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Thread: Experimental mandola with internal horn, neutral tension bridge

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    Default Experimental mandola with internal horn, neutral tension bridge

    I just finished building an experimental mandola.
    I was worried… this whole instrument was an experiment, in two major respects:
    1) I built an exponential horn into the internal body structure (hence the name “horndola”.)
    2) I used a neutral tension bridge configuration, with absolutely no bracing on the soundboard.
    Both experiments seem to have worked pretty well!

    I posted a few photos, and here is the link to a YouTube video (" A Different Mandola") that I made about the construction process. It is a bit long... I talk about the function of exponential horns and the benefits of a neutral tension bridge:

    https://www.youtube.com/watch?v=ZYYZaYI2v1g
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Much more elegant looking than the Stroh horns.
    "Bargain instruments are no bargains if you can't play them". These are the words of J. Garber.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    That's impressive -thanks for sharing.
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Wow, these things by Stroh are amazing!!! I have never seen these!!!!!!!!

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Quote Originally Posted by John Miller View Post
    Wow, these things by Stroh are amazing!!! I have never seen these!!!!!!!!
    He was trying to resolve a specific problem

    They are really interesting. By the way, I'm a Paul Klipsch fan. I have a set of K Horns in my living room that have been there since I bought them new in the 80's. I love the concept of using horns. I used to mess with building speakers and the concept of folding them up in small spaces has always appealed to me. I haven't seen your video yet but I'll look at it this evening.
    "Bargain instruments are no bargains if you can't play them". These are the words of J. Garber.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    This is absolutely gorgeous, and great engineering. Thanks for posting this!

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Fascinating video. Innovative design work and implementation. I'm sure this will drive a lot of conversation and I look forward to it.
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Thanks John. Great video and a very, very interesting design. Also beautifully executed. In some ways I wish you had separated the horn experiment from the neutral tension bridge/unbraced top idea so that the effects of the two could be distinguished. Do you think the very long sustain of the instrument is due to the latter?

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Fascinating. From both an aesthetic and an engineering standpoint. Excellent video production, too.
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Very nice, a great idea with wonderful results.
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Quote Originally Posted by tom.gibson View Post
    Thanks John. Great video and a very, very interesting design. Also beautifully executed. In some ways I wish you had separated the horn experiment from the neutral tension bridge/unbraced top idea so that the effects of the two could be distinguished. Do you think the very long sustain of the instrument is due to the latter?
    Tom:
    Great questions. Here is kind of a long-winded response.
    I am pretty sure that the horn doesn’t directly affect the sustain at all. It affects it indirectly by increasing the overall internal stiffness of the body. The stiffer the neck, body and sides (not including the soundboard!), the less energy is dampened by dissipation into the structural components, and the more stays in the soundboard and strings. And this body is EXTREMELY stiff, with the carbon fiber back, intermediate plate, internal spiral ribbon, and sides.
    >>>BUT, I think the major factors causing the sustain are related to the soundboard and bridge setup, as follows:
    1) Neutral tension is a major factor. I can move the tailpiece down by at least a quarter inch without causing any noticeable distortion of the soundboard, by adjusting the position of the nuts on the tailpiece posts you can see in one of the last images and then pushing down on the tailpiece. As I push the tailpiece down to any degree to increase the string break angle, the sustain goes down (along with treble and base amplitude AND harmonics).
    2) The lack of bracing is also a big factor. I can “fake” a brace by double-stick-taping a small carbon fiber (or spruce) beam at a variety of positions on the front of the soundboard. Anything I do like that decreases sustain and treble response.
    3) Another big factor is how I set up the bridge: note that there is no lateral bracing across the inside of the soundboard: just 2 T-nuts and small washers (1/32 inch carbon fiber.) And though it might be hard to see from the images, the bridge is offset from the top of the soundboard by small carbon fiber spacers (1/32”). That means that nothing about the bridge or under-bridge support is acting like a lateral brace, so that none of the “cross-instrument” vibration modes are dampened. I determined that this is important, too: I can just stick another set of spacers underneath the bridge to fill in that space, forming the equivalent of a spanning brace, and the sustain (and treble harmonics) go down.
    4) I found a 4th factor that, unexpectedly, affected sustain: the tightness of the beam against the strings on top of the bridge. This bridge is incredibly complex on the inside… I went to great lengths to make the whole assembly very light-weight, but with an adjustable-height saddle made from a sliding 2-piece wedge out of ebony, captured within a square carbon fiber tube with part of the top shaved off. I needed a clamping rod to go across the top, and needed to keep weight down, so I made it from titanium. (Not so difficult to drill or work with, actually!) During the setup, I get all the strings in place over the front and rear saddles (you need one on each end of the bridge to neutralize tension!) and then screw the Ti rod down onto the strings ove the groove between the saddles. And the tightness really affects sustain. Yikes. I can tighten to a degree where the sound is clean and good and the instrument is playable, but as I tighten further beyond that, sustain goes up for a few screw turns. Hmmm. Still need to figure that one out.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Nice results, thanks for sharing your experience on this interesting experience. Truly a fascinating subject. Your post #11 above is equally enlightening. Love the sustain you get with this construction.
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Very very very fascinating!... Great work!
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    John, I have a whole bunch of questions about this one. Not even sure where to start since you have put so much thought into the project. I guess the sound made an impression and that makes me very interested in hearing the instrument with Thomastic Prazision 174 flat wound strings. This type of string has less of a 'metalic' buzzing sound especially on the C course.

