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Sub Construction has Begun! Pics and Questions Inside...

Discussion in 'Archived Threads 2001-2004' started by Jeff Elliott, Mar 2, 2003.

  1. Jeff Elliott

    Jeff Elliott Stunt Coordinator

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    Thanks to the wealth of knowledge contained in this forum, I have begun construction on my first sub; a ported AV-15! If you care to take a look at the progress, a few pics can be found here: link.

    Now for the questions:

    I've been using WinISD Pro for my calculations. However, I'm confused by the port information it's providing. I tell it I have a net 7 ft^3 box that I want to tune to 18hz. It tells me to use a 4.02 (ID?) port that's ~ 12" long. If I tell it to use a 5.5" port, it says the length needs to be 24" long! This seems a bit counterintuitive to me so...

    1) What are your suggestions on port size? One or two 4" ports? How about a 6" port? I've got plenty of room so go crazy!

    2) Am I reading WinISD incorrectly or is the data it's providing inaccurate?

    Thanks for any help / wisdom you can provide!

    Jeff
     
  2. Ronnie Ferrell

    Ronnie Ferrell Second Unit

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    I love the mini-van seat clamp![​IMG] I am using two 40lb. cans of cat liter and a 5-gal. can of driveway sealant right now as clamps for a custom center channel I am building! I am actually building it into my entertainment armoire.


    Ronnie
     
  3. MichaelAngelo

    MichaelAngelo Stunt Coordinator

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    yes you're reading it right. If you increase the dia of the vent, you must increase the length too, to remain at the same tuning frequency.
    hope this helps

    FWIW, I'd use a 6" vent.
     
  4. Jeff Elliott

    Jeff Elliott Stunt Coordinator

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  5. Dustin B

    Dustin B Producer

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    With an AV15 I'd want a 6" flared port, but given your box I don't see that working.

    Twin 4" flared ports will still be underported but likely better than a single straight 6" port. And no it isn't divided between the two ports, each port needs to be that long. Adding more ports is just like using a single larger port (well except for more surface area to cause frictional loses), you've increased the cross sectional area and they need to be longer. If you can use various elbows to get the required length in there it might work ok (tricky to get the length right with elbows though).

    You got any money in the budget left? I think your best option would be to get a pair of 18" passive radiators and use those instead of ports. Will add considerable cost but won't hamper the AV15 in anyway.
     
  6. Jeff Elliott

    Jeff Elliott Stunt Coordinator

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    No more money for PR unfortunately. I did change a few things that may help.

    If I go with the 5.51" vent, WinISD says my port will need to be 19.67" long. I have 23" inside the box to work with. Questions are:

    1) Is 5.51 = the ID of 6" PVC?

    2) A 6" flared port was recommended; can I manufacture one using 6" PVC surrounded by MDF and a radius roundover router bit? If not, who sells such a port?

    3) Given that I have 23" ID to work with, is there some rule about clearance between the box walls and the mouth of the port internally? That is to say, is 3.5" enough room between the top of the enclosure and the port?

    Also, should I be lining the inside of the box with batting?

    Thanks for your help!

    jeff
     
  7. Bryan Michael

    Bryan Michael Supporting Actor

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    polly fill is a must i lined my box and put some extra by the ports. also the more you stuff the box the bigger the box seems to the sub. and the bogger the box the shorter the ports.

    compare 2 4 to 1 6 is a 29 vs 26 m/s air volisity.


    i have a 740l box tuned to 17 hz with 4 flaired vents and that gives me 42 m/s


    i have had it up to 110 db at 2 meters and had no noise. exept for sweet bass.
     
  8. Allen Ross

    Allen Ross Supporting Actor

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    props on using biscuts, isn't it a lot easier?
     
  9. Chris Keen

    Chris Keen Stunt Coordinator

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    Not knowing how ports are working on the math front of things here, I'm gonna ask a few questions and take a stab in the dark.

    Aren't the ports really a function of their volume? If that's the case, then could he just do the following?

    PVC and sand are both cheap commodities. Using a hypothetical length of 31" using 6" PVC take a section that is 31" in length and fill it with sand. Keep this to the side as it will be your known value that you're trying to reach. Take another section of 6" PVC in whatever length you can extend, place on your elbow, and then another length to be in some excess of your goal. Pour the sand from the first container (straight 31" section) into the other container (your PVC with the elbow). You can now see where to makr the pvc for cutting that leg beyond the elbow.

    I know those aren't the best instructions, but they would seem to work. Now the question is - does this meet the principles for how a port works and sizing it correctly? Probably not, but I figured if it does then at least it gives a low-budget common sense way to make an equal volume for the bent port.
     
  10. Jeff Rosz

    Jeff Rosz Second Unit

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    chris,
    just using the volume to arrive at an equivalent length doesnt work. the elbow adds resistance resulting in an even greater effective length than its volume would suggest.
    look in this thread for what an expert says in post #13

    for a 6" flared port try looking at the aeroports. i know for sure ThomasW and Jack Gilvey have used them.


    ported subs should be lined and not stuffed.

    a tuning hint: where in a ported sub's bandwidth is cone excursion the least?
     
  11. Owen Bartley

    Owen Bartley Second Unit

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  12. Geoff L

    Geoff L Screenwriter

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    Own

    Idealy your ported box volume should be (correct to begin with) and then just lined. But saying stuffing a ported box is a absoulte no no, is not realy true.

    It can help to make up for some small amount of lost volume, but heavier or maxed stuffed sould be avoided certainly. A small amount should not hurt things at all as long as it dose not interfear with the port/slot fuction and driver. Air needs to move freely inside the cabinet and it is best to have your needed cabinet volume correct to begin with.

    Regards
    Geoff
     
  13. Randy Noel

    Randy Noel Stunt Coordinator

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    Why do ported subs need to be lined as opposed to stuffed? And if this is the case, why does Adire's site recommend stuffing the ported AA Tempest? Just looking for clarification, since I'll be building a modified AA Tempest and was planning on stuffing it.
     
  14. Geoff L

    Geoff L Screenwriter

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    If your building a Adire alignment (and thats what they call for), then follow it as it is written.

    Lining the walls of a cabinet is usually for dampening and backwave of the driver as I understand it.
     
  15. Owen Bartley

    Owen Bartley Second Unit

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    Thanks Geoff, I'll fix that up when I do my finishing.
     
  16. Jeff Elliott

    Jeff Elliott Stunt Coordinator

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  17. Jeff Rosz

    Jeff Rosz Second Unit

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    #2 do a search on aeroports in this forum or on google.
    #3 i dunno. sounds adequate, but im no expert.

    you could use an elbow and attach some striaght pvc after the elbow. run some sines at a volume just loud enough to see the cone moving. as you drop in frequency, the cone movement will be least at Fb. add some straight length to lower the FB, cut some off to increase it.
     
  18. Pete Mazz

    Pete Mazz Supporting Actor

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    #2) You'd have to use a monstrous radius bit....ie: No.

    #3) Rule of thumb is no closer than one port width away, especially if it's near a side wall or two.

    Pete
     
  19. ThomasW

    ThomasW Cinematographer

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    LINK to a source for Aero-Port flares

     

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