Building my in-wall subs - need help/idea/corrections

[theq]

I am building my first 7.2 home theater (20'x20'x8' room). I have all the speakers excepts the subs.

I designed the room to have all built-in speakers and when I was about to buy the inwall subs I was shocked how much would cost me ( shy of 2k with the proprietary amp)

Therefore, without any prior experience building subs/speakers, I decided to give it a shot.

Since I wired for two subs I have 2 dedicated standard 2"x4"x8' bays available. They bays are fiberglass filled.

Here's my plan, granted I have a lot of unknows but that's why I am posting here so I can get this straight...

* approach this from infinite baffle angle (IB) - since I have those two bays available and both are open
* sealed construction (in bay, no box)
* each sub made of a Peerless GBS-250F38CP01-04 10" Paper Cone Woofer 4 Ohm
   www.parts-express.com/peerless-gbs-250f38cp01-04-10-paper-cone-woofer-4-ohm--264-1492

• Sealed Volume0.3 ft.³Sealed F3 72.1 Hz
  Vented Volume0.96 ft.³Vented F3 39.5 Hz
  Nominal Diameter10"Power Handling (RMS)200 Watts
  Power Handling (max)400 WattsImpedance4 ohms
  Frequency Response50 to 2,000 HzSensitivity85.8 dB 1W/1m
  Voice Coil Diameter1.5"
  X-max 6mm

* I am thinking I could do two driver per sub group
* Behringer NU1000DSP iNuke Power Amplifier
    www.amazon.com/Behringer-NU1000DSP-iNuke-Power-Amplifier/dp/B005EHIN3A/ref=sr_1_1?s=electronics&ie=UTF8&qid=1515520706&sr=8-1&keywords=inuke+amp+dsp+1000
    Delivers 2 x 500 Watts into 2 Ohms
    2 x 300 Watts into 4 Ohms
    1000 Watts into 4 Ohms (bridge mode)


Question/concerns

mounting
- planning to mount this on a 3/4 thick 16x16 veneered mdf and screw that on the studs.
- will that be good enough for vibrations/forces generated by the driver ? I don't think drywall would be sturdy enough.
- will protect the drivers with polyfill on the back to stop any fiberglass migrations.
- would remove all but 1/2 inch of the fiberglass right behind the driver to keep backwavers from bouncing back but keep the fiberglass in the bay above and beneith the driver
- should I install spacers in the bay to contain the open space to 1 cubic feet ?
- does it matter how high or low I install this ? For one in particular, I am concerned about protecting the driver cone from kids or other unexpected accidents


estetics, cone protection
- do you know anyone that makes covers ? I assume I can find a regular sub cover and find a way to install it.

connectivity
- how do I connect these to the amp ? Receiver subs out to amp ins and back out to each subwoofer ?
- do I need any special circuitry between the Speaker and Sub ?
- DSP should be able to provide some protection if the amplification is too high, is that correct ?!
- can I have two connected to the same DSP amp ?


Someone mentioned that could use couple of drivers per group and also should consider adding Passive Radiator Speaker.
    www.parts-express.com/dayton-audio-rss265-pr-10-aluminum-cone-passive-radiator--295-500
or
    www.parts-express.com/dayton-audio-sd270-pr-10-passive-radiator--295-494 


The X-max of the GBS-250F38CP01-04 6mm makes these pretty decent for just on passive radiator speaker. A 10mm Xmax sub would require two of these.


Like below




- what does having more than one driver per group give me ?
- without a box I need to figure out how mount them like in the picture.
- the drive and speaker will be connected in parallel ? what makes the radiator speaker passive ?
- the driver and the speaker would be connected dipole, driver facing out, speaker facing in with a bit of room against the wall/concrette foundation

That was pretty much a brain dump and I hope was a decent brain dump and not just a dump :-|

Help me out or point me to the right direction.

Thank you much!
Q

[sor]

A few things aren’t clear. Are you weighing the options between building an IB system or a sealed system?

To do IB your bay would need to be open on the backside to a large space.

