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  1. #1

    Default TangBand 75-1558SE on a Waveguide with Measurements

    So I bought a pair of these a few months ago because I was going to redesign my Delphi and wanted to use a more robust mid dome. I had a feeling these would be good performers and if you've seen Zaph's recent measurements they are really incredible dome mids. Two problems with them though, the AC is too far forward and it has a dip around 2khz. My hope is that by putting it a waveguide similar to the legendary ATC dome mid we can cure at least the first problem, and possibly the second.

    The waveguide is basically a truncated 1 1/4" roundover. I got the bit at mlcs I think. This should be very similar to the ATC roundover. Now the ATC waveguide actually ends on the outside of the surround, but I opted to bring it in closer to the dome similar to the Morel MDM55 and Seas T25CF001 I used for guidance. This is possible because, unlike the ATC, the TB uses an inverted surround. But with 1.5mm xmax and the little VC leads we do still have to be careful.

    The following is teh step by step method I used to make the waveguide. I can't just give you the measurements because this is a truncated roundover, so the process was somewhat iterative. It took 3 mock-ups to get to the final design. This is sort of the perfectionist design. I'll give a brief description of an easier design that may or may not be as good. Overall diameter is 5 1/2" inches and the dome top is just flush with the baffle plane.

    First this is based on a baffle 1 1/2" thick. Just two 3/4" MDF sheets glued together. Pretty typical for a front baffle. I will be using the backside of the baffle to drill our hole for the Jasper jig since everything is referenced to the backside and the roundover bit bearing will ride on this part of the baffle. If you drilled the hole from the front it may wonder a 1/16" and really screw the whole thing up.

    1) We'll make three cuts 3/4" deep at 5 9/16", 5 1/16", and 4 9/16". It should like this when done:



    2) Next I made another 3/4" deep pass at 3 1/16" like this:



    The 1/2" or so strip in between the grooves will be routed down at the very end. We need this for now so the roundover bit bearing has something to ride on.

    3) Now I flipped the baffle over and cut a matching 3/4" deep groove at 3 1/16" and removed the center plug:



    4) This front cut may not have matched the back cut due to the slight wander when we drilled the pilot hole. No worries though as the front will be routed away. Which we do next:



    5) Now that we don't need the guide for the router bit we can route down that strip in the second picture. But not all the way. This is where the perfectionist part comes in. I left it about 1/8" higher than the three cuts made for the driver flange:



    6) Then I took a marker and marked a roughly 1/8" (at most) line around the strip. I then routed the rest of the strip flush with flange cut, leaving the black marked portion behind:




    7) I put some gasket tape on the driver:



    8) And mounted it on the baffle:





    Now the black strip is there so that the edge of the waveguide is closer to the base of the dome, about where to where the surround attaches to the dome. The less perfectionist way would be to cut a 3" circle in step 2 and 3. In step 5 route the strip flush with rest of the flange cut and call it good.

    Measurements to follow.
    Last edited by augerpro; 06-06-2009 at 11:46 PM.

  2. #2

    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    And here are the measurements. Pretty discouraging and not what I expected at all. First, flat baffle at 0, 20, 40 degrees:



    WG baffle at 0, 20, 40 degrees:



    Flat versus WB on axis:



    Flat versus WG 20 degrees off axis:



    Flat versus WG 40 degrees off axis:



    Also sample 2 Flat baffle 0, 20, 40 and WG 0, 20, 40:




    I tried a 1 inch piece of closed cell foam stuck to the tip of the dome and it did nothing. Adding an adhoc phase shield did this:




    Surprising amount of gain from under 1khz to 3khz before everything goes to hell. Didn't expect that just like I didn't expect the big dip at 3.8khz either. 3khz wavelength is 3.5" so I wonder what is reflecting where? So I'm not sure where to go from here. Maybe a shallower WG, or maybe keep the edge of the waveguide to just outside the surround like ATC. I was hoping to move the AC back AND help the response off axis, but now I'll settle for anything that can move teh AC without jacking up the response too much. Any ideas?
    Last edited by augerpro; 06-14-2009 at 05:44 PM.

  3. #3
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    1 1/4" roundover? That is a huge bit! Patiently waiting for measurements .

    Maybe when my kids get old enough to not push in domes I will consider such a beast...

  4. #4
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Quote Originally Posted by eyekode View Post
    1 1/4" roundover? That is a huge bit! Patiently waiting for measurements .
    Patently? No way. I am waiting with bated breath!

  5. #5
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Hi Brandon,

    Nice work, but that horn looks awful small to have much of an effect on the passband of that driver. What is it's intended use (bandwidth?) in the overall system?

    cheers,

    AJ

  6. #6
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Quote Originally Posted by ajinfla View Post
    Hi Brandon,

    Nice work, but that horn looks awful small to have much of an effect on the passband of that driver. What is it's intended use (bandwidth?) in the overall system?

    cheers,

    AJ
    The TB 3" dome mid is limited to between 500Hz and 3000Hz, depending on the required acoustic roll off.


