Slot port vs circular port equivalence - diyAudio
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Old 11th August 2008, 03:12 AM   #1
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Default Slot port vs circular port equivalence

Hi All,

I'm considering building a BR floorstander design (approx 28 litres internal volume) that has a circular port: 2" diam x 3" length.

1. Can I substitute a slot port at the base of the front baffle?
2. If yes, what adjustments need to be made?

Thanks

Doug
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Old 11th August 2008, 07:47 AM   #2
MadMutt is offline MadMutt  Australia
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1 yes
2 So long as it's same internal volume.

Putting the port right on the floor might affect the end response from it though.
(room gain/room coupling ?)
Same as putting any port in that spot though.

My thoughts.
Could be wrong.
Doubt it though
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Old 11th August 2008, 04:24 PM   #3
GM is online now GM  United States
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Right, the 'floor' of the cab will increase the vent's acoustic length with increasing width, ditto if the side walls are the vent's sides and at around a 9:1 width:height ratio it will begin becoming resistive in nature, i.e. moving towards aperiodic. Unfortunately, I don't remember the math from my duct designing days to figure the various end corrections and can't find my notes ATM, but I imagine a bit of Googling or maybe this site has it: http://www.subwoofer-builder.com/

GM
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Old 11th August 2008, 04:41 PM   #4
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I've built a couple of speakers where I use two triangular ports at the bottom corners below the woofer. If they're long enough, they can serve as bottom and side panel braces as well as ports.
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Old 11th August 2008, 07:14 PM   #5
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Thanks GM for that link!

Somehow I've never seen that site before. What you said below interests me very much about the thin slot port becoming resistive.

I am working on a small bandpass design and have wondered if using thin slot ports would be better so that the ports don't have to be so darn long.

Do you have any other references concerning slot ports?

Thanks,
David
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Old 11th August 2008, 10:38 PM   #6
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Quote:
Originally posted by GM
Right, the 'floor' of the cab will increase the vent's acoustic length with increasing width, ditto if the side walls are the vent's sides and at around a 9:1 width:height ratio it will begin becoming resistive in nature, i.e. moving towards aperiodic. Unfortunately, I don't remember the math from my duct designing days to figure the various end corrections and can't find my notes ATM, but I imagine a bit of Googling or maybe this site has it: http://www.subwoofer-builder.com/

GM
Thanks GM.
I should add that I'm also planning to have these floor standers on a plinth of about 35mm height (to add stabilty) and so that should mitigate the floor gain a little.

cheers

Doug
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Old 11th August 2008, 10:57 PM   #7
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Port volume always has to stay the same for the same tuning. This means that going from a round to slot port wont change its length if the volume is to stay the same (roughly speaking). If you are designing around slot ports that are considerably shorter, then the mouth of the slot is too small, or not equivalent to the round port you were considering.
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Old 11th August 2008, 11:07 PM   #8
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Quote:
Originally posted by pjpoes
Port volume always has to stay the same for the same tuning. This means that going from a round to slot port wont change its length if the volume is to stay the same (roughly speaking). If you are designing around slot ports that are considerably shorter, then the mouth of the slot is too small, or not equivalent to the round port you were considering.
Thanks PJPOES.
I think that GM has effectively answered my real question: ie at what point does the slotted port become too "thin".
I believe I now have enough info to make the adaption with some degree of confidence.
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Old 12th August 2008, 02:13 AM   #9
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Quote:
Originally posted by pjpoes
Port volume always has to stay the same for the same tuning. This means that going from a round to slot port wont change its length if the volume is to stay the same (roughly speaking). If you are designing around slot ports that are considerably shorter, then the mouth of the slot is too small, or not equivalent to the round port you were considering.
I would have to politely disagree with the above statement.

It is not solely the port volume which dictates the tuning frequency. Case in point: when a larger port diameter is used the length required to maintain the same tuning frequency is increased.

Specifically, the tuning frequency is a function of the ratio of area / length of the port as described in the following equation:

Click the image to open in full size.

where fH is the tuning frequency, v is the velocity of sound in air, A is the port area, L is the port length, and Vo is the enclosure volume.

Changing the values of A and L while keeping the product of A*L constant (constant volume) does not result in the same resonance frequency.

While it could be true that for equivalent port areas, the same overall length must be used, it is not necessarily so. The correction factors mentioned above are just that, correction factors which account for the different behaviour of slots vs circular ports.

What GM is implying above is that a slotted port with both sides and bottom being those of the enclosure will have an effective length longer than the divider used to create the slot itself would measure. There are empirical correction factors determined for different geometries of ports, and I was inquiring to locate any references GM may have on the subject.

What I have found so far comes from the archives of this very forum. Below is an image which contains correction factors for various port geometries:
Click the image to open in full size.
The associated equation in which to use these correction factors follows:
Lv = 10 * c^2 / (16*pi) * D^2 * Np / (fb^2*Vb)-k*D

Lv=length of vent
c = 344m/s or 13400 in/s
D=diameter of vent in cm or in
Vb=volume of enclosure in l or in^3
fb=tuning frequency
Np=no. of vents
k = correction factor

This information was gleaned from this thread.

My reason for inquiry was as stated, to find a way to cheat the system and squeeze a 6th order bandpass into a smaller than normal volume by using slot ports to effectively increase the length of my ports.

I have a suspicion that there will again be no free lunch, and that the flow resistance which causes the length to be effectively longer in a thin slot port will cause increased losses in the port. This may very well render inutile any benefit to the smaller enclosure to my design, but that remains to be tested.

Thank you all for your contributions.

Respectfully,
David Malphurs
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Old 12th August 2008, 02:48 AM   #10
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Quote:
Originally posted by gtforme00


I would have to politely disagree with the above statement.

<SNIP>

Click the image to open in full size.

<SNIP>
Click the image to open in full size.
<SNIP>

Thank you all for your contributions.

Respectfully,
David Malphurs
Would someone be kind enough to post English translations of the German captions please?

Thanks in advance

Doug
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