If you’re closing it in, it’s just a sealed box as you mention and I’m not sure how much help IB forums will have for that.

If you close in both sides of the stud bay with 3/4” MDF and add a cross member to give a specific volume, that should succeed as a basic sealed sub enclosure. I might worry about vibration going up through the studs and the rest of the wall, but I don’t know enough about that.

A passive radiator is passive because it usually has no magnet or voice coil. It is just a dummy speaker. Perhaps people use actual subwoofers in some cases and don’t connect signal, not sure. I believe it is much like a ported sub where the idea is to use the air movement within the box to create a resonance, in your image the active sub is wired and pushing the passive cone with air pressure. You’d likely need to do some math to figure out the proper resonance unless someone sells resonators ready to go with provided specs.

I think most people on this forum would probably tell you to go IB if you have the volume of air to do so. If your stud bay is enclosed on three sides and you intend on closing it up then sealed or ported are probably your only options.

[theq]

I thought that an IB is any enclosure that does not allow the backwave of the woofer to wrap around the baffle to interfere with the front wave. The insulation helps with that.
The bay is 4872 cubic inches that is almost 3 ft.³ and that is 10 times the recommended volume for Sealed Volume 0.3 ft.³ and I think that would qualify as infinite baffle - am I dead wrong ?

I will have to run simulations for the specs numbers but my understanding of the passive speaker is that would help the sub to move the air(breathe) and there is a formula for volume displaced and from the specs of the passive-radiator--295-494  results that for a xmax of 10mm I'd need 2 passive radiators however this xmax for the given sub is 6mm. I assume is half bit since is volume might not be a linear adjustment. I am talking about idea, would correct as I move along with the concept.

Thank you for replying sor !

[sor]

I think something is missing from that definition of infinite baffle, that there's enough volume for the cone to move freely without pressure. Otherwise it is not infinite, just baffle :-)

The FAQ recommends a volume between 10-20x Vas to approximate the 'infinite' in IB, which would be roughly 8-16 cubic foot for the PartsExpress sub you mentioned.

Note Vas, not displacement or recommended box volume.

It's also recommended to have a large Xmax, because without any sort of pressure the drivers are going to move a lot and there's a risk of bottoming out or overdriving if there's not a lot of stroke available. 6mm is not really much.

I do have some experience with thin subwoofers (particularly for car audio applications but they can work anywhere), I can point you to a few that have a larger Xmax, but you're still going to have the Vas problem. I also can't vouch for these being good IB drivers, but in general they have some of the key features. They're also pretty expensive and you'll likely want more than two. Probably a moot point if you don't have the volume available for IB but they would be good candidates for a sealed, thin box as well. If you do want to pursue these options I'd suggest shopping around because the retail prices can vary quite a bit.

Alpine SWR-T12. (Xmax 15mm, Vas 1.2cuft)

JBL 12TW3-D4 (Xmax 15.2mm, Vas 1.72 cuft)

Rockford T1S2-12 (Xmax 15mm, Vas 1.14 cuft)

Regarding the radiator, a port is an opening that allows the sub to breathe as well, but it is tuned to resonate and that adds some feedback/stiffness to the system. I've never built a radiator system but I'm under the impression that the behavior is similar, though not as open as a port of course, and its resonance is going to push more air, I'd think.

[theq]

My constraints are related to the space I have in the bay and that includes the depth of the bay which limits me to the type for the driver I am choosing.

That alpine looks interesting, clearly 12 might be too big and expensive. Response frequency looks great thou. 

[ThomasW]

Unless you buy drivers by the dozen you're going to be bottoming out then destroying short throw 12" units.

Typically we don't deal with 'sealed box' installations on this forum. We primarily focus on installations where the rear wave coming off the woofers is vented into an adjoining room, basement, or attic. Doing this means we typically use Very high excursion 15"-18" driver units with an Xmax of ~30 mm.

Your room is pretty big and that means things will be problematic if you want high output at very low frequencies and are limited to using modest 12" driver units

[theq]

I appreciate your thought and input in spite of the fact this is off-topic. Thank you!