Hello all,
This is my first post on this forum, although I have been lurking for a while. Usually I find what I need by reading other posts, but now I have a few questions.
I've started a speaker build using GPA 416-8A and 802-8G drivers with 511B horns.
First off, the cabs are under construction but not yet glued. They will be 9.0 Ft^3 net.
Im planning for bass reflex tuned to 32Hz, with (2) 6 inch diameter ports 12 inches long for each cab. I haven't cut the port holes so this is not set in stone. Does the port tuning sound correct, or can I do better.
Also, the question of port placement. I want them located on the front baffle, and was thinking a few inches up from the bottom edge on each side. I could raise the ports higher on the baffle closer to the woofer?
A few pics for a better idea:
This is my first post on this forum, although I have been lurking for a while. Usually I find what I need by reading other posts, but now I have a few questions.
I've started a speaker build using GPA 416-8A and 802-8G drivers with 511B horns.
First off, the cabs are under construction but not yet glued. They will be 9.0 Ft^3 net.
Im planning for bass reflex tuned to 32Hz, with (2) 6 inch diameter ports 12 inches long for each cab. I haven't cut the port holes so this is not set in stone. Does the port tuning sound correct, or can I do better.
Also, the question of port placement. I want them located on the front baffle, and was thinking a few inches up from the bottom edge on each side. I could raise the ports higher on the baffle closer to the woofer?
A few pics for a better idea:
Attachments
If the enclosure's dimensions were tall enough that some quarter wave action could start to dominate tuning, driver and port locations can become critical, but in this case, it's likely mostly aesthetic. Why not at least mirror image the port locations?
Thanks for response,
Would 1 six inch diameter port be sufficient, or is two per cab overkill?
I like mirror imaged pairs, and will mirror the crossovers when I get to them.
BHamm
After looking more closely at port diameter vs driver diameter, I will use 4 inch tube flared on baffle end.
I'll just in here just to muddy the waters. I has been my experience with efficient, light cone woofers like these, that they like shallow, wide ports.
I do not know why this is, you won't find it in the T/S parameters and it won't simulate that way, but that's what works best. More modern, heavier woofers do fine with tubes. Woofers like the Altecs just sound cleaner and more even with a shallow port. IME.
I do not know why this is, you won't find it in the T/S parameters and it won't simulate that way, but that's what works best. More modern, heavier woofers do fine with tubes. Woofers like the Altecs just sound cleaner and more even with a shallow port. IME.
Just any arbitrary port dimension won't guarantee good results. Chances are it will do just the opposite. You need the T/S parameters and a design program like this one. It's free.
Loudspeaker Design Software
Loudspeaker Design Software
WinISD sims...
1) There are a few different T/S versions of the GPS 416. This model has Qtc = 0.245
2) For a 9cuft volume tuned to 32Hz, two 6" diameter ports of 12" length with one flared end models correctly, and has a 560Hz first resonance.
3) For a 9cuft volume tuned to 32Hz, two 4" diameter ports of 4.45" length with one flared end models correctly, and has a 1529Hz first resonance. This port resonance is far above the crossover, and should sound cleaner than the 560Hz resonance of the 6" port.
4) Using two ports near the bottom and near the sides is common because it evens out the internal air pressures, which prevents the suspension from any potential voice coil rocking created by only one low pressure exit. The GPS 416 has a less robust suspension/spider than a subwoofer with double spiders and thick rubber surround.
Don't forget to include good panel-panel cross bracing on the large area boards.
1) There are a few different T/S versions of the GPS 416. This model has Qtc = 0.245
2) For a 9cuft volume tuned to 32Hz, two 6" diameter ports of 12" length with one flared end models correctly, and has a 560Hz first resonance.
3) For a 9cuft volume tuned to 32Hz, two 4" diameter ports of 4.45" length with one flared end models correctly, and has a 1529Hz first resonance. This port resonance is far above the crossover, and should sound cleaner than the 560Hz resonance of the 6" port.
4) Using two ports near the bottom and near the sides is common because it evens out the internal air pressures, which prevents the suspension from any potential voice coil rocking created by only one low pressure exit. The GPS 416 has a less robust suspension/spider than a subwoofer with double spiders and thick rubber surround.
