What to build with a few JBL GTO1502D? - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-20-14, 06:11 PM Thread Starter
Shackster
Steve

Join Date: Jun 2009
Location: Edinburgh
Posts: 74
Re: What to build with a few JBL GTO1502D?

Not sure I am getting this slot port math correct. Never done a slot port before, nor ever concerned myself with converting a volume into square inches.

If I do a slot port right in the middle of the front baffle, full length from top to bottom would be 28" high.

But to calculate the required width of the opening, and depth of the port for a 22hz tune, the width of that opening multiplied by the height should not exceed 35 square inches? What about port depth?

I thought it would be something like 4" wide, 8.8" deep, 28" high, or 5" wide, 7" deep, 28" high. IE per inch of height?

I am guessing 1.25 X 28 = 35 is not what you mean.

Or are you meaning the port length remains 10" long, but calculation would be total port mouth area, IE 5 x 76mm = 15.2" = slot port surface area of 181.22", or 6.5" wide, 28" high? 10" deep? Which seems pretty big?

With a slot port do I need to concern myself with port speed/velocity or anything?

Edit, I found a port calculator! So it seems the narrower the port the less depth required, a 28" high port 2" wide requires a 12.89" depth for 22Hz. Yet the same calculator gives me -0.11 if the height is 2" and width 28"? And would the roof and floor not effect actual port length?

I think the slot port would be more economical, and help brace the cab. the port material I have is difficult to cut, and I only have 4x27". 4 76mm + 8.18 give the same tune as 5 76mm 10" tubes, but I guess port speed etc is not as good?

Last edited by Parts; 01-20-14 at 06:36 PM.
Parts is offline

Old 01-21-14, 05:39 AM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: What to build with a few JBL GTO1502D?

Just to be clear, the slot port opening needs to be 35 square inches. Whether it is 2" x 17.5" or 1" x 35", the port opening, which is the width x the height, needs to equal 35 square inches.

The length of the port determines the tuning frequency.
If the cabinet has a net volume of 440 liters the the port needs to be 7.5" long for a 22 hz tuning.
The thickness of the cabinet wood is part of the port length.
Mike P. is offline
Old 01-22-14, 05:08 PM Thread Starter
Shackster
Steve

Join Date: Jun 2009
Location: Edinburgh
Posts: 74
Re: What to build with a few JBL GTO1502D?

Well this budget project is going all swings and roundabouts.

One of the problems of storing stuff in different locations within a huge warehouse. One of the mechanics built himself a new workbench with my freebie MDF. My chipboard was on a pallet on the top Dexion, so it was safe. Some of my timber framing is slightly warped, cheap stuff with knots in it.

I am going back to simple with this, since materials are now limited.
I only have hand tools, router and a circular saw, with a long metal bar for a guide, no clamps or big flat surfaces. And only me at night doing this.
It's rather heavy and cumbersome this 1" chipboard. And I am not happy with it's brittleness, so will be using timber baton along the inside of all outer edges to help keep the screws n gluing secure.

My last pair of budget subs managed so far without grills, so I am going to forget about recessing the front baffle for a grill. I am not going to bother with a removable baffle either.

I have still got some timber lengths, around 37mm by 80mm in width and thickness. I am thinking of using this for bracing, a length right across from side to side, with two attached lengths going from mid to back, and one central. and two going from floor to ceiling. Not sure if I will use the 30" or 24" as height. Not sure if it will be standing upright or laying low.

I will need to get a router bit to give me a round edge big enough to cover the 1" of chipboard. Then I will fill any little holes and rough chipboard edges before giving it a good coat of tough cab.

I plan to round off the ports too. So will need a smaller router bit for that. I may also put the ports at each corner of the front baffle, around 3" from edges. I may have to stick to only four round ports per cab.

I have wheels on order, 75mm, two fronts will lock and swivel, rear just swivel. The cab is going to be too heavy to lump around, and wheels were cheaper than handles.

Once I have the bracing in place, I guess I can recalculate the internal volume and re-evaluate the port length for 4 ports.

But at least it is started, it will be rough n ugly looker, with less than optimum construction, ports and volume, but it was only ever intended as a budget sub, built with free timber and whatever drivers I could get at a reasonable cost.

I have put the sides and front/back or top/bottom (undecided) together for now, mainly to try and give it as square a shape as possible and allow me to move it around as I can only spend an hour or so on it at nights, over the next few days.

Now, will this cab need any dampening? I have some materials I could use if needed, egg box style foam or white BAF wadding?

And once again, thank you Mike P for your time and advice!
Parts is offline
Old 01-22-14, 05:37 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: What to build with a few JBL GTO1502D?

You are welcome. Don't skimp on the bracing, that's a large cabinet.
Mike P. is offline
Old 01-26-14, 09:01 PM Thread Starter
Shackster
Steve

Join Date: Jun 2009
Location: Edinburgh
Posts: 74
Re: What to build with a few JBL GTO1502D?

I am now considering the selling of my three JBL GTO 1502D, and replacing them with four JBL GTO 1514 instead. More Xmax, similar specifications I think.

I still have a budget of £300 for four drivers if needed. Still not found a fourth GTO1502D, well I have found one second hand, but it looks like £70+ inclusive of delivery if he delivers. Seems pointless as I can get a new boxed JBL GTO 1514 15" driver for that amount.

Also found a UK dealer advertising RE Audio SE15's at £192 each. Probably too nice for the budget cinema set up.

Both cabs are now at the same stage as seen above. But with castors added to the 30"+24" side so I can keep them upright for now. Not quite as square as I would have liked, if I had the space to leave them sitting out, and more hardware, I would have done it a little different.

Next stage is bracing, then rear panel. Front baffle will be last. Will have to make a circle jig for the router too.
Parts is offline
Old 01-27-14, 01:31 AM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: What to build with a few JBL GTO1502D?

The GTO 1514 would defienrly offer more output compared the GTO1502D.
Mike P. is offline

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