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How Do I Determine My Box Tuning Frequency?

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  • How Do I Determine My Box Tuning Frequency?

    I know I knew this at one time, but I've honestly forgotten how this computes. Here's the story:

    I'm currently building a "heritage" speaker pair that are reminiscent of 1950s Altec designs. I'm at the point where I need to install my port. The enclosure has a volume of 5.5 cubic feet, which I can easily enlarge to 6.5 cubic feet by opening up the pedestal area into the main enclosure. The woofers are Altec 416Z with the following T/S parameters:

    Xmax: .15 inches
    Re: 6.4 ohms
    Vd: 19.2 cubic inches
    FS: 23.9Hz
    VAS: 22.74 cubic ft
    REF: 3.3 %
    Qts: .25
    Qms: 6.1
    Qes: .26
    ​VID: .20 liters

    So, how do I now determine what my box tuning frequency should be? Thanks in advance, folks.

    GeeDeeEmm

  • #2
    That depends on what "alignment" you desire.
    ( the LDC explains this )

    Edit: The a ( Alpha - ratio of system compliance or box volume ratio ) could vary from 3.5 - 4.13 depending on which box volume you select
    Can't point specifically - but most modeling software allow for simulation of such "What if" scenarios.
    "Not a Speaker Designer - Not even on the Internet"
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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    • #3
      The best answer is to get some familiarity with a modeling program. WinISD has a free version that will do this sort of thing.

      However, I did just plug that into Bass Box Pro 6, and got this for you:
      5.5 cu ft
      Tune to 32Hz with a 4" x 2" port
      That gets you to 48Hz at -3dB, 37Hz at -6dB

      Moving to 6.5 cu ft doesn't gain you anything, the FB and F3 remain nearly the same. That woofer simply has a HUGE Vas to overcome.
      Electronics engineer, woofer enthusiast, and musician.
      Wogg Music

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      • #4
        Thanks, Wogg! The numbers you came up with look good. Thank you for going to the trouble of plugging in the numbers for me. I owe you one.

        GeeDeeEmm

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        • #5
          Oops! One more question. I checked my port leftovers today and all I have left is an 8 foot section of 3" diameter tube. Can anybody tell me how to convert the 4" diameter X 2" length into the 3" diameter tube I have on hand?

          Sorry. That's one conversion I haven't learned yet. :( Again, thanks in advance for going to the trouble.

          GeeDeeEmm

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          • #6
            Here's something you could do. Three equal length ports. Plug one or plug two for different tuning.
            For 5" long 3" diameter ports. The box tuning is 22hz, 31hz, or 38hz. (Two ports is for 31hz tune)

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            • #7
              Originally posted by gdmoore28 View Post
              Oops! One more question. I checked my port leftovers today and all I have left is an 8 foot section of 3" diameter tube. Can anybody tell me how to convert the 4" diameter X 2" length into the 3" diameter tube I have on hand?

              Sorry. That's one conversion I haven't learned yet. :( Again, thanks in advance for going to the trouble.

              GeeDeeEmm


              For that size of woofer, a 3" port would be quite small.
              I am wondering if even a 4" port would do the job.

              RPB gives a viable option.

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              • #8
                I'll second the 3" port suggestion for 3 ports, each 5" long. F3 of 44Hz, can take 50w RMS before exceeding Xmax. Not bad for an old beast.

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                • #9
                  You guys are great. Thanks so much for the options.

                  GeeDeeEmm

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                  • #10
                    Chris Roemer, if I may impose on your generosity one more time -

                    I've spent a lot of time sitting in my workshop and gazing at the speaker cabinets I'm building, as noted above. From a purely visual perspective, I think that I would be better pleased by venting these boxes out of the pedestal than out of the main cabinet itself. With that in mind, could I get your suggestion on what size rectangular (slot) port would work best to maintain the cabinet tuning you and others suggested - F3 of 44Hz.

                    Here are the cabinet measurements after making the pedestal integral with the main cabinets:

                    - Cabinet internal volume will increase from 5.5 cu. ft. to 6.0 cu. ft.

                    - Pedestal side walls will allow me to cut a rectangular vent no taller than 2," no wider than 20." Additional area is available on the sidewalls, if needed, but the 2" height is all I have.

                    - While the majority of the pedestal is made of braced 3/4" mdf, the area where the vent will be added is 1/2" mdf, so that will be the vent depth unless the depth needs to be extended.

                    - If it matters, the vent will be covered from the inside with the same antique-looking grill cloth as used on the grill. The cloth hasn't been chosen yet, so I don't have any porosity numbers to give you.

                    - Same speaker parameters:

                    Xmax: .15 inches
                    Re: 6.4 ohms
                    Vd: 19.2 cubic inches
                    FS: 23.9Hz
                    VAS: 22.74 cubic ft
                    REF: 3.3 %
                    Qts: .25
                    Qms: 6.1
                    Qes: .26
                    ​VID: .20 liters

                    I've run this thru a couple of online box designers, but each designer gave me different measurements. I'm much more inclined to trust your recommendations.

                    Thanks in advance. I'll try not to impose on your kindness again. (At least not soon, anyway!)

                    GeeDeeEmm

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                    • #11
                      And, just for grins, here are pictures of my progress so far. Panel thickness ranges from a minimum of 1 1/4" to a maximum of 2 1/2." Every square inch of these will be covered in a beautiful mahogany veneer that I've been saving. The pedestals (bases) are already made of mahogany. (The vent location I've asked about will be on the front of the pedestal.) Plans are to eventually pretty them up with various mahogany adornments that will mimic those great old 1950s designs.

                      When these are finally finished, the big 24" green hammertone Altec 511b horns will be mounted on top, with a similarly-hued University super tweeter in the cabinet, just above the woofer. (Yeah, I know all the inherent problems, but these are being built as little more than a trip down memory lane - not monitors!)

                      GeeDeeEmm Click image for larger version

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                      • #12
                        (The pressure.)

                        Here' goes . . . I'd make the vent (in the base) 1.5"h x 14"w x 3" deep. Should be plenty of flow. Fb =37Hz. F3=42.

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                        • #13
                          Chris, you are a lifesaver. And a gentleman. And a great helper in need. (What other superlatives can I think of before beginning to grovel?) My sincere thanks.

                          GeeDeeEmm

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