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Thread: first post/ Bob Smith horn w. horizontal "CD" contour : Is such a thing possible?

  1. #1
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    first post/ Bob Smith horn w. horizontal "CD" contour : Is such a thing possible?

    Hello all,


    I only recently discovered this great site and forum and spent the last two weeks browsing through quite some threads (until my wife and kids started feeling neglected and made me stop...). Some of you guys out here are pretty serious, indeed!


    Quick introduction:
    I have been an avid musician for all my life (playing the piano and organ) and I also listen to a lot of recorded music of all periods and cultures, with Bruckner and Mahler having for long been the main staple of my daily diet.
    I became interested in pro audio technology in the mid 1980s when I did a six-month internship at a large public broadcasting station.

    In 1989 I bought a pair of 4430 monitors, cheaply, from the above mentioned station, and this eventually got me into the rewarding hobby of DIY loudspeaker building.


    My approach on building loudspeakers has, for the last ten-odd years, been centered around three-way systems consisting of 15in / 10in / 1in + 90deg. CD-horn, electronically crossed at ~300Hz and hi-level XO / CD-EQ at ~ 1500Hz. (i.e. DIY 4430s with add. dedicated mid-driver and adjusted mid / hi XO)


    Current (since 2006, I think) driver compliment: Beyma SM115/N in a SBB4ish 200 l. (sorry, i'm a metric guy...) 30Hz enclosure; Beyma 10Mi100 in a 20 l. sealed sub-enclosure; Beyma Cp350Ti on Beyma TD194 (bought last year) horn on top (no, I have no affiliation with Acoustica Beyma, but there's this guy, friend of mine, you know, who usually gets... ah, never mind...)


    Now, to the subject of my post:


    There is one thing I find particularly sexy about the Bob Smith horn (besides it beeing comparatively easy to build), and that is the narrow vertical dimension. This would allow for, given the 10in midrange and 1500Hz XO of my speakers, to get the vertical spacing of the acoustic centers (mid and hi, that is) to approach lambda/2 at f(xo), which should, taking Linkwitz' expression of [arcsin (lambda/2d)] as an approximation, result in a neat vertical lobe and an acoustical point-source behaviour around f(xo).


    However... the Bob Smith design and its commercial JBL adaptation are simplified multicell exponential horns, and I am a „constant directivity“ guy and a big admirer of Keele's papers on the subject.


    So, this is what I have been thinking about for the last year (without actually trying it yet): What do you think would happen, if one threw out the distributing vanes of the Smith design and instead would incorporate some constant directivity contour to the sidewalls (simply a circular arc joining the throat with 90deg conical „straight“ sections [as suggested by Hughes] and some secondary flange at the mouth (out to 150deg. or so) to reduce reflections?


    I know, it's a bit weird to mate a design that tries to avoid diffraction at all costs (like Hughes') with another, that takes diffraction as an integral part of its functioning principle (Smith's), and also Keele's math doesn't really allow for a vertical expansion rate of zero, but the idea somehow intrigues me....


    Maby one of the many guys here that are more knowldgeable than myself can set me straight on this one?


    Anyhow, it's great to be joining this fascinating forum and I hope that time (read: wife, kids, job, thesis...) will permit for my regular participation.


    Thanks for having me,
    best regards,
    Chris

  2. #2
    Super Moderator yggdrasil's Avatar
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    Interesting idea.

    I would be careful with the curve on the side walls, as this will limit horizontal dispersion. Didn't I read somewhere that the CD dispersion is effective to the point where you no longer see the driver exit?

    Regarding the secondary flange - you could e.g. try (at least) 5cm/2" top and bottom, and take the Westlake'ish approach with a straight cut. (can make a drawing later if this is unclear).

    Johnny

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    Johnny,
    thanks for your reply,


    true, every part of the horn that is not „illuminated“ as seen from the center of the driver's exit / i.e. the apex of the angle that defines the basic coverage pattern of the respective plane (in my case, horizontal) is, at least according to Geddes, lost for directivity control.


