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In the thread on bridge positon, Dennis Leahy wrote:
"I read the Siminoff book too, and he showed how he determined that the bridge was primarily rocking (fore and aft), and my fuzzy understanding (don't really know where I got it) is that archtop guitars (but not flattop steelstring guitars) primarily transmit vibrations via a pumping action. Again, I know where I got the notion that "stopped" (typically bridged) steelstring flattop guitars primarily rock the bridge - it was Siminoff's book.
So, I'm betting that Al Carruth has set up his own experiment and came to a different conclusion - and I'd love it if Al would show us how he arrived at that. Maybe if we keep bugging him... "
Im sorry i missed that the first time, and have been asked by another poster to reply specifically to this. Sadly, the 'reply' button seems to be broken, so I'm taking the liberty of starting a new thread.
I have not read Siminoff's lastest book. I've heard a lot about it, and I probably should spend the money to get it, but being a typical impecuneous luthier, I'm loath to do so.
I did spend 'way to much time, spread over several years, measuring the forces that a plucked string puts on the top of the saddle as it vibrates. Part of the reason it took so long was that I can't afford to spend lots of time or money on this stuff, but mostly it was just that I was sort of feeling my way along, and didn't have a faculty advisor to help figure out the problems that came up. This was compounded by the fact that strings are not nearly as simple as we are often lead to believe, and I kept seeing stuff that wasn't supposed to be there. Many of the gory details (minus some of the more gory gore) are to be found in the article on string forces I published in 'Guitarmaker' magazine after the '07 Symposium. Sadly, we got in a bit of a hurry, so some of the charts are hard to see, and one or two are missing. Stuff happens, I guess.
Summarizing in desperate haste(supper's almost ready):
I made up a heavy beam of persimmon wood, with a 'nut' and 'bridge' on it. These contained piezo elements, set up to read the transverse (vertical) and tension change forces of the string as it moved. The string was 'plucked' by looping a fine (~#44) wire behind it and pulling upward until the wire broke. This gives a constant force pluck at a known place on the string.
What I found was that the transverse force, when the string is vibrating vertically, is, for most strings, much greater than the signal from the tension change. Of course, the ratio varies depending on several parameters, which I discussed in the article. It is also true that real plucked strings don't simply move 'vertically' relative to the soundboard, so the proportions between the two forces can vary over a wide range in practice.
I also measured the 'admittance' at the top of the bridge saddle on a steel string and a classical guitar. I looked at the magnitude of acelleration of the saddle top when it's driven by a constant voltage signal in three directions: vertically relative to the soundboard, horizontally across the top, and horizontally along the length of the strings. There's a lot of detail in this information, of coutrse, due to the complicated resonance structure of the guitar, but basically the admittance was least across the guitar, as you'd expect, and greatest in the vertical direction. Driving along the axis of the strings was harder below abnout 350 Hz on both guitars than driving vertically. Above 350 Hz they tended to swap back and forth. This fits well with the data from similar experiments done by other researchers that I'm familiar with, such as Fletcher and Rossing.
Several years ago I did a very quick and dirty experiment, using a magnet and my signal generator to drive a string in transverse vibration with a pure tone. By altering the orientation of the magnet I could drive the string in any direction from vertically to horizontally relative to the plane of the soundboard. When driven vertically the string can push the soundboard in and out, the loudspeaker motion. When it is moving horizontally it cannot push the top effectively in that way, but it will rock the bridge owing to the tension change. I used my dB meter to read the output of the guitar. Sadly, I did not take good notes of that one: it was just a quicky to answer a post on another forum. What I do remember is that the instrument produced much more sound when the string was vibrating vertically then when it was going horizontally.
So, from my data, it seems as though it's easier to drive the top vertically, like a loudspeaker, than it is to rock the bridge. The strings themselves produce a stronger signal in the vertical direction than they do along their length in tension change. Finally, the top is more efficient in producing sound when driven like a loudspeaker than it is when rocking the bridge.
This, then, is the basis for my disagreement with the statements attributed to Siminoff. I'm perfectly willing to say that there are most likely difficulties with all of my experiments. However, they're the best I could do. I take some comfort in the fact that my results do agree with those of the best authorities I know of, but argument from authority is one of the weakest sorts. If anybody wants to duplicate my experiments, and improve them, I'd be happy to help out: I can tell you about a lot of things that you shouldn't do! But until somebody comes up with solid results that contradict me, I'm sticking with my opinion.
I hope this does not come across as ill-tempered. As I say, supper's ready, and I'm in a bit of a hurry, but I wanted to clear up that point since I was specifically asked to.
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