Building a new cabinet for my AV12, a few questions

Discussion in 'Home Theater Projects' started by Stephen Hopkins, Nov 5, 2004.

  1. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    I'm starting the design for a new cabinet for my orignal AV12 driver. My current cabinet is 5.35 cu ft tuned to around 23hz (not optimal, left over from the shiva that it housed before). Over Christmas break I'm going to build a new cabinet. I'm planning on going smaller (3.23 cu ft is what I'm looking at now).

    It's going to be an SVS style box (down firing, port and amp on back, legs and base plate). I'm planning on the actual box having an ID of 16.5" w x 16.5" h x 20.5" l. The OD will then be 18" w x 18" h x 21" l before adding legs and base. I'm hoping I can get away w/ 2.25" legs so the overall height after base will only be 21". Is this enough space between the bottom (driver) and the base? The SVS subs don't look to have much more than 2" in-between.

    I'm looking at using a single 4" dual flared port at 17" long for a tune of around 22.8hz and an anechoic F3 of around 21.2hz. WinISD says the port match is good enough (port match of .12, .16 being the first red/bad port match number). Will I hear any port noise using a 272 watt PE plate amp? How about if i upgrade to a Rythmik 380w later?

    I'm also considering a 16.5" x 1.25" slot port at the same or similar tune. I would keep the same inner volume by making the sub larger to compensate for the volume of the ports. I've read, however, that the port width/height ratio should be no greater than 8:1. What would I risk by using a slot port w/ a 13.2:1 ratio? The slot port gives me a better vent match in WinISD, but are its estimates valid for high ratio slot ports? How about something in the 2.5" x 7.5" range (similar length, same tune, similar vent match)? Are there any overall risks or performance losses associated w/ slot ports?

    Thanks in advance [​IMG]
     
  2. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    Anyone have any advice, especially on the port noise issue for the slotted and single 4" port scenarios?

    Thanks [​IMG]
     
  3. ThomasW

    ThomasW Cinematographer

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    Long narrow slot ports can have greater 'wall resistance' (air wants to 'stick' to the sides of the port = increased resistance)
     
  4. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    So WinISD's port match numbers for long narrow slot ports aren't accurate?

    I assumed port noise/compression was a function of how much air was being pushed through the port, making it a function of cone excursion, making it also a function of power. Wouldn't port noise be less at shorter cone excursion (and therefore delivered power) and greater at longer cone excursion since it's pushing more air through the port?

    Do you think I can get away with the 16.5" x 1.5" x 27.25" port with out getting noticable port noise? How about w/ the single 4" x 17" round port?

    Thanks [​IMG]
     
  5. ThomasW

    ThomasW Cinematographer

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    I played with WinISD a few years ago, and found it to be problematic. If you need freeware, and have MS Excel download a copy of Unibox.

    Yes port compression is a function of the amount of air being moved (power and excursion). You should design a port to work at the highest excursion and high power levels, not the other way around.

    4" dia flared ports are a good idea for high excursion 12" drivers
     
  6. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    I'll probably go w/ the single 17" long 4" diameter precision flared port. Does anyone have any opinions on if 2.25" is enough space between the bottom of the box and the base plate?

    Thanks [​IMG]
     
  7. ThomasW

    ThomasW Cinematographer

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    That maybe a little tight and create a resistive load.

    SVS uses a close bottom plate to increase the output at certain frequencies. This isn't something I've experimented with.
     

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