Help with figuring port sizes.

[Hamilton]

Hey guys,

I'm half way through building my center speaker and I'm ready to cut the two 4" ports.

The cab will house two 2118s, a 2426 w/2370A horn, a 4430 crossover, plenty of bracing with 3/4" baffle. Dimensions are 11"h | 56.5"w | 16"d with 3" insulation.

Also, how many walls should receive insulation?

I'm very grateful,
Hamilton

[Zilch]

Your box is 5.75 cuft. gross. After deducting dual 2118 volume and .25 cuft for the 2370A and 2226 (a guess), your net volume is 5.41 cuft., about 5 to 10 times what those drivers want.

BassBox6 Pro suggests 0.53 cuft. net tuned to 89 Hz for "High Fidelity" (black), or 0.917 cuft. net tuned to 78 Hz for "Extended Bass" (violet). In both cases, you're displacement limited to 100 Watts at 60 Hz.

Perhaps I've got it wrong, but where'd you get the idea to build such a big box for these?

Lemme work up 4612 specs here and see what JBL did with paired 2118's....

[Later tonight, maybe....]

[Hamilton]

Hey, Zilch.

I didn't *purposely* build that size box for the speakers, it was built because the plasma is going to sit on it.

It is no problem for me to "minimize" the cabinet by bringing the back farther inside. I'm all ears, just tell me what to do.

Thanks again,
Hamilton

[johnaec]

BassBox6 Pro suggests 0.53 cuft. net tuned to 89 Hz for "High Fidelity" (black), or 0.917 cuft. net tuned to 78 Hz for "Extended Bass" (violet).

Zilch - is that recommended size for two speakers in a single cabinet, or is that the volume if using two individual cabinets?

BTW - 'stopped by your house Thursday on the way back from visiting my friend's studio, but no one home... :dont-know

John

[GordonW]

A 4612, to a close approximation, was about 1.35 cubic feet total (or .675 cubic feet per 2118J), tuned to around 68 Hz (two 2" holes bored into the front of the cabinet, for an effective vent length of .75").

I'd probably go a bit smaller volume than that, after doing the calculations again. Maybe 1.2 cubic feet (.6 cubic feet per 2118J), tuned to right about 68 Hz. That'd give an f3 of around 72 Hz, before the contribution of series inductance in the crossover was considered (which would lower the effective f3, probably, to something in the high-60s). To do this, I'd use one 3" diameter vent per woofer (two vents total, in a cabinet with two 2118Js), each vent being about 4" long. Use just enough fiberglass insulation, to cover the walls (every wall except the baffle, but only 1 or 2 inches thick, max). My calculations also indicate, in this box, that the woofer will not be significantly volume-displacement-limited, until about 60 Hz. With a standard 80 Hz "small speaker" setting on a HT receiver/processor-amplifier, it should be just fine, to play as loud as you want (within reason )...

Regards,
Gordon.

[Zilch]

Zilch - is that recommended size for two speakers in a single cabinet....

Yup, two 2118's. Sorry I missed you Thursday; at a seminar in Stockton....

I'm all ears, just tell me what to do.

Hang on a bit for other forum members to confirm my results. In the meantime, I'll take a closer look at 4612 here. I think the answer is to block the box internal size down with a second set of side walls closer to the center.

I'll run the 4430 crossover on 2370A and post the results for you here; I don't think that's gonna work well. You need a 2344A, probably, but that's too tall for your 11" box height. 2342 might work (it's smaller, and will fit), but I've never played one on a 2426 with 4430 crossover. That'll take a little more effort, but I can try that, too....

[Zilch]

Regarding the box and ports, it looks like Gordon and I are honing in on the same design.

Here's the story so far on various horns with the N3134 (4430) crossover:

1) With 2344A, the 4430 horn, controls at "0". It's about the 10th time I've independently measured and posted it on the forums; always the same, so I'm confident it's a reliable reference here.

2) With 2370A, same settings. How I'd expect your design to play on axis. 2370A narrows the vertical dispersion, so it's concentrating HF energy. JBL lit on that horn states that it requires no compensation when used with 2425/6. That's stretching it, but the compensation provided by 4430 crossover is clearly wrong for it.

http://www.jblpro.com/pages/pub/components/2370a.pdf

The vertical beamwith AND frequency response both collapse in the VHF region.

3) Adjust the N3134 crossover controls to try to flatten the response with 2370A, and you lose the HF extension because you have to raise the mids and lower the HF to make it happen.

4) Same with $38 OASR horn. (I'll find a suitable application for these ultimately. )

5) With $10 PTF95 Progressive Transition waveguide. Retains the HF extension, but hardly "flat". Would probably sound the best of the "wrong" horns illustrated here. Been a while since I played program material through this combination; I recall it being pretty good, but overly "crisp" from the 10 kHz peak there.

I'll access a 2342 for you tomorrow.

The red thingies are reflections from an LED on the front of the camera. There's a way to turn it off, but I'd have to RTFM....

[Hamilton]

Thanks for the reply, Zilch.

Hmmm....what to do. I own a horn that doesn't work well....... I decided on that horn because of the SR4725Xs, and because of space requirements.

http://www.jblproservice.com/pdf/SR-...es/SR4725X.pdf

Well, at least I can finish the cab. Each 2118 will have its own box, what would you shoot for in cubic feet, around .6?

Thanks again.

[boputnam]

Also, how many walls should receive insulation?

Hey...

This question appears to have been overlooked. SOP is to line the cabinet but not put batting on the baffle. There some discussion of this throughout the Forum - try some searches...