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Box tuning (mains)

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  • Box tuning (mains)

    Until recently, I've mostly used sealed speakers. About a week ago, I modeled a box for my Peerless 830874 6.5" mid-basses, and decided to assemble one to see if I liked it. Most of my curiosity is related to the positioning of the driver, and port for optimum sound quality, if there is any logic to it. I had a 1 cu-ft box that I cut a new baffle for. The woofer is very close to the middle of the box. My first port was about 11" long. It was a leftover, and my box sim indicated a very low EBS type of tuning. In the past I have not liked ported mains because I could hear something like a port resonance that I found bothersome. With this speaker, I'm not hearing that (so far.) The tuning is 27hz. The port exits the bottom of the speaker, and the 11" length places the other end close to the middle of the box. So far, I really like the bass, but the box looks strange,and I may redesign the box. Any comments on driver and port being close to center?

  • #2
    WinISD shows an F3 of 90Hz. That's awfully high for a 6-1/2".
    Try using up internal space to get 0.8cf w/a 2"id x 4" long port. Fb mid 40s/F3 low 40s.
    Then try 0.6cf w/a 2"x 5" port. high 40s/ close to 50Hz F3.
    For music, 40Hz is most of what you need. Some box modelers will show you power vs excursion w/a particular box/tuning.

    Generally you don't want the interior end of a port right behind a driver (don't want it blowing air across the tube end like a pop bottle).

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    • #3
      Originally posted by Chris Roemer View Post
      Generally you don't want the interior end of a port right behind a driver (don't want it blowing air across the tube end like a pop bottle).
      Funnily enough, that's around where the entrance to the vent will end up, if you use something like Hornresp to model the vented box and try to use the position of the driver and the position of the vent to minimize the "organ-pipe" effects caused by the vent. Yup, the locations of both the vent AND the driver, as well as the dimensions of the box, have a significant impact on the amplitude of those resonances - and the lowest ones are the most annoying because they're only slightly impacted by lining the box.

      Brian Steele
      www.diysubwoofers.org

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      • #4
        Originally posted by Chris Roemer View Post
        WinISD shows an F3 of 90Hz. That's awfully high for a 6-1/2".
        Try using up internal space to get 0.8cf w/a 2"id x 4" long port. Fb mid 40s/F3 low 40s.
        Then try 0.6cf w/a 2"x 5" port. high 40s/ close to 50Hz F3.
        For music, 40Hz is most of what you need. Some box modelers will show you power vs excursion w/a particular box/tuning.

        Generally you don't want the interior end of a port right behind a driver (don't want it blowing air across the tube end like a pop bottle).
        The in room bass is nice with this tuning. My room will boom at 45hz if I'm not careful, that's why I have mostly used sealed boxes, and also why I was going for an EBS alignment. I will try tuning higher eventually. The PVC port I used was already 11" long,
        so I didn't have to cut a piece. I was going to start off with a tuning of about 33hz, but when I found the pipe, I was up and running sooner, and I was anxious. I only have the one box currently. I took a near-field measurement that showed a dip centered at 27hz.

        The box is rather deep, so the port is likely 7" from the woofer.

        My thinking is that it is harder for the woofer to excite a standing wave if located at the center of the box height. Perhaps higher frequencies will be more easily excited. My math may be faulty, but it looks like the lowest standing wave would be around 700hz. The box height (internal) is 19".

        And for the port, the center should be a hard place to excite that same 700hz standing wave.

        I didn't design the box for these reasons, it's just how it worked out. I was surprised that I liked it so much. Now I want to know why. Did I just get lucky?

        I need to make new better looking boxes, but I don't want to mess up a good thing by moving things around.

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        • #5
          I'm curious about damping as well. I have some dense fiberglass that can stretch across the box. Pete S. mentioned placing it in the middle in some thread I read a few days ago. I'm thinking that placing it at about one third of the height might be better. Right now there's only some bubble wrap in the box, so I expect sound quality to get even better.

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          • #6
            Here's a close mic measurement on the woofer, and on the port. The f3 looks to be about 80hz. Looks like there's an issue centered at 550hz. If that's an internal reflection, it should be easy to absorb. If it's a port resonance, I might need to get fancy with the port like 4th try did on his design. As a 3rd option, I suppose making this speaker a 3-way would greatly reduce the 550hz resonance. Honestly, I don't hear it so far. The port points downward toward the floor.

            Click image for larger version  Name:	ported response.JPG Views:	1 Size:	501.3 KB ID:	1421361

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            • #7
              I found a 5" pvc port to try. The tuning went up to mid thirties, and that resonance at 550hz is much lower. Guess I'll give it a listen now. I'm going to loose a lot in the 20hz to 25hz range, but above 30hz is stronger.

              Click image for larger version  Name:	port comparison.JPG Views:	1 Size:	420.4 KB ID:	1421365

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              • #8
                I did a little reading about port resonances, and it does appear that the peaks I'm seeing are due to the port. I added a piece of egg-crate foam to the top of the box, and the peak dropped considerably. I then switched to a 7" port length, and I like it a little better than the 5".

                Click image for larger version

Name:	port resonance.JPG
Views:	1
Size:	504.6 KB
ID:	1421475

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