    Did you check the modes on the spruce top? Without braces I would think that the bridge itself creates it's own set of influences on the spruce.

    And the bridge 'footprint' is large for a standard size mandola. How was the size of the bridge determined? And the materials it is made from e.g. ebony, differ from the usual dampening properties.

    The chamber between the spruce top and carbon fibre horn, why is it a certain distance apart?

    Could a rod transfer the top vibrations to the horn as well as, or as a substitute for air pressure?

    Enough! (for now at least). Great project John. I can't wait for the next one.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Thanks John. Your observations on sustain give a lot of food for thought. The effect of tightening the bar on the bridge is certainly a mystery to me.

    Another question: have you checked the body or Helmholtz resonance? If so, is the frequency what you would expect from a deep body (ie. both chambers combined) or a shallow body (ie. just the top chamber)?

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Quote Originally Posted by DougC View Post
    ... the bridge 'footprint' is large ...
    If I'm interpreting the OP's post #11 correctly (the relevant part is quoted below), the bridge footprint is actually much *smaller* than normal:

    Quote Originally Posted by John Miller View Post
    ... the bridge is offset from the top of the soundboard by small carbon fiber spacers (1/32"). That means that nothing about the bridge or under-bridge support is acting like a lateral brace, so that none of the "cross-instrument" vibration modes are dampened. I determined that this is important, too: I can just stick another set of spacers underneath the bridge to fill in that space, forming the equivalent of a spanning brace, and the sustain (and treble harmonics) go down. ...
    He also discusses bridge stuff in the video starting at 14:23, and from 14:28 to 16:03 there is a side view of the bridge where, it looks like to me anyway, that there is very minimal contact between bridge and top. See the thin washers/spacers at the ends of the bridge - here's a zoomed-in view of a screenshot of the video, you'll have to click each pic several times to make it full size, you can see the small spacers:

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    It looks to me like the area of contact, the thin shims underneath the bridge, is considerably *less* than that of a regular bridge.

    (I hope I'm not crossing any lines about copyright or etiquette or something, by posting the two zoomed-in screenshots of the OP's video, if I've done something wrong please let me know so I can try to correct it.)

    Anyway, awesome instrument! I like the sound, and the idea of it is fascinating. Very innovative and scientific at the same time, good to see new workable ideas in instrument design, and very cool!

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Quote Originally Posted by DougC View Post
    John, I have a whole bunch of questions about this one. Not even sure where to start since you have put so much thought into the project. I guess the sound made an impression and that makes me very interested in hearing the instrument with Thomastic Prazision 174 flat wound strings. This type of string has less of a 'metalic' buzzing sound especially on the C course.

    Did you check the modes on the spruce top? Without braces I would think that the bridge itself creates it's own set of influences on the spruce.

    And the bridge 'footprint' is large for a standard size mandola. How was the size of the bridge determined? And the materials it is made from e.g. ebony, differ from the usual dampening properties.

    The chamber between the spruce top and carbon fibre horn, why is it a certain distance apart?

    Could a rod transfer the top vibrations to the horn as well as, or as a substitute for air pressure?

    Enough! (for now at least). Great project John. I can't wait for the next one.
    Hey Doug: Just got on and saw these questions, here and below.... will answer all of them that I can now.
    As for the Thomastic strings: Haven't tried any! But I will order some on your recommendation and post some sound files in a while. Will find somebody here in Bozeman that actually knows how to play the mandola... I am a rank amateur.