    Hughes (2000 : http://www.freepatentsonline.com/6059069.pdf)
    and
    Geddes (2003 : http://www.freepatentsonline.com/7068805.pdf)


    both "waveguide" approaches, mate, to my understanding, the throat with the straight walls of the conical portion that defines the coverage angle by using a circular arc from the throat perimeter to a point on the sidewalls where it is congruent with the coverage angle. The plane acoustical wave at the driver's exit is thus „bend“ into a spherical wave that is at right angles with the sidewalls and the entire conical section is thus illuminated.


    As for the secondary flange, in Keele's as well as Henricksen and Ureda's approach on CD-horns it is used for pattern control and to cut down somewhat on overall dimensions, while Geddes' „prolate spheroid“ waveguide uses a flare at the mouth only to reduce reflections.
    But anyway, as you have pointed out, the secondary flange is shadowed by the first section and thus no longer illuminated as seen from the driver's exit, and, maby most importantly, the soundwaves are no longer in right angles with the horn walls.


    I'm not sure if any of this is applicable to the basic geometry of the Smith diffraction slot horn, however, as it is more of a, so to speak, two-dimensional approximation of a circular sector than a three-dimensional approximation of a spherical sector, as horns with a vertical expansion rate >zero can be described as... and I somehow don't like the idea of building something of which I don't have at least a superficial understanding... but then – maybe I should just give it a try and see what happens....


    If i got you right, your suggestion is to simply use a pair of wedges or vanes between the top and bottom plates as sidewalls and using the 45deg. straight cut that Westlake uses in the vertical plane also in the horizontal as secondary flange?
    Would certainly be very easy to make, and would give the whole thing a cool „mantaray-ish“ appearance...


    I think I'll start doing some drawings....


    best regards,
    Chris

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    ... so, obviously my little idea doesn't seem to stir a lot of interest here,


    anyway, those who would like to join this thread (hopefully some of the more knowledgeable HF-horn-people might drop by...) will find the following of interest, as it was these designs that somewhat got me started into thinking about a „horizontal constant directivity waveguide / vertical diffraction slot horn“ design that could be both, of higher performance and even easier to construct than the Smith horn:


    1. of course, the Bob Smith „Distributed Source Horn“/ JBL 2397; well known, I think, to most contributors here, a lot of info available on this site and on the web. Relatively easy to DIY. Principle: Exponential multicell horn in the horizontal plane, diffraction slot based on the Huygens-Fresnel principle of wave propagation (mostly associated with optics, but also largely valid in acoustics) in the vertical plane.


    2. Electro Voice 8HD and 6HD; also a 1950s design, as used in the EV Regency II and Patrician IV, respectively. Principle: Exponential horn in the horizontal plane, as above in the vertical.
    The data sheet below recommends to use th 8HD „upright“, that is, using the diffraction slot in the horizontal plane, like the JBL 2405H tweeter. The earlier designs mentioned above used it the other way round.


    See: (http://archives.telex.com/archives/E.../8HD%20EDS.pdf)




    3. JBL 2370A; Keele-type „Bi-Radial“ horn, slot loaded, compound conical/exponential CD-contour in the horizontal plane; very narrow, somewhat-exponential contour in the vertical plane, that, to my reading of the vertical polars and the construction diagram, could probably also rely, at least to some degree, on the diffraction principle below 8-or-so kHz in the vertical plane.


    See: (http://www.jblpro.com/pages/components/2370a.htm)




    So, any comments on this would be highly appreciated.


    Ah, yes, another idea about this. Earl Geddes has been using 30ppi foam in his waveguides to reduce what he refers to as „higher order modes“; that is, soundwaves bouncing arround more or less arbitrarily inside the horn bell have to travel a longer path than the „direct“ sound and are thus attenuated more by the foam plug then the direct sound.
    The same principle could, I think, be employed to reduce the standing waves and flutter echoes that appear in a horn with large parallel top and bottom surfaces (Smith) at very high (>~12kHz with 25mm slot) frequncies....maybe....


    comments, anyone?


