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  • Port tuning measurement

    Hi All
    It's been many years since I have posted about speaker building by my son now has an interested in the topic.
    I have built a new Modula MT box that will fit in the corner of the room but when it comes to measuring the port's resonant frequency it has been nothing but frustration.
    I dusted off the all wallin jig and speakerworkshop but all the results were a failure.
    I can not seem to get an impedance measurement going.

    So anyway what is the preferred way to get an impedance vs freq plot in order to tune the port?
    I'm getting very close to just measuring the port length on the original box and using that.
    It's been over 10 years and I can't remember how I got any of this stuff going. *sigh*.

    Any advice would be welcome

  • #2
    The easiest way to get a nice impedance plot is a low cost tool like the DATS tester from Parts Express. There are similar older and slower ones, but DATS is what I use now. You do have to have a PC and install simple software.

    https://www.parts-express.com/Dayton...System-390-807





    OTOH, if you have a signal generator and a AC DVM you can figure out the tuning just by connecting the signal generator through a 100 ohm resistor to your speaker, and sweep the frequency, and look for where you have the minimum voltage in the low frequency area.

    Trust me, DATs is easier. I can do it both ways... it will generate a plot like this for a reflex system:

    But if you'd rather not spend any money and work out an answer, Unibox for Excel will tell you how to size a port for a given volume box, and you can adjust the port parameters (diameter) and it will calculate the length. But you need to have a PC with Excel. Man, the things I take for granted....

    So if you want, post the box size, and your port tube diameter, and the target FB, and I'll calculate it for you. And post the Unibox results. If needed, I can go back and look up the design... that was a few revolutions around the sun ago...
    the AudioWorx
    Arvo Pärt
    Isiris MLBL
    Modula MT
    Natalie P
    M8ta
    Modula Neo D CC
    Ardent
    Modula MT XE
    Modula Xtreme
    Isiris
    Wavecor Ardent

    SMJ
    Minerva Monitor
    Calliope
    Ardent D

    In Development...
    Isiris Mk II updates
    Calliope CC
    Obi-Wan
    Saint-Saëns Symphonique
    Modula PWB
    Calliope CC Supreme


    Resistance is not futile, it is Volts divided by Amperes...
    Just ask Mr. Ohm....

    Comment


    • #3
      Thanks again Jon.
      I will try to rig something up.

      Comment


      • #4
        I recently built an oversized boom box for use at the lake mainly.

        I used WinISD to calculate the port size and length. I don't have any equipment to measure it to see if it's correct, but it sounds good, so it has to be close...
        CADman_ks
        - Stentorian build...
        - Ochocinco build...
        - BT speaker / sub build...

        Comment


        • #5
          Originally posted by jquin View Post
          Thanks again Jon.
          I will try to rig something up.

          Unibox is very accurate in it's predictions, so if you just give me your enclosure volume, I can do the rest. If you already have a port tube, I can calculate what you need, but I'll also look at what makes sense based on port velocity and length required.

          For instance, that plot above was a target design for as passive radiator, which is a much more difficult analysis, and I was shooting for an Fb of 30-32Hz, (with Dayton RS225), and as you can see, I pretty much nailed it. With those kinds of calculation tools, verifying by measurement is pretty much optional.
          the AudioWorx
          Arvo Pärt
          Isiris MLBL
          Modula MT
          Natalie P
          M8ta
          Modula Neo D CC
          Ardent
          Modula MT XE
          Modula Xtreme
          Isiris
          Wavecor Ardent

          SMJ
          Minerva Monitor
          Calliope
          Ardent D

          In Development...
          Isiris Mk II updates
          Calliope CC
          Obi-Wan
          Saint-Saëns Symphonique
          Modula PWB
          Calliope CC Supreme


          Resistance is not futile, it is Volts divided by Amperes...
          Just ask Mr. Ohm....

          Comment


          • #6
            Modula MT enclosure calculations for original PE 22L enclosure

            For you and the benefit of anyone else revisiting this design, here are the enclosure parameter calculations for the vented design of the Modula MT using the RS180-4. The RS180 can provide even more extended and flatter response in a larger enclosure, but the 22L nominal original chosen is a good tradeoff between size and bass capabilities.

            First, we'll show the more optimal but more difficult to build 3" port version, then the more commonly used 2" port version, and show the tradeoffs.


            Click image for larger version

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            Note the port inner diameter is 7.5 cm.


            Click image for larger version

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            The box Fb tuning and the -3dB point are at 34 Hz, and response is fairly flat down to 40Hz, though obviously not at room shaking SPL. This is with an input of 20 watts, which is about the low frequency limit.


            Click image for larger version

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            Excursion is within the driver capabilities


            Click image for larger version

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            Port air speed is comfortably below the preferred limits; from this, its likely that a 2-1/2" version would work OK, too, without port compression or chuffing.

            But note that the port length is about 17-1/4", not including a port flare, so depending on the enclosure configuration and depth, this could pose a problem for a front or rear venting port; the port opening should be about 3" minimum from any enclosure wall. Possibly a bottom mount if the enclosure is tall enough, or a rear mount if a deep form factor like the Q Acoustics 3030i uses, or the one planned for the Obi-Wan.
            the AudioWorx
            Arvo Pärt
            Isiris MLBL
            Modula MT
            Natalie P
            M8ta
            Modula Neo D CC
            Ardent
            Modula MT XE
            Modula Xtreme
            Isiris
            Wavecor Ardent

            SMJ
            Minerva Monitor
            Calliope
            Ardent D

            In Development...
            Isiris Mk II updates
            Calliope CC
            Obi-Wan
            Saint-Saëns Symphonique
            Modula PWB
            Calliope CC Supreme


            Resistance is not futile, it is Volts divided by Amperes...
            Just ask Mr. Ohm....

