thread about a pretty large pair of sonosubs I will be building. I'm going to start by only building one. 28" diameter, 70" tall, the new 18" avalanche driver that ascendant audio will be coming out with (I'll have to redo my calculations), 450 watts, tuned to 14hz, with a very thin layer of batting inside, and then just when I thought I was sure about the ports I'm second guessing myself again.

I originally was going to go with 2, 6" ports, close to 3' in length, spaced 5.5" from the sides with 5" between each other. I was going to go this way because I thought I would have to use pvc pipe. But when I started looking around to find sonotube, I found there is a place very close to me that sells it in diameters from 6" to 56". So now I am thinking about using a single 10" port, about 44.25" in length. This dramatically reduces port velocity from what used to be about a 23m/s peak to about a 14.8m/s peak at the highest point. The material for the 10" port will also take up less internal volume than the 2, 6" ports. Are there any down sides to going with such a big port?

The end caps will be made from 3 layers of 3/4" mdf, two inside, one outside. I'm thinking of using some of the extra circles that will be cut out for the driver space on the bottom layers as more supports for the 10" port on the inside of the top cap. What I mean by that is that instead of only giving a 44.25" long port 2.25" to be caulked to the top cap, I can give it 4.5" at the expense of loosing a few cubic inches of internal volume. I'll finish that off with a 12" parts express steel mesh speaker grill. Any other potential down sides to doing this?

I've noticed that many people have problems when cutting the mdf for the end caps, and I don't have a router, so I am taking my mdf to a woodshop to have all the pieces cut for me by a cnc router. I would think that would eliminate most of the problems right there. I am going to take the leftover 4" piece of sonotube with me to make measurements and make sure the caps fit while I am there.

For the legs, I notice that most people use 3 to keep it balanced easier. I was thinking of using 4 to give this heavy sub more support. Also, most people use legs only about 6" long, I'm thinking of going 12" to give the driver a lot of breathing room. For baseplate I'm thinking of using 2 layers of 1/2" plywood that I already have. I'll be using small rubber pads on the bottom of the base plate and then use a couple layers of rigid foam insulation under that to isolate the sub from my floor. Any forseeable problems?

In terms of assembly, I will be using t nuts for the driver with a ring of weatherstrip, wood glue and screws to connect layers of the caps, silicone caulk for sealing the caps in place along with a few small nails around the sides of the sonotube, weatherstrip around the grill mounting, spray adhesive for the batting on the inside and around the port, and silicone caulk and a few small nails for attaching the port to the top cap.

Finally, with the binding posts, the bottom cap will be too thick to go through, so I'm thinking of going through the lower side of the sonotube, just above where the end cap reaches inside. I'm going to space them a couple inches apart and go around the inside and outside contact points with hot glue to ensure the sub stays air tight.

Aside from those questions, does anybody else have any suggestions or tips that might save me some headaches during the building of these subs? Any and all comments are appreciated. This will be my first time building a subwoofer.

 

[ThomasW]

At most the opening between the driver and the floor only needs to total the Vd of the driver. 4 legs are a good idea IMO.

 

[SteveCallas]

Thanks for the quick response.

"Ports don't need that much support. Ports longer than ~30" potentially have issues with port resonances. A search of this forum (or a google search) should turn up multiple threads about this"

Just to be sure, I am not using any type of flare or any mechanism with which to screw the port onto the top cap. All I will be using is caulk and a few small nails. Would you still say that 2.25" is enough to support the 44.25" length? It's not much work at all to add the extra support, I'd rather be safe than sorry. Unibox says the port resonance is at 159hz - I'll be crossing over at 80hz and LFE has an upper limit of 120hz - do you think that is enough of a gap?

Would you agree that a single 10" port is better than 2, 6" in this case? Is it better to have a port resonance higher than 159hz for a sub or to keep port velocity under 20 m/s?

 

[Brian Fellmeth]

I would not do this. With your very thick caps you would gain nothing over using beefy wood screws or deck screws into pilot holes. The benefit of t-nuts is if the driver will be removed and replaced many times- doupt you will need or want to do this. They are a huge pain in the ass to align and don't like MDF much. If they strip, you have a mess.

 

[SteveCallas]

Hmmm, I thought the purpose of using them was to get a tighter, more solid fit because MDF does not like to hold screws very well. I have worked with MDF for a few other DIY projects and can verify that unless you're using wide drywall threads, it doesn't want to hold. Also, the screws won't get the full 2.25" because I am mounting the driver flush by using the third layer as a countersink area, thus giving the screws only 1.5".


"They are a huge pain in the ass to align and don't like MDF much. If they strip, you have a mess"


I was planning on aligning them by resting the driver in its countersunk area, marking the holes, drilling very thin pilot holes all the way through, then drilling holes large enough for the T nuts on the reverse side. This way the screws will get a very tight fit against about 3/4" MDF and the rest will go into the T nut. When turned over again, it will look like this - disregard the periods.
......._________
____|_.........._|______
.........|........|
.........|........|

 

[ThomasW]

T-nuts aren't worth warm spit in MDF. If you want to use T-nuts put a layer of ply or OSB as the inner most layer of the end cap.

Port resonances don't only occur at the actual frequency, they can occur at 1/2 or 1/4th the wavelengths as well. A good rule of thumb to avoid resonances is keeping the port 30" or less

 

[SteveCallas]

Ok, tested a few more variants, a single 8" port would be 28.5" and cause air velocity to go over 20m/s below 14.5hz, which I don't expect to happen too often. I guess this would be my best option, yes?