    I DID checked the modes on the soundboard before and after mounting bridge and strings. Just went back and found the laser vibrometer movies. I will make another YouTube showing the modes. And you are right: adding the bridge changed the mode shapes.
    Give me a day or 2, though, to get the movies into a YouTube.

    The span of the bridge was simply determined by construction constraints, not by any fancy calculations. I just extended the feet of the bridge assembly far enough out from the span of the saddle itself to allow incorporation of small attachment bolts. I have a new better design for a bridge assembly in mind, though... next gen. Or I encourage everybody out there to come up with their own. Look back at Richard Toole's original discussions and designs to get the basic idea, though:
    https://patents.google.com/patent/US7795516B2/en

    The materials used for the bridge assembly include a small ebony block, 2 small end-pieces of aluminum I machined, the titanium rod, 2 tiny bolts to hold the Ti rod down, 2 small nylon bolts to attach the bridge to the soundboard, and a short section of very stiff carbon fiber square tubing (1/2 by 1/2 inch) to enclose all of the parts. That is the most important part... a very stiff envelope for the saddle and aluminum end pieces. All efforts were aimed at making an assembly with the absolutely lowest possible mass. Hard to describe... if people are interested, I could mount a few pictures.

    The top chamber is 1 inch deep. I could have made it shallower (maybe 1/2 inch?) without changing the physics of the whole thing, but I made it that thick because of the way I decided to mount the neck. I needed the fingerboard to be elevated sufficiently above the soundboard for string clearance, but didn't want the neck to stick too far up. Figure I needed an inch thick base for the neck itself, so the obvious choice was to make the top chamber about that thick. I think the images near the end of the movie about the area around the neck mount will make that more clear.

    Hmmm... transfer rod? you mean between the soundboard and the intermediate plate? Don't quite know what you mean, but anything attached to the soundboard will cause mechanical damping.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Quote Originally Posted by tom.gibson View Post
    Thanks John. Your observations on sustain give a lot of food for thought. The effect of tightening the bar on the bridge is certainly a mystery to me.

    Another question: have you checked the body or Helmholtz resonance? If so, is the frequency what you would expect from a deep body (ie. both chambers combined) or a shallow body (ie. just the top chamber)?
    Tom: I have some cool laser vibrometer movies and spectra that I will collect into another YouTube sometime within a couple of days, that show the low-end resonances. And I will record and analyze a few taps. But I am not so sure that it will be very straightforward to interpret any of this within the standard "Helmholtz" context. Hmmm. The horn complicates that, I think. But I DO know the 2 lowest "twin" resonance frequencies: 125 and 192 Hz., which bracket the low C at 131Hz.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Quote Originally Posted by JL277z View Post
    If I'm interpreting the OP's post #11 correctly (the relevant part is quoted below), the bridge footprint is actually much *smaller* than normal:



    He also discusses bridge stuff in the video starting at 14:23, and from 14:28 to 16:03 there is a side view of the bridge where, it looks like to me anyway, that there is very minimal contact between bridge and top. See the thin washers/spacers at the ends of the bridge - here's a zoomed-in view of a screenshot of the video, you'll have to click each pic several times to make it full size, you can see the small spacers:

    Click image for larger version. 

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    It looks to me like the area of contact, the thin shims underneath the bridge, is considerably *less* than that of a regular bridge.

    (I hope I'm not crossing any lines about copyright or etiquette or something, by posting the two zoomed-in screenshots of the OP's video, if I've done something wrong please let me know so I can try to correct it.)

    Anyway, awesome instrument! I like the sound, and the idea of it is fascinating. Very innovative and scientific at the same time, good to see new workable ideas in instrument design, and very cool!
    Yes, you are right! These are very small contact areas relative to a normal (tensioned) set up. I could get away with small contact areas, because there is zero tension onto the soundboard. In a normal setup, the string pressure caused by the standard break angle would probably be enough to crush the soundboard wood under those tiny feet! Like stiletto heels on sod! And as I said above in another response, if I make that contact area larger (or push a shim under the middle of the bridge between the end washers), the treble gets damped a bit, and sustain goes down.

    >>> this means that with a neutral tension bridge, we could start using other cool cuts and species of wood that would normally be impossible. Like maybe some flat-sawn quilted maple? Might sound tubby, though.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Very interesting, John. You obviously have been spending much time studying stringed instrument acoustics and the complexity of the build is very interesting to me. I know it is near impossible to truly tell how the mandola really sounds from a video.

