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About winISD end correction |
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ssbb1fire 
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 Topic: About winISD end correctionPosted: 23 April 2021 at 9:29pm |
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When designing an enclosure using a vent, changing the parameter "end correction" (or using a number other than 0.732) doesn't seem to do anything.
Why ? It doesn't work ? That's odd... I reinstalled winISD twice to be sure Edited by ssbb1fire - 23 April 2021 at 9:29pm |
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DMorison
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Posted: 24 April 2021 at 1:25pm |
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Unfortunately that's a fault that's been present since the current version came out.
I use a manual formula to recalculate that I got from a poster over at diyaudio.com thus:  HTH, David. EDIT: THE ABOVE IMAGE CONTAINS ERRORS IN THE WAY THE FORMULA IS LAID OUT, DO NOT USE IT. THE SPREADSHEET LINKED LATER IN THIS THREAD HAS THE CORRECT FORMULA.
Edited by DMorison - 12 June 2021 at 6:01pm |
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ssbb1fire
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Posted: 24 April 2021 at 7:26pm |
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thanks a lot, that's exactly what i needed.
Lv is lenght of vent I assume ? And what is in german describes the situation above them I suppose ? I'm also assuming that Rek. (bottom right) is the recommended minimum lenght for a cylindrical vent ? I don't know what duct diameter is... Can you explain what is shown on the right of the image ? I can see there is a formula for the diameter of a circle, I can see a schematic of a rectagular vent, those parts are fine. What bother me is the tabular and the 2 formulas with the duct diameter. I have tried to apply your formula for a cylindrical vent of 6.8cm of diameter. Here's what I put in : c=340m/s D=6.8 cm Np=1 k=0.732 fb=79 hz What surprises me is that I have to know the volume of the vent beforehand. Using the data from winISD (and the lenght of the vent from winISD) I have Vb = 2,32 L. So here goes : (10*c**2)/(16*pi*D**2*(Np/(fb**2*Vb-k*D)))= (10*340**2)/(16*pi*6.8**2*(1/(79**2*2.32-0.732*6.8)))= 7198847,84 What is this ? I doens't seem like the lenght of the vent... Edited by ssbb1fire - 25 April 2021 at 12:18am |
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DMorison
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Posted: 25 April 2021 at 1:46pm |
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Hi,
Yes, the right hand side is about min vent sizes for certain conditions (slot ports in particular work much less well if they're very wide & skinny). Duct = tube = cylindrical vent for the purposes of this. Vb is the net box volume, not the vent volume - ie the main box size parameter in WinISD. I put it all in a spreadsheet a while ago in case people find it useful: Link Cheers, D.
Edited by DMorison - 25 April 2021 at 1:46pm |
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ssbb1fire
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Posted: 27 April 2021 at 11:20am |
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ok I've made a few tests wich seems to work : thanks a lot !
Do you have a formula about bent vents ? Or any advice ? I have in almost all my projets vents (even tho I recalculated their lenght using k=2.227 in your formula) that will have to be bent to fit some restrictions I applied to my projets. My latest one has a rectangular vent of 17.3 cm (width) by 2 cm (height) by 36,59 cm (lenght), and my box has inside dimensions of 24.2 cm (height) by 20.2 cm (lenght) by 17.3 cm (width). As you can see, my vent cannot fit inside straight. I'll bend it. Is there a formula to simulate new lenght ? Apparently it is shorter because of the bent that lowers the resonant frequency and also causes turbulence. How do I verifify vent noise will be ok and lenght of the vent knowing it'll be bent ? Any book/research/formulas available ? |
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DMorison
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Posted: 27 April 2021 at 1:36pm |
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I don't have any experience building bent vents, sorry.
Generally, for things like horns & transmission lines, people use a technique called the "Advanced Centreline Method" for working out actual path lengths through a bend, it generally results in the effective path length being about 0.86x the actual path length through the centreline for a 90 degree bend. If you're careful to keep the simulated Vent Airspeed good & low, I wouldn't think bending the vent would add much noise, but if you're not sure, best to build a prototype box and check. You could do the prototype with a removable back so that if you needed to adjust the vent length it would be easier to get access for that too. Cheers, David.
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ssbb1fire
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Posted: 29 April 2021 at 1:32pm |
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I have found the original thread and played a bit with your formulas.
https://www.diyaudio.com/forums/multi-way/97204-correction-formula-rectangular-port.html But the documents inside are not available. Where does this image with the formulas come from ? I'd love to have the explanation behind it... |
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DMorison
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Posted: 29 April 2021 at 2:17pm |
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One of the former users on that forum was a very experienced designer who had done work with DynAudio amongst others, he provided it. Unfortunately he passed away a year or so ago so isn't around to ask for any more detail.
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ssbb1fire
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Posted: 09 June 2021 at 11:51pm |
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Hey there, I've ran into a problem. In order to write it properly I used latex to write it. Here's the link to this pdf :
https://drive.google.com/file/d/1FMOfOcBetMTZzl0g7R3XX4HV9e4qQZpl/view?usp=sharing Can you check it out please ? Thanks ! |
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DMorison
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Posted: 12 June 2021 at 5:59pm |
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Hi, Good Catch! Took me a while to figure out what had happened, sorry about that. So, the image I included in the spreadsheet (and earlier in this thread) was an older version of what Bjorno had shared, and included errors in the way the equation was laid out. However, the actual equation I used in the spreadsheet was based on a newer version of the info he shared, which corrected the layout. I think I must have saved the older image as I perhaps thought it looked clearer, without realising the error it contained. The correct equation should be:  I’ve updated my spreadsheet accordingly. Hope that helps, Cheers, David. PS, you might want to recheck your conversion from rectangle area to equivalent circle diameter ;-) Edited by DMorison - 12 June 2021 at 6:11pm |
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ssbb1fire
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Posted: 13 June 2021 at 11:25am |
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Thanks for correcting !
I still have a problem : why does my 2.227 rectangular vent, of 20 cm by 2cm, for my 115 Hz 12.76 liters, have a negative length ? I checked by the new google sheets and it's still negative |
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DMorison
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Posted: 13 June 2021 at 1:57pm |
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Negative vent length can happen with any calculation, because the calculation doesn't know what reality you're trying to model - it's just arithmetic.
Increase the vent area, or the number of vents (eg stick a divider down the length of the vent to make 2, each ~ half the area), and you'll end up with a positive length.
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