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Suggestions on first build - UM10-22 ported subwoofer, 1.7 cuft, 20Hz f3

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  • #16
    Originally posted by LOUT View Post
    What kind of problems might someone run into with a subwoofer in a ported enclosure with modelling showing a decently flat frequency response down to the port, and the port size/length optomized to avoid chuffing while the woofer will stay under Xmax?
    I don't mean this in a snarky or challange-y way, and it doesn't have to specifically be about the UM10", I'm honestly curious what kind of problems a subwoofer can run into with a ported box when FR/extension, port-noise and Xmax are kept in good shape. Are there some other problems to look out for, or is it a situation where SIMs/models tend to get things wrong?
    Good question!
    It appears that you are conversant with modeling / simulation of box tunings.
    In regards to ported alignments, the driver will unload below Fb.
    What does this mean?
    Above Fb, the driver is constrained by the "springyness" of the air in the box and the "springyness" of the port air mass.
    The sim will accurately model max SPL and excursion of the driver for that frequency range.
    Below Fb, even if the model shows no excursion issues, the sound can become "flabby" and the driver can show distress (high distortion) since it is "unloaded".
    During MWAF sessions, the judges can and do play music with a lot of bass just to see how a design will react.
    Make sense?

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    • #17
      For some reason I keep reading "Fb" as "Fs" instead and had to look-up what Fb is to make sure. For anyone brand new here (or clueless as me):
      Fb - The resonant tuning frequency of a vented speaker cabinet.

      Does the unloading below Fb mean Sim statistics like F6 or F10 on ported designs might end up being a somewhat meaningless spec for many drivers?
      In other words; sure the sound is "there" but it's probably not a very good sound.

      Is this the kind of thing where drivers that are good choices for OpenBaffle subwoofers don't really need to worry, but most other drivers will suffer some form of flub below tuning,,or can it affect basically any driver to some extent?
      My first 2way build

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      • #18
        Originally posted by LOUT View Post
        For some reason I keep reading "Fb" as "Fs" instead and had to look-up what Fb is to make sure. For anyone brand new here (or clueless as me):
        Fb - The resonant tuning frequency of a vented speaker cabinet.

        Does the unloading below Fb mean Sim statistics like F6 or F10 on ported designs might end up being a somewhat meaningless spec for many drivers?
        In other words; sure the sound is "there" but it's probably not a very good sound.

        Is this the kind of thing where drivers that are good choices for OpenBaffle subwoofers don't really need to worry, but most other drivers will suffer some form of flub below tuning,,or can it affect basically any driver to some extent?
        If there's a lot of material below the port tuning, the driver cone will move a lot since it's basically working into a box with a hole in it. That's a poor idea with any driver. One solution is a high pass filter. If the port tuning is low enough that little content is present below the tuning, you're fine.
        Francis

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        • #19
          Originally posted by fpitas View Post
          If the port tuning is low enough that little content is present below the tuning, you're fine.
          I disagree somewhat. If the port tuning is too low and VB >> Vas, the cone excursion will keep increasing as frequency goes down until it gets to the region near Fb. The more Vb >> Vas, the greater the peak "before" Fb will be. So one can still get excursion problems above Fb, as the cone excursion is not being curtailed a whole lot. In other words, the region (in frequency) where the port tames cone excursion becomes narrower and narrower the larger the enclosure is (compared to Vas), so above tuning it is working a lot like a sealed enclosure. The smaller the enclosure the wider the "dip" becomes and the more the port dampens cone excursion in terms of a wider range of frequencies centered at Fb

          I would eliminate the silly folds (just IMO of course), and open up the port area. If done with the slot port using 3 enclosure walls as the walls of the port, the final end correction factor becomes so influential that you can cut the port length by 35-40% and still achieve your desired tuning. I discovered this with a ported enclosure that I built for a JBL W10GTi that I built 15 years ago. I still have it in fact. Right now it has a CSS SDX10 for subwoofer duty in the garage.

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          • #20
            Originally posted by guitar maestro View Post

            I disagree somewhat. If the port tuning is too low and VB >> Vas, the cone excursion will keep increasing as frequency goes down until it gets to the region near Fb. The more Vb >> Vas, the greater the peak "before" Fb will be. So one can still get excursion problems above Fb, as the cone excursion is not being curtailed a whole lot. In other words, the region (in frequency) where the port tames cone excursion becomes narrower and narrower the larger the enclosure is (compared to Vas), so above tuning it is working a lot like a sealed enclosure. The smaller the enclosure the wider the "dip" becomes and the more the port dampens cone excursion in terms of a wider range of frequencies centered at Fb

            I would eliminate the silly folds (just IMO of course), and open up the port area. If done with the slot port using 3 enclosure walls as the walls of the port, the final end correction factor becomes so influential that you can cut the port length by 35-40% and still achieve your desired tuning. I discovered this with a ported enclosure that I built for a JBL W10GTi that I built 15 years ago. I still have it in fact. Right now it has a CSS SDX10 for subwoofer duty in the garage.
            You're right, although I wasn't thinking about lowering the port tuning in a given design. That would goof up the frequency response, for one thing. If you substantially lower the port tuning and EQ, like some JBL designs, your observations certainly apply.
            Francis

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            • #21
              I wasn't talking about lowering his desired port tuning either.

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