Don't forget to include good panel-panel cross bracing on the large area boards.
Attachments
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Pano,
Thanks for the advice but I feel much more confident making round holes. Not sure how cleanly I can cut a slot at this point.
LineSource,
This has been very helpful. I'm going to lay the holes out symmetrically and start cutting. Braces will be next. My goal is light and stiff.
BHamm
Thanks for the advice but I feel much more confident making round holes. Not sure how cleanly I can cut a slot at this point.
LineSource,
This has been very helpful. I'm going to lay the holes out symmetrically and start cutting. Braces will be next. My goal is light and stiff.
BHamm
I just measured my model 19s (which are 9cu ft boxes) and the ports are 11 x 2.375 x the 3/4 panel thickness. That's about 26 sq in. The stock model 19 is tuned to about 32Hz. Your 6" dia hole would be about 28 sq in. A 5.375 hole would get you close enough to the Altec tuning if your panels are 3/4" thick.
Once the cabs are done you should think about lining and the crossover. Altec was quite clever with the design but it doesn't model well with modern software.
Once the cabs are done you should think about lining and the crossover. Altec was quite clever with the design but it doesn't model well with modern software.
BHamm - if you can machine the material as cleanly as the dadoes and driver cut out appear in your photos, I think you could easily do a very tidy job of slot or rectangular ports, and I'd certainly take Pano's experience under advisement.
Thanks for the compliment, and I have been thinking how I would do this. I will need the rite router bit and would need to build a template for the cutout.
So as Pano said, "Shallow wide ports."
To my eye, I would want it at least as wide as the woofer--16 inches, and only the thickness of the baffle--3/4 inch. Then I won't have to build an internal port.
I'm tuning the box to 32Hz. I need to check the dimensions for the port height.
BHamm
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Since you have built a cabinet the size of a model 19 and are using model 19 components, why are you second guessing Altec?
The obvious answer would be to make the port the same size as a model 19.
11 x 2.375 x .75 (panel thickness) or 26 square inches. With this size and shape the port doesn't chuff and your box is tuned to 32Hz as per Altec. That's your goal so why work against it?
The obvious answer would be to make the port the same size as a model 19.
11 x 2.375 x .75 (panel thickness) or 26 square inches. With this size and shape the port doesn't chuff and your box is tuned to 32Hz as per Altec. That's your goal so why work against it?
It's a process. See how I went from round ports to thinking about slots. I'm going to lay these dimensions out on the baffle and see what they look like. My goal is not to lay glue to wood until everything is worked out.
Thanks,
BHamm
Thanks,
BHamm
Looks good enough - although the original had the port to the left side of the woofer, with XO controls to the right.
Make an MDF template (see, it is good for something 😀) and rough cut with jig saw, then clean up with pattern following bit . I find that works much better with BB plywood than plunging by hand - unless using a massive router such as the Porter Cable 7519. If you want to get really fussy, the radiused inside corners can be squared up with a Douzuki draw saw .
Make an MDF template (see, it is good for something 😀) and rough cut with jig saw, then clean up with pattern following bit . I find that works much better with BB plywood than plunging by hand - unless using a massive router such as the Porter Cable 7519. If you want to get really fussy, the radiused inside corners can be squared up with a Douzuki draw saw .
Looking good! Chris does great woodwork, so I follow his advice. 🙂
True that a random port won't do the trick, as mentioned in a previous post. My advice with these drivers is to find the tuning you need with a port that is the baffle thickness, in this case 3/4". If you are doing sims, make that your depth and work from there.
But the idea to go with the original model 19 ports is a very good one. There is a lot that I DON'T like about many of the stock Altec designs. But with the 19, Altec seems to have really gotten it right.
True that a random port won't do the trick, as mentioned in a previous post. My advice with these drivers is to find the tuning you need with a port that is the baffle thickness, in this case 3/4". If you are doing sims, make that your depth and work from there.
But the idea to go with the original model 19 ports is a very good one. There is a lot that I DON'T like about many of the stock Altec designs. But with the 19, Altec seems to have really gotten it right.