  7. #7
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    This configuration is appropriately called a waveguide. It is small and shallow compared to a traditional horn. However, you can get significant changes with much less than wavelength scales.

    For comparison sake look at Zaph's 1/2" radius on a 1" tweeter:

    http://www.zaphaudio.com/tidbits/D26...horn-stock.gif

    The boost is in the 2 to 6 khz range. I think this effect is more from the throat restriction increasing the acoustic impedance (gives the diaphragm more resistance to push against).

    It will be interesting to see how low in frequency the boost is here. I am guessing that there will be significant changes above 1 khz, but I am waiting to see what happens in the 500 to 1 khz zone. I am also curious if the dispersion is affected. I think the 3" dome needs a little help from the waveguide:

    http://www.zaphaudio.com/temp/Tangba...5-30-45-60.gif

  8. #8
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Hi Brandon,

    Those DIY waveguide steps look familiar. I'm very interested to see your measurements. I had intended to test this myself with a mid dome at some point, but I'm glad you decided to do it. I suspect the boost will be too high in frequency to benefit low end performnance, but you are not looking for that benefit in this case anyway. The boost with a 3/4" deep WG (3/4"R roundovers) on a 1" dome is centered around 3-4khz and is around 5-6 db, IIRC.

    I assume you will be taking on and off axis measurements, hopefully with and without the WG, to give a more complete picture.

    Are you thinking about testing different WG depths as well with the 1-1/4" roundover? I think that might help you fine tune the results to what you are looking for.
    Dan N.

  9. #9

    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Hey Dan! Main thing was to put the AC in a better place, but I did want to see how this affected the 1.5-2khz dip also. I don't think it will be deep enough for that, and hopefully it doesn't go the other way and make it more difficult to shape the response. The dip is actually in a good place. Being in the middle of the passband when we apply a bandpass filter it will make the dip a peak and smooth right out. I'll just have to see what the measurments say and decide if a further test is needed. If I do go deeper I *may* use the big 1 1/2" bit since I like to truncate the roundover so it is more normal to the dome.

    I'll be doing on and off axis measurements of both waveguide and stock.

  10. #10

    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Measurements added to post 2

  11. #11
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Random thought. When you mount the speaker assembly from the rear, I assume you are loosing some mounting area to clamp against. Does the assembly have a resonance freq. at 3.8k and you are not clamping it enough to suppress?
    Thanks,
    Zach Tripp
    http://home.comcast.net/~zach_beth/diy.html
    FOLLOWMY529.COM - Subscribe to get monthly updates on performance.

  12. #12
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Brandon,

    I think I would try some different waveguide configurations; different depths, different roundover radii, different throat sizes, etc. But of course, change only one item at a time! As you do so, compare to your original measurement. Eventually you might dial in on something that works for your needs. It will take experimentation, but that can be fun.

    I think I would try different roundover radii first. Keep the depth the same, but make a larger and smaller radii for comparison. If you keep the same throat opening, you will therefore change the angle at the throat as you change radii. You will have to do some calculations (or use cad as I did) to determine what size your original bored hole diameter needs to be to result in a given throat opening at a given depth and radius. Obviously, the starting hole size will be different for each radii for the final throat dia. to be the same.

    From the knowlege gained from those, try different depths with your favorite response from the first group. Then perhaps try different starting throat diameters. You might take a look at the results graphs in my tweeter waveguide experiment posted at HTguide. They might give you some idea of what to try next.

    The good news is you got some good boost down lower than I would have expected and you filled in that dip in the response at 1.5khz both on and off axis, so you did get some improvements if you were to cross low enough.
    Dan N.

  13. #13

    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Yeah the dip at 1.5khz did fill in, and I did get gain but damn...cutting wood now for new baffles...

    I'm going to make a spacer to make this waveguide deeper. Then route it out so it is shallower. Then a new one with basically the same roundover and whatnot, but a larger cutout, so the edge comes to end at the outside edge of the surround, instead of close to the dome. Maybe try this with a smaller roundover too.

  14. #14
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Quote Originally Posted by augerpro View Post
    Yeah the dip at 1.5khz did fill in, and I did get gain but damn...cutting wood now for new baffles...

    I'm going to make a spacer to make this waveguide deeper. Then route it out so it is shallower. Then a new one with basically the same roundover and whatnot, but a larger cutout, so the edge comes to end at the outside edge of the surround, instead of close to the dome. Maybe try this with a smaller roundover too.
    I like the effect below 3KHz for sure.

    How tough would it be for you to try a simple conical waveguide?

