Can you cheat slot port ratio?

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B

bulten

Member
Joined 2012
#1
Can you divide a very wide slot port into multiple ports to avoid the problems of a high ratio slot? The slot on the left has a ratio of 1:12 and each slot on the right is 1:4.
9KJK8L4.png

My gut feeling is this doesn't work, but then again why not?
 
#3
I don't have a proper source on it, I've only seen it written about around different forums. The problem as I understand it is that as the aspect ratio of the slot gets bigger and surface area of the port increases, friction against the port walls start to become an issue. The tuning of the port changes at high air velocities. I've seen different ratios recommended, < 1:12, 1:8 and even 1:4. Have you heard about this problem?
 
#4
On a more complex built, if i understood well, master Bjorno talk about it :
bjorno said:
Hi All,

FYI:

If you want to interface a Circular Stub to a Slot-Port: You should first calculate the equivalent Hydraulic Diameter for your intended Slot-Port Area and make equal to a corresponding Circular Entrance that is placed centered(middle)
to the Slot-Port if the Slot CSA Ratio is equal or less than 2/1 but if > than 2:1, consider to add Slot CSA Dividers.

Your suggested Slot has a CSA Aspect Ratio of 36.4/6= ~6.07/1 so in order to keep the Ratio below ~2/1 you need at least 3 Dividers:
Then 3 Circular Stubs should be used perpendicular to the 3 Dividing Slot Areas.
Always calculate the required Stuffing needed to control the Stub Q+anti Pressure Level and try to make the individual interfacing Stubs total Hydraulic Area as close as possible to the sum of the calculated Slot-Port Hydraulic input Areas/ # of divided Areas( Suggested here: # =3).
Tapering the Stub is also an option.
See the submitted example Picture where calculations are made to improve a Slot-Port that was too wide(=LF HOM Generator).

b 🙂

PS: I modified the HR simulation made by papasteack into a QW BR optimized for one Decade of BW; f-3 dB= ~24 Hz to f-6 dB at ~240 Hz.
Click to expand...
So yes it can be done.
 
#5
You'll have to regard two things:
- To keep the port cross sectional area costant, the cabinet needs to be wider by the sum of the dividers' thicknesses. Or you'll have to increase port height instead.
- The dividers will increase the port's superficial area, hence chuffing noise will be increased somewhat by the increased air friction.
Best regards!
 
#6
I have not heard the argument of higher order modes before, with respect to ports. I think that they are not a problem for typical ports, because ports suffer from a resonance along the port length before (= at a lower frequency) these higher order modes can exist, as ports usually are longer than wide. Resonances along the port length start at a frequency at which 1/2 wavelength equals the port length. These will be excited if the box is not filled with fibrous material. I think higher order modes are supported, but not necessarily excited, at a frequency at which 1/2 wavelength equals the port width. Not completely sure though.

In your drawing there is horizontal symmetry, meaning that the higher order mode at 1/2 wavelength cannot be excited. The first one that might be excited occurs at 1/4 wavelength.
 
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#9
TBTL said:
II think that they are not a problem for typical ports, because ports suffer from a resonance along the port length before (= at a lower frequency) these higher order modes can exist, as ports usually are longer than wide.
Click to expand...

Are we talking about the same thing here? Im talking about cross section area, port height to width ratio, not length
 
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