    Quote Originally Posted by John Miller View Post
    Wow, these things by Stroh are amazing!!! I have never seen these!!!!!!!!
    Honestly, I think your innovations are very far afield from Stroh-viols. I have one and it intended for focusing the sound of a bowed instrument into a microphone for acoustic recording. The violinist who plays with Tom Waits uses one of some cuts on some of his CDs. The tone is very hornlike. It doesn't sound like that was your intention.

    I think your innovations relate much more to the multiple tone chambered instruments, for instance the Paramount guitars built by Martin, the Martin Model America and the French Gelas guitars and mandolins:

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    The Gelas instruments have an inverse bridge (as opposed to neutral).

    I also refer you to Brian Dean's Grand Concert mandolin built for Joe Brent. That one has multiple tone chambers and a spruce internal soundboard.

    I wonder what it would sound like with a mostly wood construction even a solid wooden nautilus horn. Yours is really a carbon fiber instrument and the wood veneer on the body would be more for aesthetic purposes.
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Wow, more cool instruments. I had seen photos of the French Gelas inverse bridge instruments before. With the clamp setup, they really could have been configured for neutral tension, too. I wonder if he ever tried it?
    And Brian Dean's 10-string is absolutely stunning. My next instrument might actually be a 10-string horndola with a few changes I have in mind, based on things I learned in this first build.
    Yes, it would be straightforward to build one totally with wood... though very tricky to do a tight spiral, I bet. I had considered doing the whole thing with wood, but worried about internal stresses and strains, and didn't even know if the whole idea would work anyway. And since the whole body is essentially epoxied together, there is no way it is going to separate along any seams. I could use this thing as a war club to fight off a grizzly.
    A wooden one would definitely have more "character": not as stiff, and so would have a more complex spectrum.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    My guess about the longer sustain when the strings are clamped from the top is that it sets up a more sensitive 'reverse' transducer. Actually pulling up as well as down.

    I wonder how far away a top could be in order to energize the horn?
    The consideration for attaching the neck makes sense but I would think that 'distance from the horn' would be a bigger concern.

    As for a mechanical post from the bridge to the horn, that idea is used on Stroh violins and is a whole different system than this one. A much different design of the top of the John's horn would be needed to incorporate this idea.

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Quote Originally Posted by John Miller View Post
    Wow, more cool instruments. I had seen photos of the French Gelas inverse bridge instruments before. With the clamp setup, they really could have been configured for neutral tension, too. I wonder if he ever tried it?
    And Brian Dean's 10-string is absolutely stunning. My next instrument might actually be a 10-string horndola with a few changes I have in mind, based on things I learned in this first build.
    Yes, it would be straightforward to build one totally with wood... though very tricky to do a tight spiral, I bet. I had considered doing the whole thing with wood, but worried about internal stresses and strains, and didn't even know if the whole idea would work anyway. And since the whole body is essentially epoxied together, there is no way it is going to separate along any seams. I could use this thing as a war club to fight off a grizzly.
    A wooden one would definitely have more "character": not as stiff, and so would have a more complex spectrum.
    Amazing invention! I think Jim's thoughts about an all wood version are an exciting thing to think about. In that regard you mention the difficulty of bending the wood in tight curls to make the mouth end of the exponential horn. That is something anyone who has bent mandolin sides can appreciate. But I suppose you could go to some very thing strips of wood to make the horn thus making the bends easier because the horn would not have to be a structural part (or would it??).

    Also, I was wondering how loud this instrument is compared to a conventional mandola? The Stroh violins were actually louder (and of course more directional) than a normal wood violin thus helped the sound engineers trying to make early recordings. From your video it sounds like it makes adequate projection.
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    Default Re: Experimental mandola with internal horn, neutral tension brid

    Quote Originally Posted by Bernie Daniel View Post
    Amazing invention! I think Jim's thoughts about an all wood version are an exciting thing to think about. In that regard you mention the difficulty of bending the wood in tight curls to make the mouth end of the exponential horn. That is something anyone who has bent mandolin sides can appreciate. But I suppose you could go to some very thing strips of wood to make the horn thus making the bends easier because the horn would not have to be a structural part (or would it??).

    Also, I was wondering how loud this instrument is compared to a conventional mandola? The Stroh violins were actually louder (and of course more directional) than a normal wood violin thus helped the sound engineers trying to make early recordings. From your video it sounds like it makes adequate projection.
    All good ideas... will really have to figure out exactly how to proceed next.
    As for relative loudness: This one is pretty loud, I think. I don't have another mandola, but I do have a beautiful Weber 20" scale length octave, with oval hole. I have done a direct comparison by putting a capo on the 5th fret of the Weber, so it matches the range of my Mando. My mando is louder and brighter. But that might not be a fair comparison. I will take this down to Music Villa here in Bozeman when they get a nice Mandola in, and do a direct comparison...