  15. #15
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Quote Originally Posted by augerpro View Post
    Surprising amount of gain from under 1khz to 3khz before everything goes to hell. Didn't expect that just like I didn't expect the big dip at 3.8khz either. 3khz wavelength is 3.5" so I wonder what is reflecting where?
    3.8K is about 3.6", 1/2 of that distance for total path length gives the maximum interference. The dome radiates as an integral of all of the surface area. The roundover reflection distance is an integral of its area as well for any individual point on the dome. All points on the dome that have a direct line to the roundover contributes. There's apparently an average of the integrated area of the dome to the roundover integrated surface area of approximately 1.8" for the delay that results in the 3.8K dip.

    Not surprising for a 3" dome recessed in a roundover. It should change rather dramatically in the off-axis as your measurements show that it does. This is a good way to verify that it's an external reflection of a driver rather than an internal one.

    I suspect that the dome has to recessed a LOT more to minimize this. The odd peaks/dips of a lot of tweeters is due to this same phenomenon.

    dlr

  16. #16
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Since this driver is really best used no higher than 3KHz, this waveguide may not be such a bad thing after all.

    The response in the usable band, 500Hz - 2500Hz is improved over the non-waveguide version, and you get a bit of a sensitivity boost as well. The off axis response looks good too.

    I wouldn't throw this one out yet . . .


  17. #17

    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    I does do some things well, the gain across its usable passband, and the 1.5khz dip is smoothed out. I think I see what Dave is saying. I had planned on these large roundovers to get it to flare away from the dome as quickly as possible but if we think of the walls of waveguide as a radiator-and out of phase from the dome- maybe the answer is to get the walls nearer the dome. Is this what you were referring to Dave by recessing it more? What I'm thinking is it will probably always have that dip, but maybe I can push it up in frequency. Maybe get lucky and knock out that 7khz peak acoustically.

  18. #18
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Quote Originally Posted by augerpro View Post
    I does do some things well, the gain across its usable passband, and the 1.5khz dip is smoothed out. I think I see what Dave is saying. I had planned on these large roundovers to get it to flare away from the dome as quickly as possible but if we think of the walls of waveguide as a radiator-and out of phase from the dome- maybe the answer is to get the walls nearer the dome. Is this what you were referring to Dave by recessing it more? What I'm thinking is it will probably always have that dip, but maybe I can push it up in frequency. Maybe get lucky and knock out that 7khz peak acoustically.
    Push it too far back and you lose the AC offset you were attempting to "correct." You'd then have to also offset the tweeter, which I suppose isn't too bad as it could go in a waveguide as well.

    Still, I kind of like what you have there already. I was planning on crossing lower, simply because of CTC with the tweeter.

    I'm looking forward to your further experiments!!

  19. #19

    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    I'm curious as to what happened to HD with the guide? I bet F2 and F3 are way down from 1khz to 400hz.

  20. #20
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    Default Re: TangBand 75-1558SE on a Waveguide with Measurements

    Quote Originally Posted by augerpro View Post
    I does do some things well, the gain across its usable passband, and the 1.5khz dip is smoothed out. I think I see what Dave is saying. I had planned on these large roundovers to get it to flare away from the dome as quickly as possible but if we think of the walls of waveguide as a radiator-and out of phase from the dome- maybe the answer is to get the walls nearer the dome. Is this what you were referring to Dave by recessing it more? What I'm thinking is it will probably always have that dip, but maybe I can push it up in frequency. Maybe get lucky and knock out that 7khz peak acoustically.
    In looking at pictures of the ATC (I haven't seen it up close for years) it seems to be a bit more recessed (could easily be wrong there) as well as the walls having a bit less curvature. The latter actually seems counter-intuitive at first. The problem may have more to do with the damping in the dome. The ATC always looked to be extremely damped. This limits its top end extension, but at the same time it reduces the output higher up the dome where the reflections they create are more exacerbated. Damping that reduces the amount of reflected energy. They're down 3db at, what, about 5K? My guess (and that's all it is) is that they roll off the dome output early to minimize radiation in the top area for this reason.

    Were it to be recessed more, it would be closer to a horn loading, but it may push the reflections/dips higher in frequency. The closer the baffle rollover is, the higher they'll be, as you suggested. This kind of problem is the major problem for tweeters in coaxials. I hesitate to call it a waveguide, I don't see it as such in this application, especially given the provenance of that descriptive in audio. As Pete said, keep the lowpass at 2.5K or so and it should work reasonably well there.

    If you spread out the opening, you may just push the interference down in frequency, though I would guess that it will be a bit lower in amplitude as well. It's a tough call.

    I'd consider trying to sketch it out in profile. See what the geometry looks like from the side. In the end I can really only make guesses as to what might work.

    dlr

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