Vent dimensions when 3 of 4 sides are the enclosure's wall - diyAudio
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Old 19th June 2012, 07:51 PM   #1
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Default Vent dimensions when 3 of 4 sides are the enclosure's wall

I have read in past somewhere that if an enclosure's vent is part of the walls of the enclosure then the length should be counted as having the extra half of the vent's height.For example if a vent has dimensions 40(L)x10(W)x20(D) then the total depth should count as D+1/2(W) or 20+1/2(10)=20+5=25.In simple words it it should be counted as longer than it measures.

Do you agree with this?It sounds like a rule of thumb but seems to have some validity since the 3 walls are extended into the enclosure and only one side,the single wood part is of specific length.
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Old 19th June 2012, 08:02 PM   #2
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It makes sense, though I don't know if the exact correction is right.
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Old 19th June 2012, 08:28 PM   #3
OscarS is offline OscarS  United States
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with a 1:4 slot ratio, I have typically seen about a ~40% reduction in required port length for a given cross sectional area. Conversely, only ~60% of what theory predicts for length needs to be used in such a situation.

I have here a 1.75 ft³ box (after all displacements) that with a 3"x12" slot port (using 3 enclosure walls) that is 32in long. See if you can find out what frequency it should be tuned to, and then I'll surprise you with what it's actually tuned to.
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Old 19th June 2012, 08:37 PM   #4
bjorno is offline bjorno  Sweden
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FYI:

b
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Old 19th June 2012, 08:54 PM   #5
OscarS is offline OscarS  United States
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Quote:
Originally Posted by bjorno View Post
FYI:

b
Even that K=2.227 is not that accurate
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Old 20th June 2012, 03:54 PM   #6
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Quote:
Originally Posted by OscarS View Post
with a 1:4 slot ratio, I have typically seen about a ~40% reduction in required port length for a given cross sectional area. Conversely, only ~60% of what theory predicts for length needs to be used in such a situation.

I have here a 1.75 ft³ box (after all displacements) that with a 3"x12" slot port (using 3 enclosure walls) that is 32in long. See if you can find out what frequency it should be tuned to, and then I'll surprise you with what it's actually tuned to.
Hi OscarS how do you measure correctly the tuning frequency of the enclosure?I can measure the tuning watching the excursion of the woofer or touching the woofer and feeling the excursion and when the excursion is at the minimum point i consider this as the tuning.But this is not possible with tunings over 40 or 45 Hz where the movement is very fast and the excursion much lower.I have found this way very accurate thought for tunings under 40 Hz.I am very close to what i have simulated before.

Also how true is this 60% number?Have you measured lots of similar enclosures?Have you also measured normal vent enclosures?What were the results for them?

Can you explain the slot 4:1 ratio?
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Old 20th June 2012, 03:55 PM   #7
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Quote:
Originally Posted by bjorno View Post
FYI:

b
Can you explain how to use this numbers for vent size?
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Old 20th June 2012, 04:08 PM   #8
OscarS is offline OscarS  United States
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This is how I measure tuning:

Dayton Audio DATS Dayton Audio Test System 390-806

As to the 60%, I'm basing it on emperical evidence on enclosures I have built, including the one I mentioned.

by 4:1, I'm talking about the opening of the port. AKA height x width. I mentioned it because it is typically the way I set up my slot ports. I suspect that the reduction in port length will be different for different ratios, aka: "skinnier" ports or more "squareish" ports.

The "K" is called the end correction. It has a certain place in the equation for port length. It modifies the final length calculation to account for different scenarios. You can easily google this equation.

Last edited by OscarS; 20th June 2012 at 04:11 PM.
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Old 20th June 2012, 05:25 PM   #9
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Now i see the ratio meaning

My vent is 37,6cm x 9,2cm so the 1:4 is very close!

I have the WT3 software and the hardware too of course.Can i use this to measure the Fb?

This DATS tool seems very similar to my WT3.Maybe it is a replacement for it?Is the WT3 still sold in Parts Express?

Thanks!

Last edited by luv2bass2; 20th June 2012 at 05:28 PM.
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Old 20th June 2012, 05:55 PM   #10
OscarS is offline OscarS  United States
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Yup, the DATS is the WT3 with slightly different hardware and much improved software. Essentially the same thing for the purposes of this discussion.

If you use a software like WinISD to compute your tuning frequency, you'll see that there will be a huge discrepancy between what it says is required for port length and what you actually have for your particular Fb. Just run an impedance sweep and look at the local minimum in between the two impedance peaks.
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