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The SideTowers - An F6 Corundum Build

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  • 4thtry
    replied
    My port tuning came out a little low. My original, modelled FB was projected to be 36Hz, but my actual Z curve measurement came out to about 32Hz. This was using my original 8.3 inch long slot port.

    I listened to it for a while and felt that it was probably drifting a little bit too much into an extended bass shelf type of mis-alignment, with the deepest bass being a little on the anemic side.

    So I shorted the port by about 1.5" by cutting a rounded curve on the end of the slot. This curve should help to slow down the release of internal slot port pressure at the terminus, similar to what happens with a conventional flared type port. FB now measures roughly 37Hz on my impedance curve. I did a NF cone + NF port summation in OmniMic posted above (post 58).

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  • 4thtry
    replied
    I was curious as to whether or not the small tweeter protection grill was affecting my measurements. Attached are my findings. Because OmniMic's phase curve sometimes bounces up and down a little, I ran the measurement three times to confirm my findings.

    Result: There was a very small difference in frequency response from 7 to 17kHz, as you can see in the graph. Phase angle shifts just a tad from 3.5 to 15kHz. As far as I can tell, nothing to worry about here.

    I should note that I ran this same comparison test before attaching foam to the tweeter flange and discovered that there was no measurable difference between grill and no grill. So, the above differences may have been created by the foam rubber.

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  • 4thtry
    replied
    Here are OmniMic's CSD and Toneburst graphs. I think the CSD plot may be contaminated; I think I may have forgotten to lower the gate before switching over to this screen. I will have to double check and re-run, but something does not look right to me.

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  • 4thtry
    replied
    Here are the mirrored images of the horizontal plots. The contours seem fairly smooth and the 4.6kHz on-axis dip problem is gone.

    Also attached are the vertical polars from 0 to 30 degrees in 5 degree increments. The positive set shows the difference between a seated (0 degrees) and standing position (+15 degrees) and appears very uniform. The plus 30 degree vertical dips down a bit around the crossover, due to the phase difference between the two drivers in the vertical direction.

    The negative set shows the difference between a seated position and the position of someone sitting on the floor (-15 degrees). Again, the 30 degree vertical dips down quite a bit around the crossover. In addition, the microphone is only 36" above the floor for this measurement, causing a bit of floor bounce contamination.

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  • KEtheredge87
    replied
    Congrats Bill! Looking forward to seeing these and competing in April!

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  • 4thtry
    replied
    Finally done. The 4.6kHz problem has disappeared, probably due to the J-rolled facet curves. The on-axis frequency response at 1 meter is extremely flat (looks like plus or minus 1dB or so from about 300Hz to 20kHz). Polars are very broad and uniform from 500 to 20,000Hz, all the way out to 90 degrees. I can see just a little bit of bunching in the 2.5 to 4kHz region, which is probably why the speaker has a fairly crisp and detailed type sound quality.

    Not bad for 5 parts! See attached graphs.

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  • Kevin K.
    replied
    Your tenacity is awe-inspiring Bill! I've noticed it in some of your other builds as well. Really cool solution to address the issue. Good luck getting it all ironed out. Opps, I meant rolled out.

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  • KEtheredge87
    replied
    Wow Bill! That's a labor of love and highly impressive to boot! I really dig the new baffle shape with the variable radius. I made a comment a while back on my own build log where sometimes the DIY guy is as much a sculptor as a carpenter. You definitely fit that bill right now. Makes me think there was some value in all that time I played with PlayDoh and clay as a kid! I'm really excited to see how this turns out.

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  • ernperkins
    replied
    Very cool and ingenious Bill!

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  • tomzarbo
    replied
    Darned if that foam doesn't look like suede in the pics. Pretty cool stuff you're doing with the round-overs. This is science at work!
    TomZ

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  • jhollander
    replied
    Bill that is impressive, nice work!

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  • 4thtry
    replied
    I will be back next week with some finished pictures and revised polars. Hopefully, they will measure (and sound) much better this time around. But even if they don't, I can now simply get out my J-roller and reform the facets to fix any glitches.

    Since these pictures were taken, I have peeled back the foam a couple times to fill in some small mortite pot holes. After pressing the foam back in place, I was able to quickly smooth everything out again with the J-roller. It is a very flexible system. Maybe I should change the name from "SideTower" to "FLEXO-BAF" or something like that!!

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  • 4thtry
    replied
    Here are a few more shots showing how I J-rolled the mortite into shape. I used pizza box templates to check my work as I went along. The top corner is a 4" radius, which quickly reduces to smaller radii as you move around the baffle.

    I glued the foam sheets to the baffle and the side panels with double stick carpet tape. As I went along, I feathered (overlapped) each 7.5 degree slice by roughly 1/8th of an inch, pressing it firmly into the mortite and carpet tape. When done, I installed an elastic head band to hold everything firmly in place.

    Note: I was originally going to glue polyester fiber fill to these areas instead of mortite, but this did not work. The fiber fill was very lumpy and difficult to work with. It would not stay in place as I tried to wrap the foam skin around it. The nice thing about mortite is that it is very sticky, stays in place, and helped to hold the foam skin material in position as I streached it around the baffle edge.

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  • 4thtry
    replied
    After grinding the facets down with a belt sander and painting the baffle, I fitted some mortite caulking cord to the faceted areas and J-rolled it into a continuously variable radius. I cut a large number of radius profiles from leftover pizza boxes to use as guides during the fitting and J-rolling process.

    The result was a very smooth edge radius going around the tweeter, which changed continuously from 1/2" radius near the woofer to a full 4" radius at the the top corner, then back to a 1 3/8" radius at the top middle.

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  • 4thtry
    replied
    Here are a few pics of how I marked & cut the foam rubber. This material is available from my local hobby/craft store in inexpensive 12 x 18 x 1/16" sheets. I cut the foam in 7.5 degree slices along the top 180 degrees to give the sheets the flexibility to wrap around the baffle corners.

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