Contemplating a headcase build

[lucas303]

Having built a Fearful 15 Sub, I'm thinking about building a headcase to go with it. What I have in mind is the 18 Sound 6NM410 without a tweeter. Questions...

1. My 15 Sub has no crossover or HPF built in. I'm thinking I could mount the crossover in the headcase, plug the jack from the amp into the headcase, and then have the signal from the crossover that goes to the 15" driver come out the back of the headcase. Is that feasible?

2. In all likelihood I'd get the crossover pre-built from Speakerhardware. I'm a bit confused about which one I'd need though, because the listing for the 15/6 true mid crossover says a woofer crossover is also required. Can I not use just one crossover to "split" the signal between the 15" and the 6" drivers?

3. Does anyone know the minimum chamber size for the 18 Sound 6NM410? Also I understand the chamber has to be packed to the brim with fill, right?

4. Does the chamber for the mid driver need to be airtight?

Thanks in advance.

 

[sclift]

1. That should work.
2. You need the 15/6 CBG low-pass crossover board for the 15" driver and the 15/6 CBG midrange crossover board for the 6" driver. Two boards are required to do the split - the low-pass board allows frequencies up to about 900 Hz to go to the 15" driver and the high-pass board allows frequencies above 900 Hz to go to the 6" driver.
3. The chamber size is not critical but you might as well aim for the same size chamber as in the 15/6 plans. It should be packed with fill (dacron fiber, Ultratouch, or something like that). The fill cleans up the sound out front by absorbing sound from the rear of the cone, preventing it from reflecting from the chamber walls and passing back through the cone.
4. In a normal enclosure, yes, because you don't want pressure variations generated in the main enclosure by the 15" driver leaking into the mid chamber and pushing the 6" cone around. In a headcase, not so much.

 

[lucas303]

3. The chamber size is not critical but you might as well aim for the same size chamber as in the 15/6 plans. It should be packed with fill (dacron fiber, Ultratouch, or something like that). The fill cleans up the sound out front by absorbing sound from the rear of the cone, preventing it from reflecting from the chamber walls and passing back through the cone.

In that case maybe a larger chamber would be of benefit because it could fit more fill and move the walls further from the driver?

 

[Foz]

The 15/6 chamber dimensions are efficient - I used those and it worked a treat.

 

[lucas303]

Is there any reason, other than making the construction more complicated, not to slightly angle the baffle upwards a bit so the 6" driver isn't pointing at my knees when it's sitting on top of the 15 Sub?

 

[Foz]

Might just make it rectilinear and fiddle around with wedging with some folded paper or some such for a bit of tilt if you feel it is needed - I don't think it is necessary as that small driver is not very "beamy", but no reason not to try it. If you like it that way then add something like a pair of short rubber feet towards the front - or glue some layers of old mouse pad to the bottom to get the tilt you think you need. Or you could build it so it could be set square when you will be a few feet away or laid on its side when close and do the side aspect with a bit of tilt to the box.

OTOH something you should perhaps consider is mounting the driver low on the baffle of the bird house so it is as close to the woof as possible.

 

[808State]

In that case maybe a larger chamber would be of benefit because it could fit more fill and move the walls further from the driver?

Deeper enclosure can help. But you do not want it too large.
Whatever volume is needed for a simple Quasi Butterworth alignment should be fine.
Sealed enclosures shouldn't have leakage or yes be airtight.
And the polyfill helps with absorption in the enclosure.
1/2 fill or Full fill is pretty standard.
Large baffle will effect off axis response. Its common in HiFi for the enclosure
to be not much wider than the speaker. But very deep. pressure leaving the cone is going rearward inside the box.

You can see the effects of absorption in a model with advanced parameters
absorption is Qa
Usually a model is set at Qa of 100 which is unlined.
A lined box might be around 50
half fill around 30
and a heavy full fill have a Qa of around 7 to 15

 

[lucas303]

I ordered all the parts for this today, other than the wood. I ordered the crossovers prebuilt if for no other reason than for convenience. Once I get all the parts in hand I'll do some measurements and figure out the exact dimensions I want for the cabinet. I'm definitely going to make it as wide as the 15 Sub....depth and height are the things I'll want to double check with the parts in hand. Once I have the measurements I'll try to get the wood locally. I don't think I'll end up angling the baffle. Instead I think I'll just put something under the front feet if I feel like experimenting with angling the driver upwards. I will likely make the chamber for the driver the same size as it is in the 15/6 plans.

 

[lucas303]

One thing I didn't order yet is the fill for the driver chamber. What are some good options for the fill material?

 

[808State]

One thing I didn't order yet is the fill for the driver chamber. What are some good options for the fill material?

Polyester Fiber

I think a well known brand name is Poly-Fil

Poly-Fil is safe and not itchy like old school use of pink fiberglass insulation.

Fiberglass is for insulation and air or sound pressure does not pass through it as easily.
plus its itchy and horrible to work with.
Many like using foam or foam mattress toppers.
It will lighten the " box" sound. but again air pressure does not easily pass through it.
The tiny hair like fibers in polyester fiber are ideal for sound absorption

Polyester Fiber allows the air pressure to pass through it easier.
Aside from the audio brand names it is basically stuffing used for stuffed animals or pillows.

Which is a loose fill material, more common for sealed type box or midrange.

Ported application a loose fill would blow out the ports.
So you still use polyester fiber, but its formed into a sheet.

And you just line the cabinet.
Often called Batting available in 2 thickness values usually.
Usually used for making comforter blankets, or light padding for furniture.

So usually a fabric store or super walmarts with fabric sections will have loose poly fill or poly/batting sheets/rolls available