    Best regards,


    Chris

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    Senior Member 1audiohack's Avatar
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    I'm not so sure it's a lack of interest, you have given a fair amount to think about and it is obvious that you have thought about it more than casually. There are many rules to live by when designing horns and it seems to me that often the "next generation" breaks some of the previous rules and or borrows from the past to boot.

    JeffW stuck a 1.5" driver on the back of a 2397 recently, his adapter replaced the throat and it has radius' on the side entry not unlike what you are thinking about though it still has the conical side walls. The 2397 entry without the throat is about 4" wide, if you were to fill in the outermost entry slots it is 3" wide. You may be able to experiment with some rigid foam or something by making some curved wedges that fill the ends and contour out to the end as you wish and make an adapter like Jeff's with the calculated radius as described by Charlie Hughs, and just see what you think.

    I haven't read Keel in years but listen through his lenses daily, I think I have every bi-radial made by JbL haha. I have studied Charlie's quadratic horn papers and from my point of view that throat geometry just looks right.

    Maybe Jeff will tack a picture here.
    If we knew what the hell we were doing, we wouldn't call it research would we.

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    A pic of what, my fabulous plywood adapter?

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    Senior Member 1audiohack's Avatar
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    Yup. Or a look down it's mouth, I thought it was cool!
    If we knew what the hell we were doing, we wouldn't call it research would we.

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    1audiohack,
    thanks for your thoughts on my idea. You don't seem to have any theoretical objections against my basic concept. That's encouraging. I somehow feared there would be a number of horn pros jumping up, shouting "combining a CD-waveguide and a diff-slot? Nonsense!" ... but until now nobody seems to have any moral, ethical or physical objections against this approach...

    JeffW,
    I'd certainly be interested in seeing a pic of your custom 2397 throat!


    So,
    instead of wasting other people's time in a, to this point, merely theoretical evaluation of my ideas, I will get to work and see if I can make a useful simulation of the thing in either MatLab or HornResp. As soon as I've got the basic maths straight (shouldn't be that big of a problem, other people, Smith, Keele, Hughes, Geddes, et al., have done the hard work long before me, and I'll just have to basically pick what I need from their equations to get some reliable predictions), I'll build a prototype to measure response. This will certainly take me several weeks, maybe months (so much other stuff to do...), but after that I'll be back with, hopefully, something more substantial to talk about.

    'till then,
    best regards,
    Chris

  9. #9
    Senior Member Hoerninger's Avatar
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    Quote Originally Posted by ChrisKoehn View Post
    I'll build a prototype to measure response.
    That will be the main task and flattening the frequency reponse.

    There is no generalized theorie for horns. All these different design ideas need validitation in reality. For example tractrix horns have quite a directivity at higher frequencies - the tractrix is first of all a mathematical approach and not a acoustical solution.

    Good luck, laß Dich nicht entmutigen.
    ____________
    Peter (close to Hamburg)

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    OK, here's the pics.

    First off, I wouldn't recommend using plywood. It's what I had handy at 10 o'clock and I was mainly interested in just giving these drivers a listen. I had these horns handy and a couple of dimensional checks showed that the 435AL would be pretty easy to mount up to this 2397-ish clone.

    The pictures give the illusion that the slope on the inside of the adapter is rough or stepped, but it's actually pretty smooth. The angle of the slope matches the angle on the inside of the rear face of the horn pretty closely. With a 1.5" holesaw to make the hole and tapering down to as thin as practical with the plywood, the 3/4" thickness of the ply worked out pretty nice.

    While I still haven't gotten any measurements, I have really come to love the sound of these drivers and adapters on these horns. They are crossed in at 1KHz, crossed out at 8KHz, basically replacing the 175Nd driver/horn on the 800 Array. Even with the 435AL way too far back to be aligned with anything, they just sound great. They have gained semi-permanent status for now.
    Attached Images Attached Images      

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    JeffW,
    thanks for the pics and description, neat and simple, like it!

    Peter,
    Danke fuer den Zuspruch!

