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Thread: Calculating vents

  1. #1
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    Calculating vents

    Hi,

    for my upcoming 4430 project I'd need two ports of 104 mm internal diameter and 210 mm length. Tubes of this diameter appear to be unobtainable here in Germany. Next standard diameters would be 96 mm or 121 mm.
    How would I calculate the correct length with other diameters? Does the cylindrical volume count, i.e. longer ports of smaller diamaters and vice versa?

    Best regards!

  2. #2
    Senior Member RMC's Avatar
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    Hi Kay,

    The original vents on the 4430 are 4 1/8" X 8 1/4" according to Drew Daniels Notes on porting.

    JBL in France published in the past the chart below to help figure out port dimensions. In case you don't understand it let me know I'll explain it to you. This should be simpler than doing the math. For your dimensions in between some on the chart, well you "adjust" logically somewhere in between (near the closest).

    If that chart doesn't do the trick for you let me know then we'll go with the math...

    Generally it is preferrable to go with the next larger dimension available (instead of smaller), but this will depend on larger dia. vent's length (larger dia. vent = longer) VS acceptable space available in the box... Regards,

    Richard

    EDIT: From the top of my head (I haven't calculated) maybe 3-4 of the smaller dia. ones could get the job done, instead of two larger ones...


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  3. #3
    Administrator Robh3606's Avatar
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    Don't do a clone to match the tuning. It's a bit too high just get a double flared 4" or 3" port tube and look for about a 30hz Fb

    Rob
    "I could be arguing in my spare time"

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    Quote Originally Posted by Kay Pirinha View Post
    Hi,

    for my upcoming 4430 project I'd need two ports of 104 mm internal diameter and 210 mm length. Tubes of this diameter appear to be unobtainable here in Germany. Next standard diameters would be 96 mm or 121 mm.
    How would I calculate the correct length with other diameters? Does the cylindrical volume count, i.e. longer ports of smaller diamaters and vice versa?

    Best regards!
    I've tubes of 105mm diameter on my TAD 2402 clones, instead of 100mm accordingly to the plans. I've clone of exact dimensions for all the rest, especially the interna volume, and I used glasswool for the damping as in the originals.
    I first calculated the corrected length using formulas as well as simulating with WinISD. Logically when increasing the diameter the length has to increase when keeping the same internal volume and tuning frequency.
    As a result my tuning was a bit to low. My advise is to start from a computed new length, and to be ready by using impedance measurements to cut a bit, reaching precisely the targetted tuning frequency. My port length are finaly 5mm shorter than the original ports. May be my damping is a bit more than the original. As a conclusion if targeting precise results when cloning, fine adjustments are necessary helped by measurements (JBL 4430 is tuned to 34Hz for 140L as stated in the datasheet: http://www.jblpro.com/pub/obsolete/443035.pdf)

    Just to help: It seems that the glasswoll I used (Isover Soniroll) has absorption similar to glasswool used by JBL.
    Think also moving to active crossover.

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    Senior Member Horn Fanatic's Avatar
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    [QUOTE=Kay Pirinha;420768]Hi,

    for my upcoming 4430 project I'd need two ports of 104 mm internal diameter and 210 mm length. Tubes of this diameter appear to be unobtainable here in Germany. Next standard diameters would be 96 mm or 121 mm.
    How would I calculate the correct length with other diameters? Does the cylindrical volume count, i.e. longer ports of smaller diamaters and vice versa?

    Greetings -

    If you have the baffle area to do so, I highly recommend using larger ducts. Forget using flared ducts. They will not be necessary if you use a larger duct diameter. The objective is to keep the mach number as low as possible or negligible.

    Might you have access to a 15 centimeter or so PVC or ABS pipe in Germany? Sewer or water pipe?

    I typically like to us six inch ABS for my ducts. Sometimes a single, sometimes a pair depending on the volume, driver, and application. Large ducts allow for more articulate, solid, defined bass. Keep in mind, a larger duct diameter results in longer duct length. So your enclosure internal depth would have to be considered.

    If you do not have the baffle area, it is acceptable to vent the rear panel. Technically, it does not matter where the vent is, as long as it is not obstructed. Against a wall for example.

    Since you are in the design mode, it wouldn't hurt to try a larger vent in a prototype.


    When calculating, there is the matter of end correction. End correction is an acoustic issue which pertains to the effective length of the port, which is not the physical length. The effective length extends past both ends of the duct. Picture the air emanating from both ends of the vent as that of a mushroom. The length and expansion of the mushroom will not be the same, as one end of the duct is baffled, and the enclosure volume end is not. The length and expansion are dependent on the tuning frequency. I can only assume that canned box tuning programs have taken end correction into consideration.

    Good luck with your project. I think you may be happy with the larger vent.

    H.F.

  6. #6
    Senior Member RMC's Avatar
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    Hi Kay,

    dn92: No big deal but 4430 tuned to 34 hz in spec sheet dated 9/95, simply because I had to check Vb/Fb for an idea I got for the present post.

    In case you decide to tune your box to 30 hz as mentioned in post # 3.

    I have a long list from JBL of box volumes/box tuning/vent area/vent diameter/duct length, these are already calculated or pre-determined from 0.3 cu.ft. up to 10 cu.ft. with about every size in that range, along with the most common tuning frequencies for each box size, giving vent area and diameter plus duct length. So the math is already done for those. Sort of a reference table.

