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Thread: drawbacks w/ constant directivity horns

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    drawbacks w/ constant directivity horns

    Hi,

    So called constant directivity horns for example narrow its radiation between 500Hz and 1kHz, but widen it again above 1kHz(1). With the 1.5" "optimized aperture" design off axis level exceeds the on axis level by several dBs between about 3kHz and 12kHz(2). The latter combined with the narrower radiation below causes a serious inbalance.
    As most people wouldn't listen on reference axis this may be an issue. At home and especially with horns the reverberant field can't cure that. To tow the speaker in isn't practical due to sheer size of common (to us) cabinets.

    Any idea?

    Thanks a lot


    (1) This is true with the groundbreaking EV HR90 up to now with JBLs PTWaveguides. Not to forget E.Geddes - his famous design shows the effect too, which makes "higher order modes" a/k/a HOM apparent at a glance to the educated reader 8-]

    (2) http://www.jblpro.com/pub/obsolete/23525354.pdf , "Horizontal Off Axis Response"
    Last edited by mini; 11-18-2008 at 04:44 AM. Reason: sorry, my US English nowadays is worse than ever

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    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by mini View Post
    Any idea?
    Crossover filter "fixes" the dip at the low end.

    EQ the high end where you listen.

    I'm not seeing the actual on-axis frequency response published anywhere there....

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    Quote Originally Posted by Zilch View Post
    I'm not seeing the actual on-axis frequency response published anywhere there...
    Hi,

    I've posted a reference to JBLs datasheet regarding the 235x series.

    (2) http://www.jblpro.com/pub/obsolete/23525354.pdf , "Horizontal Off Axis Response"

    With the 90°x50° waveguide off axis level exceeds on axis level by several dBs between 3kHz and 10kHz. Second to that level below 3kHz goes down with listening angle as it should with a CD design. This effect comes up with off axis angles as low as 15°.

    You don't see the inbalance e/g using this device as intendet from 600Hz up to ~20kHz? Equalization to flat response at some special angle would in any case alter linearity near besides that angle quite a lot. That in consequence gives a definite "sweet spot". Ain't it a contradiction to what "CD" is meant to do?

    I wonder why this hasn't been discussed yet regarding home applications of medium to full size CD devices. May be in this respect the older horn/lens 2..3-way combinations do better?!

    Juergen

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    RIP 2011 Zilch's Avatar
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    The on-axis response is "normalized" to perfectly flat there, so we don't know the actual frequency response. Do we suppose the response above 0 dB is coming from spontaneous generation?

    It doesn't tell us much more than how the horn behaves with the particular driver and mouinting alignment employed for the testing. Until it is used in a final system design and measured, the specs provide only guidelines for comparison to alternatives.

    Once you EQ the system to the preferred listening angle, the minor deviations shown become factors only in the reflected soundfield, in which context a variation of 1 - 2 dB would be swamped by other variables such as whether the drapes are closed or not or even if and where the dog is in the room.

    Compare these specs to the measured polar response of the old exponential horn/lens combinations, ignoring the fact that they have long since been abandoned for other reasons as well.

    You want "perfect" CD, use the Geddes oblate speroid waveguide; he tells you to EQ that on the listening axis as well as NOT to listen on-axis, for reasons most obvious. His polar measurements of the $9.90 JBL PT waveguides I sent him, using the cheapest compression drivers I could find, are posted on his website:



    Finally, recognize and appreciate that you're looking at pro SR horns and waveguides here, which we "adapt" to home use DIY. For JBL's best efforts in horns intended for home use, you must look to their TOTL consumer product offerings and clone those.

    Quote Originally Posted by mini View Post
    I wonder why this hasn't been discussed yet regarding home applications of medium to full size CD devices. May be in this respect the older horn/lens 2..3-way combinations do better?
    There are plenty of other forums rife with endless techno-babble about the comparative virtues of one approach versus another. Sure, do Tractrix, if you want, or Le C'leach, or quadratic whatever. Bottom line, at some point, the theoretical speculation must end and the rubber hit the road, or it's all just so much wank and blather.

    DO it, measure it, and listen to it....


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    Administrator Robh3606's Avatar
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    Hello Zilch

    Good post reminds me of "Just do it"

    Rob
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    "I could be arguing in my spare time"

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    Senior Member Ian Mackenzie's Avatar
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    Quote Originally Posted by mini View Post
    Hi,

    I've posted a reference to JBLs datasheet regarding the 235x series.

    (2) http://www.jblpro.com/pub/obsolete/23525354.pdf , "Horizontal Off Axis Response"

    With the 90°x50° waveguide off axis level exceeds on axis level by several dBs between 3kHz and 10kHz. Second to that level below 3kHz goes down with listening angle as it should with a CD design. This effect comes up with off axis angles as low as 15°.

    You don't see the inbalance e/g using this device as intendet from 600Hz up to ~20kHz? Equalization to flat response at some special angle would in any case alter linearity near besides that angle quite a lot. That in consequence gives a definite "sweet spot". Ain't it a contradiction to what "CD" is meant to do?

    I wonder why this hasn't been discussed yet regarding home applications of medium to full size CD devices. May be in this respect the older horn/lens 2..3-way combinations do better?!

    Juergen
    I think the questiom must be related to the end use application.

    For a stadium or concert hall where to want sound to each everyone in the audience yes there are benefits.

