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Measuring effect of port length on box tuning - odd results

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  • Measuring effect of port length on box tuning - odd results

    I am building a 2-way with a 5.25" mid-woofer (SB13PFC) in an oval box of 7L volume. This volume is a bit small to get a low tuning frequency in sealed form with this driver so I am adding a port. After some research on the web into how to calculate port length I found the troels gravesen page where he compared prediction and reality by putting the port on the outside of the box and varying it's length ( This led me to expect that I would be starting with a port based on calculations that was too long and I would be shortening it until it was right. I am using a DATS to do impedance sweeps which show where the resonance peak is for the driver in the box. In the empty box I was initially seeing a peak at a little over 100Hz, I added fiberglass stuffing and was able to move the peak down to 90 Hz. Then I tried the first test port. The port is made of 2" PVC pipe and I had attached it to the back of the box through a 90 deg elbow so it points up. The entrance to the port on the inside of the box is just the hole through the MDF with a 1/2" radius made by router, The tube I started with was about 10" long including the length of the elbow. The impedance sweep now showed 2 peaks, one at 90 and one at 30Hz. The 30Hz was shorter than the 90 Hz peak. I was trying for 40Hz so I concluded the port was too long and tried shortening it by an inch. This did not change the position of the peak at all, but it did make the 30Hz peak a little taller. I have shortened the port now twice and the result has been the same both times. 30Hz and 90Hz peaks, but the 30 Hz peak has gotten taller each time the port length was reduced. It was now taller than the 90Hz peak. This made me curious. I tried covering the end of the port with duct tape and this produced a single broader peak centered at 75Hz. If I removed the port and just covered the hole in the box with duct tape I get a peak at 80 with a very shallow slope down towards lower frequencies. If I put a 1.5" long PVC sleeve on the inside of the hole in the box, I get a 35Hz and a 100Hz peak, and the 35Hz peak is almost twice as tall as the 100 Hz peak.

    What is it telling me that any length port tunes this box to 30Hz? Is it common not to be able to change tuning frequency with substantial changes in port length? Or should I not worry about the impedance peak and just do frequency sweeps with a mic to decide what the best length is?

    I also tried listening to test tones at each port length and as I expected, near 30Hz most of the sound seems to come from the port. If I listen to some music with a lot of bass, like pipe organ music, I have the vague impression that sound quality was a little better at one of the intermediate lengths I tried, and with the short internal port, in the sense of sharpness or clarity. With the short internal port the bass sounds a little less strong and deep, maybe because that one points back instead of up. The tuning frequency isn't much different.

  • #2
    Not sure if your box is still stuffed with fiberglass, but you don't want anything but some lining on the walls for bass reflex. Egg crate foam is popular.


    • #3
      A ported cab is supposed to have two peaks, a sealed cab has one. With a ported cab you don't determine the tuning frequency by the position of the impedance peaks, you determine it by the low point in between them. While it's not a hard and fast rule, you do tend to get the best overall result when the peaks are about the same height. When that won't happen it usually indicates the box size is incorrect for the tuning you want. You should start by software modeling the speaker to see what the impedance chart should look like, then fine tune the port length to get as close as possible to that. As stated above never fill a ported cab, line it only.


      • #4
        Did you try the box with no port on it except the 1/2" radius hole? I suspect that is setting the 30 hz frequency, and may be masking other responses.


        • #5
          Can that driver even reach 30??


          • #6
            You are to be commended for your thorough, scholastic approach, but a ported box impedance has 2 peaks, and the
            box tuning frequency (Fb) is the bottom of the "trough" between the 2 peaks. Do not worry about the height of either
            of the 2 peaks.

            Example: This shows a Fb around 45 Hz Click image for larger version

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            • #7
              Do you have the T/S parameters of your midwoofer? I can whip something up using the factory specs too. That should be pretty close.

              Using a 2" port, you actually need kind of a long port because it's a bit oversized. I would recommend moving to a 1 3/8" ID port if possible- might be too late? This one:

              Anyway, 10" port (at 2" ID) tunes you to about 53.5 Hz at 7 liters internal, F3 is 55 Hz in that scenario. It's super flat through the bass region.

              If you can use a 1 3/8" port, you'll get some chuffing at high power be decent port length and decreased port resonance - (2" port has a big resonance spike at 600 Hz)
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              • #8
                Thanks everyone for the interesting and informative responses. I learned that Fb in a ported box is the midpoint between the two impedance peaks. Which means I have seen pretty much just what djkest said is predicted by model, Fb of ~53Hz. What software are you using to model this? Especially curious since you noted that there is an nasty port resonance to be aware of. I assume that was a modelling prediction?

                As for the question of fill, I thought you wanted to avoid having fill near the entrance to the port and to allow a clear passage between the driver and the port. In the oval the driver is in the middle, the port is behind it a little lower. The fill is in the pointy ends of the oval and not in the way of the port opening or driver. It lowers the box tuning some so I would think that is a good thing. The midwoofer sounds great in this enclosure. I think it is absorbing internal box reflections very effectively, which is why I built an oval enclosure. It does have ~1/2" thick carpet backing foam on most of the internal surfaces as well. That seems to use up some internal volume. I found that the closed box resonance dropped when I removed about 30% of that pad.

