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[Antone Atmarama Bajor]

     I was wondering if anyone here has any information on port choking (using some breathable membrane like Sound Wool? to reduce vent velocity).

    I was just wondering if this makes the port appear smaller and consequently lowering the tuning or if it simply lowers the Q of the port resonance?

    I was playing with some Shop vac Hose as port material to see how well it would work as a flexible port.  At High output the ports became really noisy around tuning.  I zip tied some sound wool over the tubes and it seemed to reduce the noise significantly and didn't seem to effect the low cut to drastically.  Maybe by 1 or 2dB.

    I have another cab I built that has a really nasty upper port resonance (asymptotic spike/null in the response around 200Hz) below the Xover point.  I was wondering if Choking the port may tame this behavior.  

    This behavior was predicted in the modeling software.  I was hoping that it wouldn't be as pronounced as it is.  Now that I've built the box I was hoping that I could find some way of taming this behaviour.

    Any thoughts or tips are always appreciated.

Antone-

[sheldon harris]

hello antone,
cant you just simply increase the area and length of the port to match the tuning frequency of the same port you have?

the greater port area would decrease the velocity.

[Antone Atmarama Bajor]

     Vent Velocity is not the issue with the cabinet I am currently thinking about choking.

    It is upper port resonance issues.  Sort of Asymptotic peaks and nulls in response (Tangent like).

    Bass Box Predicts this.



    Of course for some reason with this cab everything seems to be shifted lower by 2/3 of what it should be.  So instead of that first port resonance being at 300Hz its closer to 200Hz.

    I was wondering if Choking the port may reduce the severity of these resonances.

   By making the Port larger and longer I would only increase the severity of these resonances.  So that wouldn't be to helpful.  Plus the wood is already cut I'm not going to be able do anything with the port.

Antone-

[Antone Atmarama Bajor]

Here is some of what I'm getting:

   Keep in mind some of this could be room related, and I haven't actually put any damping in the cab yet.




    I could get off my ass and actually experiment with some material to choke the port but I would like to see if there is more information on the subject available.

Antone-

[Dave Rickard]

Antone Atmarama Bajor wrote on Wed, 13 December 2006 18:24

I would like to see if there is more information on the subject available.

Strat by googling "aperiodic" vent, port, and any similar term you find in those articles.  This is usually more of an audiophile technique.

[Mike {AB} Butler]

Oh, joy. Welcome to the caveats of the port loaded design..
In stuffing a port, you will definitely lose efficiency of output.. as well as change the response slope.. been there, done that. I found it's the least best way to solve the problem, and also, the force of the speaker tends to shread or alter the stuffing used (at least the materials I tried).
I have tried to deal with the Harmonics generated by the port in general, and have found several things that help a bit.
First, use the largest port diameter possible. Next, flairing both ends of the port seems to help as well, and last, use a shape other than cylindrical where possible.
No wonder the Shop Vac hose didn't work. Those ridges in the pipe really create a mess.. they "warble" at the right frequencies!
Best bet: Either seal the box.. and use some electronic boost.. or (hate to say it) scrap the design.. and start afresh. I've had to scrap many a design that SAID it would work in CAD.. but failed to deliver in real life.
Hmm.. last thought. You said the Freq's where the overtones were produced were "off" from what the software predicted? Are you sure the driver, port, and cabinet volume AS PRODUCED aren't slightly off from the calculated numbers - that would account for the radical difference in frequencies noted..
Regards,

[Tony "T" Tissot]

- a peripheral post to Mike's point about shop vac hose.

We used shop vac hose, swung by actors on-stage, to make a windstorm sound effect.

It really demonstrates clearly the sound that is made from air moving over ridged hose.

[Antone Atmarama Bajor]

The Drivers I have may have been reconed with not stock cones and coils.  They came out of some old Kintek Subs from Skywalker, back from the Early Days of Thomas Holman.

    They Are RCF15----AK.  I have a pair of them I should really test the second one to see if the TS are similar.  It is supposed to be an 8ohm driver but the one I have in the cab right now seems to be 4 which would lead me to believe the guys at A Brown Soun' Reconed them.

    I have heard from a lot of people that they have a tendency to use different coil forms and various other non OEM parts.

    Last time I had them re-cone my EVM 15B's and they came back with the 15L cones (no ridges).

    So that could be the problem right there.

I should check my other driver.  I've only assemble one of the two cabs so far.

    My thoughts on the Port choking.

    Instead of Stuffing.  I was going to put a layer of Sound wool or, Berber carpet padding over the port.  Perhaps using some sort of heavy metal mesh to keep from blowing the meterial through the port.  My ports are rectangular.  Part of the cabinet structure.

    The soundwool seems to be a very strong, nylony material.

    I guess thats all I can do is experiment and see if it actually helps.

Thanks

Antone-

[Mike {AB} Butler]

Antone,
Remember that ANY alteration of the air velocity and stiffness will change tuning, efficiency, impedance by frequency, and so forth.
It's easy to check impedance. An 8 ohm driver looks anywhere from 5.5 to 6 ohms DC resistance, and a 4 ohm driver typically is 3 to 3.5 ohms DC resistance. Another easy check is a current meter across your amplifier rails.. you WILL see spikes and dips in current consumption by frequency that are easy to translate into an impedance curve.
Yeah, sometimes the recone kits aren't right when sent to the vendor.. and they don't always carefully or fully check what they got.
Regards,

[Ivan Beaver]

PDQ bach used a dryer hose as a musical instrument to great effect.  Along with such a bicycle with cards in the spokes and balloons.  Great fun.  I've seen the show several times.

[Antone Atmarama Bajor]

     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.



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

Antone-

[Too Tall (Curtis H. List)]

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?

[Antone Atmarama Bajor]

    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-

[Mark "Bass Pig" Weiss]

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]

     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-

[Too Tall (Curtis H. List)]

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.

[Antone Atmarama Bajor]

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-

[Too Tall (Curtis H. List)]

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]

[Antone Atmarama Bajor]

     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-

[Too Tall (Curtis H. List)]

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

[Antone Atmarama Bajor]

Yes I'm interested.

    I'd like to take a look.

Thanks

Antone-

[Mark "Bass Pig" Weiss]

Antone Atmarama Bajor wrote on Tue, 02 January 2007 13:42

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-

That is true at 1 oct above the vent tuning. But since I don't use these much above 25Hz, it's moot. A 3dB roll off in amplifier power up there is all it takes to bring the excursion back under control.
My concept is to extend the Low-Q Hemholtz Resonator's (a slightly trapezoidal-shaped path) bandwidth so that it is effective over a wider range of frequencies, not just +/- 1Hz. However, the real gain should come from the massive vent area facing the room--so room coupling of the vent output should contribute the lion's share of SPL below 20Hz.