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Old 11th June 2005, 12:48 PM   #1
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Default Are multiple ports better ?

Hi,

Are multiple ports sonically superior to a single port for a subwoofer ?

Thanks in advance,
Goldy
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Old 11th June 2005, 05:03 PM   #2
Goodguy is offline Goodguy  Finland
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Hi,

I would imagine one round port being the best alternative, it gives you the largest area/circumference. Port walls always add friction to the flow, and that can't be good?

Of course if you choose multiple ports you can place their ends evenly on the box. On a large box, location of port ends might have significant effect when placed in a room.
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Old 11th June 2005, 05:11 PM   #3
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You might want to try searching. There have been many discusions on this matter.

Cal
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Old 12th June 2005, 03:37 AM   #4
KBK is offline KBK  Canada
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my limited expereince in ported boxes tells me that a single well placed port is best, and I tend to cut the ends of my ports with angle cuts, or plate them out for better loading of the port as a whole. Ie, if the port is 3" in diameter, then cut a square plate out of MDF or the like, that is 4-5" across on it's sides, and place that flush with the end of the port, inside the box. Equalizes the air mass loading when the air is in motion, or during transition to motion.

The plate on the end of the port on the inside kills diode like chuffing/farting of the port. It tends to increase fidelity in ported systems by a very noticable amount. Sometimes this effect can be so severe it blows a woofer via this diode like issue of the port air mass. The plate equalizes the air mass loading on either side of the port : CRITICAL.

Just my bit of expereince designing sub boxes for cars when I was doing such things for a local store. back when I thought ported boxes were ok.
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Old 12th June 2005, 08:50 AM   #5
Collo is offline Collo  Australia
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One port is easier but you get more flexibility with your sizes if you use two or more.

I use PVC pipe for ports and the common sizes in Australia are 50mm - way too small, 86mm (internal diameter), 102mm and 152mm.

To keep the airspeed down, the 100mm is too small and the 152mm is too large - meaning a longer port and problems with "pipe mode" resonances.

I have generally found that a pair of 86mm ports are just right!

regards

Collo
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Old 15th June 2005, 12:30 PM   #6
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Goldy, the short answer is NO there is no real sonic difference if the multiple ports are of a reasonable size.

The critical thing to watch is vent velocity, it should not exceed 34m/s where the driver is moving at the max required excursion.

However, all vents should be flared as this has a MASSIVE impact on turbulence, which is the one thing you want to avoid.

Quote:
The plate on the end of the port on the inside kills diode like chuffing/farting of the port. It tends to increase fidelity in ported systems by a very noticable amount. Sometimes this effect can be so severe it blows a woofer via this diode like issue of the port air mass. The plate equalizes the air mass loading on either side of the port : CRITICAL.
KBK,
I have no idea what you are talking about regarding a diode!

As Collo indicated, multiple ports are more a matter of pragmatics - different ways to get a decent vent area. You may find if a 100mm vent is not big enough and a 150mm vent is too big that you can find a pair of 90mm vents are just right.

*off topic* Collo, I think you should call your sub something along the lines of "lost in space" it reminds me of that robot, the vents like its arms, remember that old show with the robot which always rolls around saying "warning! warning!"
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Old 15th June 2005, 12:52 PM   #7
DIAR is offline DIAR  Finland
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How about several ports with different CSA and same lenght same lenght for easier calculation).
For Example one large and two smaller tubes. Plugging a smaller port changes the tuning frequency about 2-4 Hz. If you use two or three ports, plugging one of them easily changes the tuning frequency too much.

Any comments on this idea?
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Old 15th June 2005, 02:11 PM   #8
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Did I hear the words "different CSA" and "easier calculation" in the same sentence?

Your idea begs the question "why would you want to change the tuning of a completed vented box?" If it's a prototype, that's a different story.

Diar, nice site, very stylish
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Old 15th June 2005, 02:26 PM   #9
DIAR is offline DIAR  Finland
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Calculation is not difficult (i guess...). instead of using 5 small tubes you could use 1 large and 2 small tubes. Isn't that the same thing?
All tubes should be equally long (for easier calculation) so that only total CSA changes when you plug the tubes. Calculation is easy with WinISD, just replace one large tube with eg three smaller tubes. That should give reasonably accurate results.

I would like my speakers or subwoofers as flexible as possible. Many people plug their speakers to make them act like sealed boxes. Why not make the speakers more adjustable.

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Diar, nice site, very stylish
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Old 15th June 2005, 02:30 PM   #10
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ok now I get what you are saying. They all have the same total CSA as a single vent with a given length, which would be the basis for calculation, but you just happen to make up the area with multiple different diameter vents. Not a bad idea if you want to change the tuning just a little.
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