            Comment


            • #7
              Now, a 2" ID port version is examined.



              Click image for larger version

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              This results in a much shorter port, about 7" not including flare.


              The tradeoff is port air speed and compression at higher sound levels:



              Click image for larger version

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              If we look at the 2.5" option, it splits the difference and is the minimum port diameter meeting the port speed requirements.



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              Click image for larger version

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              But this does require a port length of 11", not including flare.
              the AudioWorx
              Arvo Pärt
              Isiris MLBL
              Modula MT
              Natalie P
              M8ta
              Modula Neo D CC
              Ardent
              Modula MT XE
              Modula Xtreme
              Isiris
              Wavecor Ardent

              SMJ
              Minerva Monitor
              Calliope
              Ardent D

              In Development...
              Isiris Mk II updates
              Calliope CC
              Obi-Wan
              Saint-Saëns Symphonique
              Modula PWB
              Calliope CC Supreme


              Resistance is not futile, it is Volts divided by Amperes...
              Just ask Mr. Ohm....

              Comment


              • #8
                Originally posted by jquin View Post
                Hi All
                It's been many years since I have posted about speaker building by my son now has an interested in the topic.
                I have built a new Modula MT box that will fit in the corner of the room but when it comes to measuring the port's resonant frequency it has been nothing but frustration.
                I dusted off the all wallin jig and speakerworkshop but all the results were a failure.
                I can not seem to get an impedance measurement going.

                So anyway what is the preferred way to get an impedance vs freq plot in order to tune the port?
                I'm getting very close to just measuring the port length on the original box and using that.
                It's been over 10 years and I can't remember how I got any of this stuff going. *sigh*.

                Any advice would be welcome
                You've got everything you need in the Wallin jig, but maybe its time to try some more modern software..ARTA or REW will both do the trick. I personally prefer ARTA, the GUI is a lot less cumbersome I find.

                All that is required for impedance & T/S measurement is any functioning sound card and a few resistors. As few as 1 resistor if you want to cheap out and make a low level un-amplified jig, similar to the old DATS/WT.
                Click image for larger version

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                I prefer to include the amplifier for greater reliability and versatility, it just needs an additional voltage divider at the soundcard input to step down the speaker level signal to line level:
                Click image for larger version

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                Of course, if you study the guts of the old Wallin jig you will find something rather similar already included. Above images are from the ARTA/LIMP manual, a good reference to get started:
                https://artalabs.hr/download/LIMP-user-manual.pdf

                Comment


                • #9
                  " I have built a new Modula MT box that will fit in the corner of the room but when it comes to measuring the port's resonant frequency it has been nothing but frustration."

                  Do you have a mobile phone? Or your son? Do you have a PC or laptop?

                  Download some freeware sound analyzer app in the phone. Download some freeware sound generator/analyzer progam in laptop (RoomEQWizard for Win). Install and learn to use them!

                  Use laptop/REW to generate and play pink noise and connect it to an amplifier and that to the speaker. Open phone's analyzer app and put it at the opening of reflex tube and you will see frequency plot, the highest peak spl is the tuning frequency! Use dB(C) or dB(Z) instead of dB(A) filtering to get valid response of low freq! Headset or webcam microphones usually have fixed highpass filter, so they don't work below 50Hz.

                  If you don't want to use REW or alike, and you have a FM receiver, tune it between stations (disable off-station mute) to get noise to the amp

                  Anyway, please invest 100$ on some USB-microphone like Minidsp UMIK-1.
                  My DIY speaker history: -74 Philips 3-way, -82 Hifi 85B, -07 Zaph L18, -08 Hifitalo AW-7, CSS125FR, -09 MarkK ER18DXT, -13 PPSL470Dayton, -13 AINOgradient, -18 Avalanche AS-1 dsp, -18 MR183w

                  Comment


                  • #10
                    Or just set the port ID and length per the calculations above! YOU DO NOT NEED TO MEASURE IT!
                    the AudioWorx
                    Arvo Pärt
                    Isiris MLBL
                    Modula MT
                    Natalie P
                    M8ta
                    Modula Neo D CC
                    Ardent
                    Modula MT XE
                    Modula Xtreme
                    Isiris
                    Wavecor Ardent

                    SMJ
                    Minerva Monitor
                    Calliope
                    Ardent D

                    In Development...
                    Isiris Mk II updates
                    Calliope CC
                    Obi-Wan
                    Saint-Saëns Symphonique
                    Modula PWB
                    Calliope CC Supreme


                    Resistance is not futile, it is Volts divided by Amperes...
                    Just ask Mr. Ohm....

                    Comment


                    • #11
                      If all you want to know is resonant frequency and the rest of the speaker is unknown, you can just lay the speaker face up and use a function generator phone app. Throw some rice on the speaker and use the generator, the frequency with least rice bouncing is the resonance.

                      Comment

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