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Post by steverino on Jun 19, 2009 19:09:43 GMT
Mel, funny you should mention the biscuit single cone cover. A few months ago I did calculate the ratio of area of the center hole and all the sieve holes to the cone's piston area at about seven to one. We deal with things like this all the time in loudspeaker compression driver design. I had noticed a slight quieting effect when installing the coverplate. Acoustically I believe it functions as a bit of a low pass filter, curtailing the highest frequencies more than the rest. It is a minor effect though, and running the guitar without the coverplate exposes the cone to being damaged.
In terms of Helmholtz resonance though, our considerations are the interior volume of the guitar body and the area and thickness of the F holes. The coverplate is on the other side of the cone, which ideally seals tightly to the body around its perimeter and so prevents the air volume from communicating with the coverplate holes. It would be a different story on a mandolin that has the holes in the side of the cone well.
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Post by steverino on Jun 19, 2009 6:54:04 GMT
The StewMac files are great, though pricey. It is possible to improvise by rolling pieces of old strings or other metal rod materials of suitable dimensions between flat files to roughen them and in effect create slot files.
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Post by steverino on Jun 19, 2009 6:47:28 GMT
Wow, what a thread. Michael, your perspectives and comments on the round and square neck tri-cones are priceless, the sort of information that is almost impossible to find anywhere. Ditto for the thread containing Marc Schoenberger's discussion of Nationals; many thanks to 1928triolian for that one.
Tark, the box volume and ports certainly do function as a Helmholtz resonator. I guess my point was that the fundamental resonance of a loaded cone is too high to interact properly with the presumably lower resonance of the body. As an aside, my first exposure to Helmholtz resonance was in high school, when we would blow across the tops of our pop bottles in the dining hall, creating a tremendously loud high pitched noise that would drive the person in charge mad trying to identify the perpetrator.
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Post by steverino on Jun 18, 2009 7:20:59 GMT
I sure enjoy these meetings of the Armchair Engineering Society. I apologize if I have dragged things off topic, or into outer space even. It is just that in the world of loudspeakers I have seen so many examples of technological avenues going unexplored, or of really valid engineering concepts being underutilized then abandoned. In the world of resophonic instruments we have three basic designs, all springing forth from one brilliant fellow, John Dopyera. It strikes me that this realm could be, perhaps should be, broader by now. If any of this "wild hair" design talk leads to something useful then it will have been worthwhile, methinks.
The basic advantage of a resonator instrument, as I see it, is the relatively high mobility of the cone(s) surface. In brief, the aluminum can move further and more easily than the surfaces of a braced wooden guitar in response to string vibrations, and so produces a louder sound. I have heard resonator instruments described by my (admittedly warped) loudspeaker enthusiast friends as "louder output, narrower bandwidth" than wooden guitars. This may be the case, but strikes me that it needn't be so. The spider instrument I modified with a Schireson type bridge retains the loudness of a resonator, but its sound is more in the direction of a wooden guitar, and very pleasing.
In loudspeakers I'm a horn guy, so I haven't studied bass reflex theory too much. As I understand it, in classic BR theory, the box (Helmholtz) resonance is tuned to approximately the same frequency as the resonance of the cone in the box. That way the box resonance, which is 180 degrees out of phase with the cone, damps what would otherwise be a large response peak at cone resonance. With this technique the overall system response is usefully flattened and extended on the bottom.
One problem (or at least characteristic) of resonator instruments, it seems to me, is that the fundamental cone resonance is too high to usefully interact with the box resonance in the manner described above. In the (few) measurements I have made, the majority of the energy from the cone seems to be in the region of 200 to 600 Hz. Most of my work has been with spider bridge instruments, which I think have a higher cone resonance than most biscuit resonators. Even with biscuits, I would expect the cone resonance to be above 200Hz. when loaded by the strings. So, the frequency extension and flattening offered by the reflex loading goes largely unused, or at least it seems so to me. This situation is probably even worse with most wooden guitars, and there are many marvelous sounding instruments of both types, so perhaps this bass reflex analogy is flawed to begin with.
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Post by steverino on Jun 16, 2009 5:29:55 GMT
This issue of bass response from resonator instruments is one of considerable interest to me. IMO we have worlds yet to conquer!
Acoustically, a piston radiator (cone) of a given size needs to quadruple its excursion each descending octave to maintain constant power output. All resos built to date lack this ability, primarily due to excursion limiting from the string load sitting on the cone and greatly limiting its movement. I have found that limbering up a cone's suspension by thinning helps somewhat. Supporting the cone from underneath with a coil spring to take part of the load helps as well, but not enough. The cone must have enough downward force exerted on its outer edge to prevent buzzing, and this amount also seems sufficient to largely stifle the bass response. Grrr! If it were not for this situation we should have bass to burn, as we have a 9.5 or 10.5" cone in a bass reflex enclosure.
I built an experimental spider bridge reso with a 5' long 50Hz. exponential horn rear loading the cone. This was an experiment to try to raise the output in the low frequencies by 10dB or so, well within a horn's capabilities. It works to an extent, and growls with authority when I strum it in passing, but the design didn't increase bass output much relative to the midrange as I had hoped it would. What I had forgotten to consider is that a horn theoretically loads to infinity above its flare frequency, so the mids are elevated in level nearly as much as the bass.