I've really gotta say I am suprised at the disregard for T Nuts. Almost every sonosub construction page I have looked at reccommends using them and the notion makes sense. It allows you to spread out the force over a wider area and gives you a much tighter and solid fit for the screws.

I'll change the port to 8" no problem, but I'd like to hear more opinions about T nuts. Anybody who used them and is still willing to reccommend them here?

Might hurricane nuts be better?

http://www.partsexpress.com/pe/showd...umber=081-1084

 

[ThomasW]

There's nothing wrong with T-nuts, the problem is that T-nuts don't hold in MDF. MDF has no grain = no strength and nothing for the little teeth to hold on to.

I use T-nuts in virtually every speaker I build. They just don't hold in MDF. So if possible I line inside with a layer of ply or OSB. If that's not possible I glue small pieces of oak for each T-nut. Hurricane or threaded inserts work well.

The 8" port sounds fine.

 

[SteveCallas]

Ok sounds good - I will add a 1/2" layer of plywood for the T Nuts to dig in to.

Any other possible concerns or tips?

 

[chuckg]

Howdy!

I've seen very nearly every piece of hardware in the world, but I have not heard of "hurricane nuts" before seeing it on this board. I assume they are not some snack food for those weathering the storm, but just what the heck are they?


Thanks!

 

[SteveCallas]

Click on the parts express link a couple posts up.

 

[Brian Fellmeth]

The suggestion of making the inner layer plywood instead of MDF is a great one whether you use t- nuts or screws. My problem with the t nuts is that the large holes from the inside must be PERFECTLY aligned or the screws from the driver side won't find the threaded chamber. This means that your thin pilot holes must be made with a drill press or somehow perfect vertical. The thin holes must start dead center in the driver frame mount holes, and the large holes must be dead center on the thin pilot holes. None of these conditions need to be met to simply use screws into the caps.

I disagree with the notion that MDF always holds screws poorly. If the screws are good sized with coarse threads, and the pilot hole is optimal diameter, and you have 1 1/2 inch to work with, screws will hold very tight and solid- so long as the driver isn't removed and rescrewed multiple times. I have even mounted 5 inch drivers into mdf baffles with only small machine screws- very easy to overtighted and strip, but with the perfect pilot hole even these thin fine threads hold such that you couldn't easily pry the screw out with a chisel.

 

[SteveCallas]

Well I guess the T Nuts won't be worth it then. I'll just use extra plywood to make the thickness 2.5" and use long, coarse thread drywall screws.

I appreciate all the help, anything else?

 

[Patrick Sun]

If you have space on the bottom endcap, I'd still place the terminal cup there, just cut out the main rectangular hole for the terminal cup for the outer bottom endcap, and then just smaller holes through the interior endcaps to feed the terminal connectors. I did this on all of my Sunosub projects. Terminal cups are usually 3/4" in depth, so it works out fine cutting the main hole in the outer layer of MDF.

I used a drill press adapter for my power drill to get the holes to line up for the holes for the t-nuts. I also used a 1/4" thick plywood laminated to the inner MDF layer, and I applied some glue to the t-nuts where it mated with the plywood. To make sure the holes lines up with the driver, I did a test-fit for the t-nuts and screws and the driver before hammering and glueing the t-nuts into the plywood.

 

[SteveCallas]

I won't be using a terminal box, I plan on using these.

http://www.partsexpress.com/pe/showd...number=091-340

1.5" length would mean that if I wanted to mount them on the bottom, I would have to drill two large (probably 1" diameter) holes in layer 1 and layer 3 of the bottom end cap and smaller holes in the middle layer in order for that to work. I would gain some aesthetic value and wouldn't have to cut two small holes in my sock, but I would be compromising the integrity of my bottom plate. I don't think that would be worth it.

Could you give me a link to a drill press adapter? I think I know what you're talking about, a cylinderical piece of plastic with a spring that attaches above the bit and self retracts, keeping the drill level, but I can't find one.

Another option would be to take the driver with me to the woodshop, let the guy there cut the circluar pieces for the end caps, line up the driver, mark the drill points, and let the cnc make perfect pilot holes.

 

[Patrick Sun]

This is what I used:

Plunging drill press adapter

I think I got it from Sears, but I couldn't locate it on their website anymore. I might be able to provide the part number, but that'll have to wait until after work today.

Here's a link to my other DIY projects.

 

[SteveCallas]

Oh, that's more complicated than what I was thinking of. I might just go ahead and get the mounting holes drilled in at the wood shop - I like the benefits of T Nuts but I wouldn't have any other use for such a piece of equipment.

 

[Patrick Sun]

I'm not crazy about having terminals sticking out on the side of the sonotube, waiting to be knocked around.

 

[SteveCallas]

They will stick out less than an inch and they will be on the back by the seam of the sock, so they won't really be visable.

I agree, it's not optimal, but neither is taking a 2.25" - 2.5" thick bottom plate and reducing it to 3/4" thick in a couple of spots. It would be two countersunk areas on the inside and outside and it would be very clumsy trying to fit my fingers in and thread some wire in the post on the oustide.

I will acknowledge that it will also be clumsy making a good solder on the binding posts a few inches up into the tube from the bottom with my proposed method. It seems to be the lesser of two evils though.

 

[Patrick Sun]

It wasn't that big a deal