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  • jhollander
    replied
    more tests of the mid response comparing multiple holes to one hole near the front edge (of the horn mouth)
    Last edited by jhollander; 02-22-2018, 12:12 PM.

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  • jhollander
    replied
    Spent the day testing and learned a lot. I focused on the dip in the mid range response and what was causing the dip. It appears that the cavity resonance of the woofer is a major factor. The mid frequency response sees the cavity resonance of the woofer which cases the dip in the mid response. The tweeter frequency response sees the cavity resonance of both the woofer and the mid.

    My conclusions are the cavity resonance is a function of the volume between the horn entrance and the driver and the size of the opening into the horn. Much like a port as the opening got smaller (with the same length) the resonance moved lower. As the opening got large the resonance moved higher. As the volume between the driver and the horn entrance became smaller the resonance moved higher. b Multiple smaller woofer openings made no difference (in the midrange dip).

    Additionally there is a factor of where the opening enters the horn. Further from the apex the lower the response dip occurs but the change is much smaller. I'll add notes to pics for those who want to draw their own conclusions over the next few posts. Mid response original over modified
    Last edited by jhollander; 02-22-2018, 12:11 PM.

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  • jhollander
    replied
    Back in post #42 you'll see the woofer port holes are on the same side as the mid range holes. Covering the woofer port holes makes only minor differences in the mid FR. The woofer response looks like it sees the woofer port holes at 450 hz.

    If you follow this, one fix would be to put the mid holes on the same side as the woofer holes. I checked and there's no room on the internal baffle to fit the woofer and the mid.

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  • bwaslo
    replied
    Another wild thing you could try is to distribute the woofer holes so they aren't all the same distance from the tweeter (so they don't reinforce each others' effects).

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  • bwaslo
    replied
    I don't have anything proven to fix that other than moving the holes or making them smaller (using frustrums, so they're bigger inside and smaller into the horn maybe?).

    Someone over at diyaudio mentioned that JBL uses high density reticulated foam in their midwoofer vents. I looked around and found that those cheap foam paintbrushes are made from 100 ppi reticulated foam, so I ordered a bunch of them (cheap from Amazon, wife has Prime!). A lot cheaper than ordering custom foam for something that only MIGHT work. Haven't gotten to try them yet, so you might be able to beat me to it if you feel like giving them a try - places like Home Depot or Lowes carry them. JBL says high ppi open pore foam can pass lower frequencies but block higher on a horn wall. I tried a while back with lower density foam (before I heard about JBL's method) and it did precisely nothing, but maybe some magic occurs when you get higher (100 ppi is the upper end of the range that JBL mentions in a patent).

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  • jhollander
    replied
    Good news I found the dip problem in the mid range response, bad news is it's the woofer ports. I tried covering one port and then the inside and outside sections of the ports, there were only minor changes in the mid response.

    Bill any other options like multiple smaller woofer holes?

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  • jhollander
    replied
    I ran out of time today. I'll save a few graphs to share next time. The 700 hz dip remain at the horn mouth and at the mid port entrance to the horn. First glance the tweeter response looks better with 1 mid port vs. 2

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  • JavadS
    replied
    Seems like non hardening modeling clay would help for experimenting on these too(or silly putty), we use clay when flowing cylinder head ports to test different port shapes and area, really helpful. Javad

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  • bwaslo
    replied
    Have you measured off axis? Tried covering the woofer holes and ports? A wavelength at 700Hz is about 20 inches, at 1200Hz it's about 11 inches. A baffle (or mouth) edge that is about a half wavelength from the throat will give a peak (because the edge inverts the polarity due to pressure dropping there, and it gets a half wave delay from the time to reach the edge and then to you). Anything in your horn geometry about 5.5 inches away? Geometry induced things tend to be emphasized when all is symmetric, so off-axis can help (or help debug).

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  • jhollander
    replied
    Thanks Bill. What we have is the baffle response making the peak, duh. Near field is flat. but with some ugly dips.

    Kind of funny with horn response the smaller mid holes actually make a similar FR to the above (peaking upper frequencies) which lead me down another path and new mid range hole (yes single hole). The 700 hz dip looks inherent to horn loading as it shows up in every driver I have modeled.

    I'm now thinking similar to what you mention previously if I can add a peak to the sealed enclosure response I can lift the 700 hz dip and offset some of the baffle step.

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  • bwaslo
    replied
    I've haven't run any models with (nor measured) that GRS mid, but your midrange holes look awfully large. You might try modifying your HornResponse model (the hole length, effective diameter, mid front volume) to see what approaches your measured mid response, then try to make some changes that go in opposite directions. Make sure you model with !*measured*! Thiele/Small parameter values of the GRD mid, not published values only -- there can be a lot of variance and with a driver this inexpensive probably not a lot of budget for stringent parameter control.

    Wood filler/wood putty is pretty wonderful stuff. It might take a few days of filling and drying in stages, but you can cover over the holes to modify them in whatever direction they need to go.

    Also - make sure to measure off-axis a little (maybe 20degrees off), that suckout in the mid might be from horn mouth effect which will be worst looking directly on-axis

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  • jhollander
    replied
    Thanks Javad.

    The in horn GRS mid response is bad. Since the flat baffle response is OK I need to look at horn response to come up with a solution for modifying the horn. This could get ugly.

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  • JavadS
    replied
    Originally posted by jhollander
    10 lbs in the 5 lbs bag. Now it's time to take it apart and finish building the rest of the box so testing can start. I'll be disappointed if the holes need to be changed.
    You're not kidding, nice work John!

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  • jhollander
    replied
    We look at off axis response. I use omni mic to create polar response graphs. I usually just check or look off axis so I don't have a lot of graphs. Here's a horizontal graph for the titan OB project. There are some room interaction issues.

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  • TN Allen
    replied
    John,

    I'm curious about the way you will evaluate dispersion and pattern control. That's been the reason I don't test guides, I haven't seen a setup that considers the particular characteristics of a guide that correlate specifically to improved sound, and that can be easily quantified. Perhaps you will include photos and explanation of your setup?

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