    For Vb 5 cu.ft and tuning frequency of 30 hz, the quick fix given in the table is: 19.6 sq.in., 5" dia. vent, duct length 8". I wouldn't open the Champagne bottle yet...

    The original vents I mentioned in post # 2 for a 4430 represent an area of 26.73 sq.in. combined (each 13.36). And the 5" one above gives 19.64 sq.in. as per my calculations (Pi X radius squared). So the quick fix vent in the previous paragraph, though larger in diameter is still short, area wise, compared to the 4430 Design Engineer's 26.73 sq.in. I assume the Engineer had a good reason to go with that larger area, specially at high power.

    Now, on to what vents you can find in Germany: 96 mm (3.78") and 121 mm (4.76"). One 4.76" (121mm) vent represents 17.795 sq.in., too small area, and two of them represent 35.6 sq.in., too large area for nothing in order to minimize excessive length [4 1/8" original vents already have 8 1/4" (209.55 mm) long].

    One 96 mm (3.78") vent represents 11.22 sq.in., two give 22.44 sq.in. and three brings it up to 33.66 sq.in., so still over, but just a bit better than the previous option. The excess area issue is important in view of the longer vents created by excessive area VS inside usable cabinet depth. I'll let you fiddle with vent length for now using the JBL chart I posted.

    4430 Cabinet dimensions (ext.) are indicated in spec sheet, depth is 400 mm and remove say 20 mm for back panel thickness, that leaves 380 mm, now remove another 25 mm or so for fiberglass thickness, you now have 355 mm left (14"). If you leave say 76 mm (3") for the vents to "breathe" inside the cab what you have left now is 279 mm (11") for vent length. Try the chart with the relevant info (e.g. 96 and 121 mm Dv) and see if the lengths you get are within the 11" available... Maybe it will work. Regards,

    Richard

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    Corrected, I wrote this without checking first. I added the link to the document.

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    Hi,

    due to some restrictions in cabinet height given by our living room environment I might have to provide triangular shaped vents in both bottom Corners instead of the tubes. Both with the original area of 13.4 sq in or 86.2 sq cm. The hypothenuses of those equilateral triangles would be mitred boards that are glued to the bottom and each side wall. How do I calculate or determine the correct lenght of those boards to get the enclosure properly tuned? I assume the original length of 8.25" could be too much, as the cabinet walls form the ducts' major part?

    Best regards!

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    Hi Kay,

    I have something in the making for you about this. Not exactly the number you want but still very related. See you later. Regards,

    Richard

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    Hi Kay,

    RE: How do I calculate or determine the correct lenght of those boards to get the enclosure properly tuned? I assume the original length of 8.25" could be too much, as the cabinet walls form the ducts' major part?


    Vents that include cabinet wall(s) as part of the vent construction are a bit tricky. I don't have the magic formula you're looking for but below are some of JBL's thoughts on this type of issue:

    "One or two sides of the box may be used as sides of such a port, but this will cause an alteration in the expected tuning. Common wall ducts should therefore be designed to allow for some length adjustment after the box is completed." (JBL, Vented Loudspeaker Enclosure Construction And Operation, P.3)

    "Overall, it's safest to locate the port somewhere on the baffle with the woofer(s) far enough away from side walls to avoid interaction between port and enclosure wall or the fiberglass insulation on the wall."

    "If you are using a rectangular port that has as one of its sides, an enclosure wall, you might have to use some correction." (JBL, The Most Commonly Asked Questions About Building Enclosures, P. 3 & 4)

    The interaction between vent/wall(s) makes it difficult to predict precisely the outcome. Could be that vent length is ok, too long or too short... I didn't see any indication about which way to go, + or - on Lv.

    So for box tuning you'll have to do with trial/error method to get to the right tuning frequency. I'd be tempted to start with Lv 8.25", see what Fb is, then adjust accordingly. It may also be preferable not to have fiberglass on walls in the vicinity of the port. Regards,

    Richard

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    Thanks, Richard!

    I'm afraid that means I'll go and buy an appropriate mike and install some suitable software .

    Best regards!

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    Or might it be sufficient to measure and plot impedance vs. frequency and adjust the ports until the 4430 datasheet specs at www.jblpro.com are met?

    Best regards!

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    Hi Kay,

    The vented box tuning verification method I use is much simpler than mic/software or impedance plot vs frequency. I use test records that have frequencies swept and announced as they go by. With a finger slightly touching the driver cone near the surround as LF go by, the frequency of least or no cone movement is the box tuning frequency. No special gear, no measurements, no effort... And it works pretty well.

    Below are pics of the two test records I have. A pretty old thick vinyl record the size of a 45 vinyl but it runs at 33 1/3 this one does 40-15,000 hz (back in the old days that was it), and a much more recent one on CD that covers 20-20,000 hz. There are numerous other audio tests on these records.

    I assume you may be able to find something similar to the CD one in Europe. However, nowadays some use their smart phone with an app for frequency generator instead of a record/CD. Regards,

    Richard


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    Thanks, Richard, I'll have a look!

    Best regards!

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    Tech Sheet

    I wish they would say more but often they are very useful - Tech Manuals : http://www.jblproservice.com/pdf/stu...ies/4430lr.pdf

    Places that sell carpet are also good for cardboard vent tubes. I prefer cardboard over the plastic. More natural to me..?

    DogBox

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