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    Default Re: Experimental mandola with internal horn, neutral tension brid

    I’ve been asked by several people now, including friends through e-mails, about the thinking process that went into the design of the horndola, and also if I have measurements of the resonant characteristics of the instrument. So here is a little about the design. I will also post a YouTube of the activity modes of the soundboard sometime soon.

    As far as the design process went, it was a combination of guesswork, intuition and calculations based on a bunch of equations related to speaker design.

    Here was some of my thinking:
    As for building in an exponential horn: that was a no-brainer. I thought it would be a cool thing to try incorporating a horn, and so I just decided to build one in. There are other types of horn designs (check “Horn Loudspeakers” in Wikipedia), but the exponential ones seemed like the best option for a stringed instrument.

    Given that choice, I needed to come up with a viable design.
    The primary consideration was overall shape of the body, and again, the “no-brainer” was to model it on the shape of a simple logarithmic spiral, like a Nautilus sea shell, which is almost a perfect exponential horn shape.

    And starting with that shape, all I had to do was decide on the orientation and dimensions. I started with the overall width and depth I wanted for the body: 12 inches wide and 4 inches deep. The depth allowed a 3 inch-deep-horn and a one-inch-thick “compression chamber” under the soundboard.

    There are a bunch of great sites on line that present details on how to build exponential horns into speaker cabinets, with equations that allow calculation of how the length of the horn, the mouth diameter, and the throat diameter will affect the frequency characteristics. I rearranged the equations to allow calculation of how big I needed to make the throat and mouth ports, given the total internal length of the horn, and the lowest resonant frequency that would be produced.
    I ended up setting the parameters to aim for a low-end resonance of about 120 Hz, because the lowest note on the mandola is 131 Hz. (C3).
    >>> Amazingly enough, the final low-end resonance was a peak centered at 123 Hz!
    So I think the basic theory is sound (no pun intended).

    The spectrum of the horndola is attached below. This was measured with my buddy Chris Jenkin’s bazillion dollar Laser Vibrometer in his lab at MSU. This shows the actual velocity of movement of the horndola face, driven by white noise from another huge speaker. The instrument was all strung up, so this is a very good representation of its mechanical resonance characteristics (not really the same as its sound spectrum.) This just shows the “tap tones.”

    Note, then, that the whole “Helmholtz resonance” approach may not apply here, or at least not nearly as well.

    Here is the equation I used for horn design, in the format that could be used in a spreadsheet:

    F = ((log(Am)-log(At))*9210)/(L)

    Where F= low frequency resonance/cutoff (should be set lower than the lowest note!)
    Am = area of mouth,
    At = area of throat
    L = length along the horn
    >>>> ALL MEASUREMENTS must be in cm!

    The 9210 number is the lumped scalar factor that accounts for all of the variables that go into the calculations… stuff like speed of sound, factors of Pi here and there, Q of an exponential horn, etc.

    The final design of the horn port on the upper left side was partially esthetic, partially practical.
    The port could have come straight out of the side facing the player, but I wanted part of the hole to face up toward the soundboard, for 2 reasons:

    First, more of the bass comes out of the port, whereas most of the treble comes off the soundboard itself. So the bass projects better with some front exposure.

    Second, another really important part of the horn design is that the horn causes a “phase delay” of the sound wave front at the port relative to the sound wave front generated at the face of the soundboard. So there is no destructive interference of the low frequencies of the sound as there is in a conventional instrument.

    In other words, there are “modes” of soundboard movement in a guitar or mandolin where the soundboard is, say, moving out, but the air is moving into the sound port (or f holes), and that decreases the volume. For the horndola, the sound wave fronts coming from the port don’t interfere as much with the soundboard wave fronts, and can actually ADD to them, through constructive interference.
    That also goes for the side-to-side modes in my case. There are modes for all string instruments where, for example, the left side of the soundboard is moving in, and the right side is moving out… rocking back and forth along the axis of the strings. In a normal instruments, this would also decrease the amplitude: the air pressure waves kind of “recirculate” across the face of the instrument, instead of radiating out as much as they could. For the horndola, the offset, phase-delayed wave fronts from the port cancel out some of the recirculation, and yield more volume for those modes (I think…).

    I really hope some of you people try to build some of these things! I do NOT want to do any bull#### patent on any of these designs… and would love to see them tested and improved!
    Just keep me in the loop with cool photos and results!

    Click image for larger version. 

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