                If skatz is right about about the hole setting the port frequency then I may be stuck with the Fb I have now. Let's say this is correct, then what change to the hole would raise or lower the frequency? I can go bigger or smaller. Would you think leaving it sharp-edged would change it as well? Do I even need a port tube?

                Overall, I take away that I may be stuck with the Fb I have now regardless of what size port I use, but I can choose a port that does not have the resonance issue and gives impedance peaks of roughly equal height.


                • #9
                  WinISD is an EXcellent and FREE program you can get from .
                  You should use your DATS to get these parms for box modeling: Qes, Qms, Qts (which is actually calc'd from the 1st two), Fs (these 1st 4 are taken in "free air" flawlessly by your DATS w/very little work on your part), and Vas - which takes more effort on your part to get usable results. Anyway, YOUR DATS parms will override the Mfr's specs. THEY will determine the box size (and tuning) you'd need to get the results you're after.

                  Assuming that the Mfr's numbers (the ones EYE have, anyhow) are correct: (Qts = 0.29, Fs = 44Hz, and Vas = 0.47cf), this driver has a "borderline" low Qts value (below 0.30) to get decent bass in a ported box (in a closed box, you'll typically lose another octave of bottom end).

                  This driver CAN reach about 40Hz, but not very smoothly, in 0.6cf tuned into the low 40s.
                  In YOUR 7L (0.25cf) box, it looks like a 1.5"id port that's 5" long will give you a tuning in the mid 50s, and an F3 in the upper 50s. I think that's about the lowest (strong) bass you can get in your box (which COULD be larger - to go a bit deeper). (This is basically the SAME tuning you had using a 2"id port when it was 9" long.)

                  You'll STILL get 2 Z-peaks (near 30, and 90, as before) at about 27 and 90Hz. The valley will be in the mid 50s. The tuning freq. is determined solely by the interior box volume (adjusted downward by any "stuff" that's inside the box, consuming that volume - like the driver magnet and port tube (unless the tube's EXternal)), and the port ID and length. Using a different driver WON'T change the box tuning - but various driver's WILL respond differently in the identical boxes (and tunings) depending on their T/S parameters.


                  • #10
                    I'll +1 what Bill said about the peaks, the port seems to be the correct length ( for a given box volume) when the peaks are equal. Changing the length will move the tuning a bit, but the volume of the enclosure has more effect on that.
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                    • #11
                      Here I am a few months later - I have now made external ports for these 7L enclosures so as not to use up any of the limited internal volume with the port itself. The ports are roughly rectangular in cross-section and have a large flare at the end. About 1.75" x 1" and 5.25" long. They may be a little longer than necessary still since the lower of the two impedance peaks is larger than the main box resonance peak, but not by a lot. The two peaks are as discussed above, at 90Hz and about 30Hz, so Fb should be in the mid to high 50's.
                      So here is my current question -
                      When I run a frequency sweep on the woofer in the box with the LP crossover section in place the FR is flat to 100 and then drops rather quickly. I was hoping for something that looks more like the box model from WinISD, which drops down near 60Hz. The situation is the same with or w/o the LP filter. What might explain this discrepancy in bass behavior? What I am seeing looks about like what I would expect for a closed box - acoustic suspension. I should say the post is on the back of the enclosure and I am measuring out in front at about 1M. Oh, I will also say it is not a microphone artifact because if I make the same measurement on the other speaker in "the shop" here, some large sealed 1976 "monkey coffins" with 10" CTS woofers I get an FR curve that is flat to 50Hz.


                      • #12
                        So your measurements are rolling off faster than the model? If this is the case it may just be the way you are measuring. I don't remember the specifics but I believe you need to measure the speaker as you have done and then measure the port output and sum them. I believe Jeff Bagby posted how to do this (a write up somewhere maybe?) a while back.
                        This explanation is goin to suck lol! What you may be measuring is where the woofer starts to hand off the sound to the port (i told you it was going to suck didn't I). The output decreases from the woofer and increases from the port. This is cause by a phase shift.


                        • #13
                          How are you measuring the response?


                          • #14
                            killa said "So your measurements are rolling off faster than the model?"
                            YES, that is what I am saying. Your suggestion that one has to measure output from the port is the sort of thing I am suspecting. As I said, I am measuring from the front of the speaker and the port is on the back.
                            I think that answers isaeagle's question as well, but I will provide some more detail about the setup. The speaker is sitting on the edge of a table so it has free air around it except for the surface it is sitting on. The area is a bit cluttered so I get quite a few reflection artifacts in my graphs, but that does not affect the low end roll off.

                            These are intended to be bookshelf speakers so I suppose it would be more realistic to measure them placed near a wall. That should return most of the port output back towards the listener, I would think.

                            Now you have me thinking about the greater distance the bass frequencies have to travel to reach the listener in front. It makes me think ports should really be on the baffle. But then, there is the phase shift business...


                            • #15
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