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Author Topic: Port Choking  (Read 8131 times)

Antone Atmarama Bajor

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Re: Port Choking
« Reply #10 on: December 15, 2006, 10:06:59 pm »

     I just reviewed the graphs of the impedance charts I made of the Shop vac cab.

    What appears to have happened is the tuning was shifted slightly lower.

    The first resonant peak was reduced a bit and the second lower resonant peak (The peaks bellow and above port tuning) was reduced significantly making it look almost (not quite) like a sealed box.

index.php/fa/6971/0/

there was about a 1dB loss in response. which seemed to even out around tuning.

Antone-

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Too Tall (Curtis H. List)

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Re: Port Choking
« Reply #11 on: January 01, 2007, 09:56:19 am »

Hey Antone,

If you are not sure what they reconed it to measure the T/S.
If you don't have anything that will do that use the demo of Praxis.
You will need an accurate scale and then buy some "silly Putty" at the local dollar store for mass and a 1% 10 ohm resistor.

Also a couple of 10k ohm resistors to keep the power amp from burning up your soundcard si a great idea.  If your soundcard is robust you can use it directly instead of a power amp.

Other then that did you keep the end of the port at least one diameter away from any wall?
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Too Tall
        Curtis H. List    
             Bridgeport, Mich.   
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Antone Atmarama Bajor

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Re: Port Choking
« Reply #12 on: January 01, 2007, 06:26:27 pm »

    No I did a really lazy thing with the port design.

    Its a 90degree port that climbs vertically up the back wall for a couple of inches.  So the back wall is part of the port.

    I probably should have tried at least doing a 1/2 round routing on it but I just left it rough cut.

Lazy Lazy.  Its just a proto type right now.

Antone-
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Mark "Bass Pig" Weiss

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Re: Shop vac hose as sound effect: Port Choking
« Reply #13 on: January 01, 2007, 11:06:16 pm »

I am working on a concept where the vent area is equal to the total cone area. I'm designing a cabinet with a very long vent path. The vent walls will be covered with 1/8" adhesive foam rubber, to damp HF resonances and the vent path will be extremely long, to achieve extended bass self tuning. The advantage will be that the vent will have a large surface area coupling to the room, and since vent response goes deeper, coupling the vent  in this manner will result in a better composite response. Also, vent velocity problems are eliminated. Disadvantage is that this will be a big box. About 76" tall, 25.5"W and 32" deep.

Antone Atmarama Bajor

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Re: Shop vac hose as sound effect: Port Choking
« Reply #14 on: January 02, 2007, 01:42:09 pm »

     Isn't the main problem with extended (Shelved) LF cabs, the lack of control over driver excursion.  So even though you can reproduce extra low bass you can't actually do it at very high SPL?

I have seen some ports lined with felt and other such materials.


Antone-
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Too Tall (Curtis H. List)

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Re: Port Choking
« Reply #15 on: January 02, 2007, 02:20:33 pm »

Antone Atmarama Bajor wrote on Mon, 01 January 2007 18:26

    No I did a really lazy thing with the port design.

    Its a 90degree port that climbs vertically up the back wall for a couple of inches.  So the back wall is part of the port.

    I probably should have tried at least doing a 1/2 round routing on it but I just left it rough cut.

Lazy Lazy.  Its just a proto type right now.

Antone-


if you use a wall as part of the port it "acts" like the port you made is longer. You have to play with the length and watch the impedance, but you know that already.

Also if it is used above 100Hz you MUST line half the walls with fiberglass or similar otherwise you can't separate the reflection garbage from other problems during proto work.

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Too Tall
        Curtis H. List    
             Bridgeport, Mich.   
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Antone Atmarama Bajor

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Re: Port Choking
« Reply #16 on: January 02, 2007, 09:17:00 pm »

Half the Walls, meaning the cabinet walls, or the room that measurements are made in?

    I know that without any damping I can get some nice cancellation.  But the funny business I'm getting was predicted as a port resonance.

    Damn so I effectively made my port longer.  Is there a way to calculate how much longer my port may have effectively become?

