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Discussion Starter #1 (Edited)
Hi everyone. First time posting here.

This is my second DIY sub but first time trying a ported design so I could use some of your expertise.

The sub is going to be 7 cubic feet tuned to around 19hz. It will have dual 4" precision ports 23" long and will use an Avalanche 15 driver. I am making 2 of these subs. Material is .75" MDF. Powered with a Samson S1000.

Here's my initial plan:



So here are my questions (maybe a bit elementary):

1. Does the port HAVE TO be supported by the brace? The precision port seems light enough that supporting it seems optional. If I do end up supporting the port with the brace, what do you guys use to adhere the port to the hole in the brace? Do you use some type of gasket?

2. How do I cut a precision port?

3. I will be lining the inner walls with .5" thick rigid fiberglass that I have laying around. Is this fine? Also, is it recommended to wrap the port? If so, what should I wrap it with?

4. I am considering several options for driver and port placement. The dual subs will live in the front corners of the room. Based off this layout, are there any pros and cons to these different driver/port configurations? I prefer the first option since I don't have to do any bends in the ports.







The subs are going behind the screen wall


Currently, they are in an IB




Any additional thoughts, comments, tips would be appreciated.
 

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1. A 23" port is long enough that it should be supported. I use hot glue to adhere the port to the brace.

2. You can cut the port with a hacksaw. Draw a line around the port making sure it is square and follow the line carefully.

3. .5" thick rigid fiberglass is fine. I personally have never wrapped a port.

4. Your first option is fine as long as the port exit isn't too close to a wall.

One question I have for you, what is the port air speed with two 4" ports in your design?
 

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Discussion Starter #4
Thanks for the feedback Mike.

Any other alternatives to hot glue for fixing the port onto the brace?

WIN ISD says 0.06Mach. I also believe that the driver manufacturer recommended a similar enclosure with 2 4-inch ports.
 

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Discussion Starter #6
No high-pass filter as of now. If there are any issues with bottoming out, it may get added into the chain after the fact. Do you think it's a necessity?
 

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You see people wrap their ports with absorbing material in sonosubs because the port is on the opposite end cap of the driver. Since you're using a box, you would want to put extra material on the wall behind the driver. I would definitely do this - there were some measurements on a different forum that show how effective this is in reducing resonances caused be reflections in the enclosure. An inch or two won't cut it - either fold it up to get it really thick or use a couple pillows.

Another option to hold the port in the brace and ensure it won't rattle is more material in the gap.
 

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Discussion Starter #9
Interesting.

My enclosure is only 17" deep. How much rigid fiberglass do you think I need to use. I also have 4" thick mineral wool that I can apply in multiple layers if I had to. The only thing that concerns me is that the rigid fiberglass is going to take up volume and alter my tuning, which would then force me to make the box larger. This would be the case, correct?

Regarding using the polyfill (pillows)? I'm guessing this would have a different affect on absorption and volume and ultimately provide a different end result than using rigid fiberglass.

Also would mounting my driver to fire the long way reduce the effect of resonances?
 

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If air can pass through the material - which it can for all of the materials you mentioned - it will actually make the driver behave as if the enclousre were larger. I'd use multiple layers of that 4" mineral wool.

Also would mounting my driver to fire the long way reduce the effect of resonances?
It would just change the frequency - the smaller the distance, the higher the frequency, the larger the distance, the lower the frequency. On the flipside, the shorter the distance, the less attenuation of the reflection. Pick your poison :D
 

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It's done to attenuate the standing wave that can form in a large sonotube. If you have a lot of material at the top of the tube, then when the sound wave works its way up, it has to go through the material, bounce off the top cap, and then work its way down through the material again. Since most of us are using 20-36" port lengths, it does the trick. You can also use pillows.

I had done this without ever having measurements to confirm the effect, but I think it was Michael Hurd who really proved it makes a difference. Some others have done it as well.
 
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