Tark makes a good point that a larger body is needed if Helmholtz resonance is to effect greater bass output. The huge bass guitarron used in the Mariachi band comes to mind here.
To some extent our hearing fills in the missing fundamental when hearing a partial series that implies a really low note. What would be cool would be a reso that could sock the listener in the gut with fundamental bass energy, but we've a ways to go yet to get there.
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Post by steverino on Jun 15, 2009 22:39:27 GMT
Excellent post, Michael. There are a lot of us out here in more or less the same boat. Thanks much.
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Post by steverino on Jun 14, 2009 22:29:37 GMT
You can use an online string tension calculator to select string gauges for different scale lengths and tunings. I have found that this one works well: www.bangzero.org/stringtension/
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Post by steverino on Jun 4, 2009 17:41:54 GMT
Hi Michael, I have spoken with John Dopyera and his wife Margaret, both lovely people as you know. Some may not be aware of Margaret's book "My Life As A Dopyera" which provides great insight of the many family members as they made their way in America. It is available from Elderly Instruments and Amazon.com. www.elderly.com/books/items/557-1.htm
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Post by steverino on Jun 4, 2009 6:03:54 GMT
Hi Michael, the last thing I would want to do is challenge anyone's authority in these matters, least of all yours. I have done a lot of reading since becoming fascinated by resophonics a couple of years ago; perhaps my enthusiasm boils over a bit. You have been at this for decades and have no doubt seen many "instant experts" come and go in that time. I have been in touch with the gentleman you mentioned and he is a terrific fellow.
I guess my conjecture was that the reason we have a 9.5" biscuit cone is that Beauchamp hastily retooled the mandolin cone for guitar after JD left National and due to the excellence of the resulting instruments it is still with us today.
Mr. Timmins' carbon fiber tricone is so cool! I can see how CF would make for a great guitar body; light, stiff and strong. It is a shame that the long cycle times and high expense of producing CF items limits its use. I wonder if costs might possibly be brought under control in China... :^)
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Post by steverino on Jun 3, 2009 6:45:16 GMT
From the reading I've done, it appears that John Dopyera invented the 9.5" biscuit cone, which was used in National mandolins and tenor guitars before his departure from the company in early 1929. One might suspect that the size was chosen as the largest that a mandolin body could accommodate. Dopyera had rejected the single biscuit cone for guitar based on its sound. Within two months of Dopyera's departure George Beauchamp had built National guitars with the 9.5" biscuit cone and filed a patent on the design. This incensed John Dopyera when he found out, and he filed a lawsuit in response.
Decades later the OMI company, run by several of the Dopyeras though John was not directly involved, again built a single cone biscuit resonator. This time the cone was 10.5" in diameter, which allowed it to be interchanged with the spider cone in some of their models.
I'm not so sure about the prospects of a carbon fiber reso cone. In my loudspeaker business I make carbon fiber compression driver diaphragms. They are light and strong (so far so good), but the material also features considerably greater self damping than aluminum. This is fine for a sound reproducer where pure piston response over a broad frequency range is desired, but not so good for a sound producer that depends greatly on a variety of breakup modes to achieve high frequency response. I would expect a CF reso cone to sound very fast, clean and dull on top. This would not apply to a CF biscuit, which might work very well.
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Post by steverino on May 31, 2009 7:50:02 GMT
Colin I'm delighted that you got the Silvatone! Pictures would be great as would a sound sample if you should have the chance. I appreciate and share your interest in these lesser known resophonics. I'm on pins and needles at the moment as I have purchased a Schireson which is in transit. There is a lot of ground to cover yet as I have yet to put my hands on a sure 'nuff vintage National. My experience to this point has been in dissecting and fixing up imports. These resonators as a class are terrific instruments though; ordinary wooden guitars seem relatively mute and lifeless since I discovered the real "top fueler" acoustics.
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Post by steverino on May 27, 2009 5:43:24 GMT
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Post by steverino on May 25, 2009 23:27:30 GMT
Stevie, mine is a Joni Mitchell story also. I was sitting in my VW before a college class in probably 1972 when "All I Want" (also from "Blue") came on the radio. It grabbed me like no other song before or since. I was late for class as I had to wait through several other songs before the disc jockey gave the titles and artists.
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Post by steverino on May 25, 2009 18:15:41 GMT
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Post by steverino on May 18, 2009 19:36:52 GMT
Most likely there is a fault with the guitar; a sharp edge, pinching nut slot or something like that. However it isn't unheard of to have a problem in music wire that exists for many feet of length. I have heard the explanation that an impurity in the ingot like a speck of carbon will be many feet long by the time the metal has been drawn through the successive diamond dies to form the music wire. I have encountered pianos that had the tendency to break strings of a certain wire size. Looking at the broken ends, there is sometimes a stair step that suggests a weakness running lengthwise through the wire. I've never seen this in a guitar string though.
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