Which is better? Port question. (1 Viewer)

[Neil Joseph]

I am building another DIY sonosub, 4 cubic feet and I am pondering using either two 4" ports at 26" long or a single 6" port at 28" long. The 4" give a vent mach of 0.04 whereas the 6" gives a vent mach of 0.08 according to winisd. I have heard other posts where the 6" was supported over dual 4" ports because of higher port surface area (or something like that) but what about the vent mach as I have above as per winisd? Is a lower vent mach not preferrable?
orangeman
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[Jack Gilvey]

Neil,
The vent mach # from WinISD is simply wrong. One 6" has more area than two 4", and, as you stated, the dual-fours are subject to more frictional losses. If possible, go with the 6".
In fact, if I recall correctly, Pat Sun converted one of his subs from three 4" ports to one 6" and preferred the 6".
[Edited last by Jack Gilvey on September 14, 2001 at 03:42 PM]

 

[Neil Joseph]

And another question, does SVS use triple 3"orts on their SVS Ultra designs? Why not use a single bigger port then?
orangeman
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[Patrick Sun]

Yah, just do the math, the cross sectional area of two 4" wide ports is the "same" as one 5.657" wide port, so it would follow that the mach would be a shade less for a single 6" wide port.
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[Patrick Sun]

The cross sectional area of three 3" wide ports is roughly one 5.2" wide port. But SVS are using three flared ports so they are getting a little more cross sectional area but keeping the port length shorter than if they used three 3" wide straight ports with no flare. As far as why SVS does it on the Ultra's, I guess it's a question for Tom Vodhamel.
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[Neil Joseph]

Ok, the math prooves it.
What kind of material do you guys use for your 6" ports, sonotube, Plumbing, what???
Thanks,
orangeman
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[Dustin B]

I used section 40 PVC pipe. Just find a place that deals with sprinklers, irrigation, plumbing etc and they should have it.

 

[Patrick Sun]

I paid $18 for a 10 foot section of 6" wide PVC (probably over 1/4" thick). I got it at a plumbing supply company. I've found uses for the remainder of the 6" PVC (speaker stands), but still have over 4-5 feet of the stuff.

 

[Robin Smith]

When using the 6" PVC pipe for a port, it is not flared, correct? What are you using for a flange and how are you mounting it to the sub? How important is a flare? I am sort of obsessing on the need for a flare.
For example, in a Shiva-based 20" sonosub is a 6" non-flared port better than a flared 4" port of the same volume? Just thinking ahead to the day I start to build this thing.
Thanks
Robin Smith

 

[Neil Joseph]

Is a 2" clearance between a port and a driver enough? I am considering this 6" port but I have to change the length and tuning slightly to accomodate the box.
orangeman
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[Patrick Sun]

Robin, I'd prefer the 6" non-flared over the 4" flared port (if you can get away with the additional port length of the 6" port. The flared ports gain you shorter port length while providing a little more throughput in the air flow. Plus, if you want, you can roundover one of the MDF layers of the surface where the port is attached to the baffle/wall.
Neil, I'd shoot for 4" (or more) of clearance between port and driver if at all possible. Remember the port is there is act like a "slug of air" that provide resistance in the overall "circuit" of the system. If the port can function properly due to lack of air flow, it fails to function as intended.
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[Neil Joseph]

My sonotube section is 33" long so the total length with outer endcaps is 34.5" long. The driver is 7" tall so this leaves a total of 27.5" for the port plus the space between port and driver. I need a single 25.5" long 6" port with no flares in order to tune it to 25Hz with this driver but if I make a flare with the top endcap plus put a flare on the other end of the port, how much shorter can I make the port? I want to try and obtain the minimum port to driver clearance that Patrick has mentioned above.
Thanks,
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[Jack Gilvey]

Don't flares actually increase the required port length for a given tuning?
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[Patrick Sun]

Just some clarification:
If one wanted to reach a specific Fb (tuning frequency of the box), and you were to use either a 4" wide port, or a 6" wide port, the 4" wide port would always be shorter in length. That's what I was talking about (when comparing which width to use). But with a smaller width, you will increase the vent noice/mach, so it's always a trade-off of width/length/vent mach.
A flared port should be 1" longer from end-to-end versus a conventional straight port end-to-end.
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[Patrick Sun]

Neil, just calculate for a 4" wide port with Fb=25Hz, and then use a flared 4" port (add 1" to the overall end-to-end length), and that should be short enough to give you enough clearance. WinISD can be used for the port length determination (don't worry too much about the vent mach if you go with the flared ports.
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[ChrisA]

One should always strive to use one single, larger diameter port over two smaller ones. Of course that means a longer tube and bigger box to give the necessary clearance (agree with minimum 4 inches, although it might need more depending on just how big the port diameter is...). And there is only a benefit to building a larger box, other than physical size (if you consider that a negative). Technically 'too big' of a box could develop some internal standing waves, but you won't get anyehere near close to big enough where that is a problem.
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[Neil Joseph]

I cut another piece of sonotube with a slightly bigger net volume (after ports, driver, endcaps etc) of 4.37 cubic ft. This will allow me to use the bigger 6" port and tune it to 24Hz and still allow for the minimum 4" clearance. Thanks,
orangeman
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[Neil Joseph]

One more question, I have had it onmy mind for sometime but never remembered to ask. With air flow being so important when it comes to ports, I wanted to use a circular protective grille like those used on some in-wall speakers. See picture of Eric M Jones's sonosub
http://www.jones4.com/
[Edited last by Neil Joseph on September 19, 2001 at 08:36 PM]
[Edited last by Neil Joseph on September 19, 2001 at 08:37 PM]

 

[Greg Monfort]

Typical speaker grills are acoustically transparent to LF WLs.
BTW, I assume that since you're tuning it so high (for HT) that you plan to EQ out the lowest 1/2 octave.
GM
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[Patrick Sun]

At most, you'll lose maybe 1dB of output with a grill on a top-ported enclosure (i.e. nothing to worry about - SVS does it, and I don't hear anyone complaining).