Antone-
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Too Tall (Curtis H. List)

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Re: Port Choking
« Reply #17 on: January 03, 2007, 12:00:48 pm »

Antone Atmarama Bajor wrote on Tue, 02 January 2007 21:17

Half the Walls, meaning the cabinet walls, or the room that measurements are made in?



I don’t follow what you wrote above.

Anyway the rule is you must keep the exit of the port inside the cabinet at least the diameter of the port away from any wall. So if it’s a 4” diameter piece of PVC mounted on the front baffle you must stop it 4” from the back wall and keep the exit 4” away from any side wall, top or bottom.

Actually I don’t know what happens when you push it too close to the back wall. You have to imagine that if the port stopped 1/16” before the back wall you will get some pretty severe side effects.

The typical setup is you cut a square in the baffle in one corner and you use the bottom and side wall of the box as two of the four sides of your "“square” port.

You have to imagine that where you stop with the two sides of the port you glued in the other two sides keep going. So it would be like using a standard port according to all the rules of keeping it 4"”away from everything, BUT you cut the end of the port at a 45-degree angle.

Antone Atmarama Bajor wrote on Tue, 02 January 2007 21:17


    I know that without any damping I can get some nice cancellation.  But the funny business I'm getting was predicted as a port resonance.

    Damn so I effectively made my port longer.  Is there a way to calculate how much longer my port may have effectively become?



Could be, but I don’t know. I thought I was doing good knowing it acted like it was longer! LOL


Antone-[/quote]

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Too Tall
        Curtis H. List    
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Antone Atmarama Bajor

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Re: Port Choking
« Reply #18 on: January 03, 2007, 04:32:33 pm »

     Actually just thinking about using the walls as part of the porting.

    If you can use the bottom wall as one of your port walls.  And keep the distance from the rear wall >port diameter.  Than it would be effectively the same as having the port which opens in the face across the bottom, then makes a 90 degree bend up the back wall and maintains the >than diameter distance from the top which it does.

 It should behve the same.  Except that a 90 degree bend may introduce turbulence.  But shouldn't change tunning unless the diamter of the bend is constrictive.  Which it can't be if anything the diagonal could be a greater diameter. for a very small section of the port.

    I almost put a small 45 degree glue block there and routed the bend to a half round but I was feeling lazy.  But I don't think that explains the lower tunning.

    I will have to pick up the other driver and see if it matches with this one. I do the Mass Added TS parameters method.  I use the blue tac style putty.

Antone-
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Too Tall (Curtis H. List)

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Re: Port Choking
« Reply #19 on: January 08, 2007, 08:21:40 pm »

Antone Atmarama Bajor wrote on Wed, 03 January 2007 16:32

     Actually just thinking about using the walls as part of the porting.

    If you can use the bottom wall as one of your port walls.  And keep the distance from the rear wall >port diameter.  Than it would be effectively the same as having the port which opens in the face across the bottom, then makes a 90 degree bend up the back wall and maintains the >than diameter distance from the top which it does.

 It should behve the same.  Except that a 90 degree bend may introduce turbulence.  But shouldn't change tunning unless the diamter of the bend is constrictive.  Which it can't be if anything the diagonal could be a greater diameter. for a very small section of the port.

    I almost put a small 45 degree glue block there and routed the bend to a half round but I was feeling lazy.  But I don't think that explains the lower tunning.

    I will have to pick up the other driver and see if it matches with this one. I do the Mass Added TS parameters method.  I use the blue tac style putty.

Antone-



I use Silly Putty with good results
Any bend will change things. I have not done one, but I know it from what others have said.

For the resistive design you are looking at model a sealed box, but set the model for a very leaky box.
There is a big discusion going on in the DIYspeakers listserv. I you want I can forward it to your email
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Too Tall
        Curtis H. List    
             Bridgeport, Mich.   
        I.A.T.S.E. Local # 274 (Gold Card)
        Lansing, Mich
Independent Live Sound Engineer (and I'm Tall Too!)
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