    So I fiddled around on paper with a few parameters and variables, and it seems that the CD-"waveguide" approach (Hughes/Geddes et al.) won't do the trick for horizontal pattern control in a "Smith"-type diffraction horn.
    It's not possible to go a lot narrower than the 140° angle of the 2397 horn, as this is the closest approximation to exponential area growth you can get with parallel top and bottom and straight sidewalls.
    With a narrower angle (<120° or so) the driver wouldn't be loaded properly and there would certainly be excessive ripple as f approaches the horns cutoff.... while with a 120°-140° horizontal angle, using Hughes' / Geddes' approach, the sound waves would most probably not "stick" to the sidewalls at higher frequencies, so it won't be constant Q / CD, and that was what my whole idea was about in the first place.... this is getting a lot more difficult than I thought ...

    regards,
    Chris

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    Now, after spending some more hours trying to save the basic concept of my initial idea, I have to admit: It sucks...big time! Even just on paper! The whole thing just won't work. No sense even in building a prototype.
    Getting horizontal constant Q with a „waveguide“ approach within the basic „parallel top and bottom surfaces“ confinement of the Smith horn while at the same time giving some proper loading, let alone plane-wave-tube-ish-mostly-resistive loading, to the compression driver is simply not possible. Either I run out of acoustic impedance or I run out of HF (~>6kHz) pattern contol. It just won't do it. Period. Sad but true...


    However....
    While the „waveguide“ approach to horizontal constant Q behaviour won't work - and the following is a fundamental departure from my initial idea/post - a more traditional (i.e. Keele-ish) „CD-Horn“ approach, employing a diffraction slot, close to the throat, also for horizontal pattern control, will, at least on paper, in all probability do the trick.


    This would save the basic idea of „even easier to construct for the audio hobbyist then the Smith-Horn“, while sacrificing the „measurable or subjective improvement in fidelity through the use of a modern waveguide“ side of my approach.


    But then, this would still mean only one diffraction slot compared to the de facto six 'slots' in the original Smith design – probably still an improvement in „sound quality“, almost certainly one in pattern control.


    So „why not use a diffraction slot for horizontal pattern control? The Smith approach already prescribes a diff-slot at the mouth for vertical!?“ certainly is a valid question here. The basic idea of initially not using the Keele approach in the horizontal plane was simply this – and this is my „pet theory“ = mostly semi-educated guessing, so please take it with a large spoonful of salt - :


    With the Smith horn, the diffraction slot works (almost, leaving back-reflections into the hornbell aside) entirely into the monomodal propagation of „free air“, while the slot in „standard“ slot-loaded constant directivity horns works into the multimodal propagation inside the horn bell, thus causing arbitrary reflections, or what Earl Geddes would probably refer to as „higher order modes“. So, to my understanding, the diff-slot of standard CD-horns is a possible source of “colouration“, while the vertical diff-slot of the Smith-design is not, or to a lesser degree.


    However, taking that approach, that is using a diffraction slot coupled to a conical/exponential flare in the horizontal plane, all the numbers now start falling into place beautifully. I get a nicer and more manageable 110° basic horizontal pattern while maintaining a pretty neat approximation of a M=0,7 HypEx area expansion which should load a given driver quite well....
    (all this is for a 2'' exit, 4'' diaphragm driver, I've got a single B&C 1050 sitting on the shelf that I'll use for proto... : 20x50 HxV mm „initial slot“, 50mm V „main slot“, 400mm axial length of horn, 700mm at it's widest).


    And, well, how shall I put this......, This thread is, with some appreciated exceptions, until now mostly a monologue. I had hoped for a little more participation and contributions at a site dedicated to those companies that made the compression driver the staple and standard of the professional audio industry for almost seven decades and that refined the basic concept over and over and over again. Now here's an approach that will redefine the very art of DIY horn building for...ahhhm... hours to come! So: participate in this endeavour, as such an opportunity will offer itself to the audio enthusiast only once in ...a week, or so. Just imagine all the girls at your doorstep, asking „aren't you that guy everyone's talking about? The one with that really BIG, rigid, powerful, acoustically inert.... HORN? May I touch it?“
    Wouldn't that be great!?


    Best regards,
    Chris

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