    But for home use the term CD is too much of a generalization for the way the current crop of JBL Consumer horns are designed .

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    Quote Originally Posted by Zilch View Post
    DO it, measure it, and listen to it....
    Sheesh, Zilch - is there *any* JBL 2-way combo you *haven't* tried yet??

    John

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    Seems I'll be waitin' a VERY long time for my 476BEs to arrive here....

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    Quote Originally Posted by Zilch View Post
    The on-axis response is "normalized" to perfectly flat there, so we don't know the actual frequency response. Do we suppose the response above 0 dB is coming from spontaneous generation?
    Sorry, I won't get this. The + does come up cause of "higher order modes" that alter the (intended) zero-order spherical radiation pattern.


    Quote Originally Posted by Zilch View Post
    Once you EQ the system to the preferred listening angle, the minor deviations ... and where the dog is in the room.
    If equalized to flat on axis at 15° off axis You have to face a step of +6dB @ 3kHz that extends up to 10kHz. That assumeably would be apparent even to the least demanding listener.

    Quote Originally Posted by Zilch View Post
    You want "perfect" CD, use the Geddes oblate speroid waveguide;
    These too show serious "HOM" - HOM may be definded simply as deviation from the intended radiation pattern (sperical, plane ... whatsowever), expressed in orthogonal functions, alike Fourier analysis. T'm convinced that the ondulations of Geddes' OSW in frequency response over angle ain't intended. They do no that much better than conventional devices. That shouldn't suppress the merits he earned with theortical findings - great work!

    Quote Originally Posted by Zilch View Post
    the $9.90 JBL PT waveguides
    O/k, The curves I saw didn't look that good as Yours here. It's a pitty they aren't useable with lower cutoff. In Germany they can't be bought too.

    BTW: all above didn't "bring the rubber to the runway" What to do if one owns the "optimized aperture" somethings already? To equalize to flat on listening axis would affect nearby listening positions. That is my practical finding with these. Remember that I swaped the diaphragms of the 2447s already to the plane ones of my 2445s.

    Thanks a lot anyhow

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    That's certainly not how Geddes defines HOM, and like "Waveguide," I would presume since he believes he coined the term he is therefore entitled to define it. Go over to the forum he frequents and suggest that his waveguides are a veritable festival of HOMage, and see how he responds.

    ********

    Those are not my curves, they are HIS curves. Go back and look at his report again, but this time, scroll down to the EQ'd comparative data; that's where those I posted above may be found.

    Have you asked Guido if he can obtain these cheap waveguides for you? They're also available with 1.5" throats, but not so cheap. You may be surprised to discover how low those might actually be used with certain drivers. There's actual curves posted for them here on LHF, and not just by me.

    ********

    What's that in the upper left of my pic above? Have you ever heard one?

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    Senior Member Ian Mackenzie's Avatar
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    Quote Originally Posted by 4313B View Post
    I hope you don't plan on bragging on about them or posting something retarded with respect to them.

    .

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    No perfect WG yet

    Quote Originally Posted by Zilch View Post
    That's certainly not how Geddes defines HOM, ...
    Those are not my curves, they are HIS curves.
    Great,

    You've sent him the w-guides, that he subsequently measured. On the other hand I would insist on what HOM has to be meant with. He describes his finding of OSW as that the solution of the wave propagation equation in special coordinates can be found in 1 dimension. In spite of solving the 3-dim equation for complicated shapes of the horn he solves the easier equation for a specific geometry. The result should be a perfectly spherical waveform at the mouth with all frequencies.

    That isn't true with his own designs. Not only is the frequency response contaminated with ondulations from internal interferences but this too depends on angle. That's what "HOM" makes, isn't it? O/k, what about perturbation analysis? Why not describe deviations from a perfect waveguide/in-wave/out-wave with a - example given - Fourier-series? Find the transformations! That is what I would expect when someone claims to deal with Higher Order Modes.

    Yes, I've got the 235-2 WG with the intended driver myself. It's not bad but I wondered if anybody had made the same observations as me.

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    I can only suggest taking the HOM issue up with Geddes himself.

    I run the #3 and #4 drivers on 4313B's above list on 2352s, and find them to be "Transcendent" using either of those.

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    Quote Originally Posted by 4313B View Post
    You like them better than those super expensive sonoglass Consumer horns that no one can get even if they wanted them!?
    Nope.

    Those are "uber-transcendent."

    Quote Originally Posted by 4313B View Post
    Why don't you hook toddalin up with some of those 2352's and the 2452H-SL's I have for sale and save him from those nasty old fog horns with the plastic lenses?
    You have 2452H-SLs?

    Would you run them on PT-F95HF (the combination I'm mostly building with) and post the curves in an appropriate thread, please, in comparison to 476Be and 435Be?

    I forget where you posted those, but you suggested that I send you 2452H-SL to try the same.

    *******

    I don't know, but I certainly would have dumped the potato mashers as well, long ago. Todd and I have had this discussion, actually, with respect to potentially switching to PT waveguides.

    I BELIEVE 2352s would generate WAF issues....

  15. #15
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    Quote Originally Posted by 4313B View Post
    I haven't opened the boxes yet. The person who ordered them lost his job and I can't return them for credit if I open them. If I get socked with a restocking fee, which is fairly likely, I'll probably keep them and run your curves.
    I can still send you a pair to measure, if necessary.

    Let me know....

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