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Author Topic: Port Choking  (Read 8130 times)

Antone Atmarama Bajor

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Port Choking
« on: December 11, 2006, 07:40:30 pm »

     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-
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sheldon harris

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Re: Port Choking
« Reply #1 on: December 13, 2006, 07:09:20 pm »

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.
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Antone Atmarama Bajor

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Re: Port Choking
« Reply #2 on: December 13, 2006, 08:10:57 pm »

     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.

index.php/fa/6959/0/

    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-
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Antone Atmarama Bajor

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Re: Port Choking
« Reply #3 on: December 13, 2006, 08:24:02 pm »

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.

index.php/fa/6960/0/


    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-
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Dave Rickard

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Re: Port Choking
« Reply #4 on: December 14, 2006, 11:42:11 am »

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.
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Dave
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Mike {AB} Butler

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Re: Port Choking
« Reply #5 on: December 15, 2006, 08:02:46 am »

Oh, joy. Welcome to the caveats of the port loaded design.. Confused
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,
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Mike Butler,
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Tony "T" Tissot

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Shop vac hose as sound effect: Port Choking
« Reply #6 on: December 15, 2006, 01:14:50 pm »

- 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.
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Antone Atmarama Bajor

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Re: Port Choking
« Reply #7 on: December 15, 2006, 06:17:21 pm »

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-

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Mike {AB} Butler

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Re: Port Choking
« Reply #8 on: December 15, 2006, 06:42:05 pm »

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,
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Mike Butler,
Principal, Technology and Operations,
Dascott Technologies, LLC

Ivan Beaver

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Re: Shop vac hose as sound effect: Port Choking
« Reply #9 on: December 15, 2006, 06:58:03 pm »

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.
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