ctanzio

Members
  • Content Count

    711
  • Joined

  • Last visited

1 Follower

About ctanzio

  • Rank
    Enthusiast

Profile Information

  • Gender
    Male
  • Location
    Philadelphia, PA USA

Recent Profile Visitors

3361 profile views
  1. Synthetic shellacs are available. Zinsser makes a variety of products that use synthetic shellac, including a clear one for use over wood. I have used the "white synthetic shellac" product and it is a fabulous primer and sealer, although pricey. You might try experimenting with their clear version in your 1704 formula.
  2. When observing spectral plots, one must be very careful to understand how the energy was put into the violin. For example, impact hammers can input a continuous spectrum of energy into the violin, but the energy drops off rapidly with increasing frequency. In other words, that high frequency roll off you are observing in the response spectrum may be due, in no small part, due to the lack of strong excitation at higher frequencies from the hammer impact. Studies of small hammers, like those used in pianos, show total impact time in the tenths of a millisecond range. So peak energy input might occur +/- the 1kHz range. The energy versus frequency input will drop off like the square of the frequency. I know some makers have setups that input energy into the bridge using a transducer, so they can control the energy versus frequency being forced into the violin across the entire spectrum of interest. If you are looking at spectrum response plots that were not generated using a similar method, your ability to make valid, quantitative comparisons will be challenged.
  3. The bow driven string forces all these modes to vibrate at the string's frequency. How strongly they vibrate at the string's frequency depends on how close the natural frequency of the mode is to the frequency at which the string is being forced to vibrate, and on the internal damping of the wood.
  4. Freedom of Speech includes the freedom not to listen to speech. That is what the ignore button is for. I didn't ban anyone. I just lowered my personal volume control on people who didn't know when to stop exercising their free speech. But to get back on topic, although one cannot hear a pure sine wave tone with frequency in the upper hertz (the 15khz whistle of some old computer crt screens used to drive me to insanity, but now I am lucky if I can hear anything over 8khz), these upper frequencies can dramatically affect the shape of the sound pressure waves over short time intervals. These rapid changes the ear can detect even when detection of high frequency pure tones has degraded. There is, currently, no way of quantitatively measuring a "good" tone versus a "bad" tone. Which means there is no systematic and scientifically valid way to determine how much 4khz+ roll off is enough, or too much.
  5. Loudness, which is highly subjective and is affected by other factors other than pure air pressure, roughly doubles ever 10db. Air pressure, which is an objective measurement, doubles every 6db. Intensity (sometimes called power), also an objective measurement, doubles every 3db. So if Curtin was talking about actual sound power, then a difference of 2.5db is significant. If by power he meant perceived loudness, then it is not so much a difference. A similar problem is encountered when considering the Fritz test. Was she measuring loudness, air pressure or sound intensity? If sound intensity, then that is a useful distinction. From Martin's excellent story about the violin test, it is not clear the difference in projection is purely a difference in loudness. There seems to be something more to it.
  6. Lime (calcium hydroxide) is highly caustic, both out of the package and after combining with water. So there is a handling issue. You should probably avoid such a reactive material as a ground, since it is likely to cause ongoing changes to the wood and the varnish. A popular mineral ground is slaked Plaster of Paris, also known as gypsum and calcium sulfate. There is a handling issue when in its initial dry state, but after thoroughly slaking it (adding excess water), it is very stable and non-caustic. It is a good wood sealer and filler and leaves a whitish haze on the wood surface after it dries. But after applying an initial layer of typical oil or spirit varnishes, it becomes completely transparent. This is because its refractive index is very close to the refractive index of typical varnishes. The white haze is caused by light dispersion of many tiny, transparent gypsum crystals.
  7. To echo Don's observation: once the strings are tensioned, the frictional force on the ball joints combined with the very small movements of the plates would render the joints effectively rigid, so no different than a simple spruce post in that regard. Mass, vibrational modes, and the very slight offset at which the plate loads are applied to the post are the substantive difference between the Hamberger and a simple spruce post. Something to keep in mind: when a spruce post is slightly adjusted for position, it inherently necessitates a change in how much it stretches the plates apart, and how the ends interface with the plates. One cannot avoid this given the plate arching unless one painstakingly changes the end fit and post length to replicate the end fit and overall post load. But when these adjustable posts are moved, they essentially replicate the snug fit and post load of the previous position. Be careful that you are not comparing apples to oranges, so to speak, when conducting tests of spruce versus adjustable posts.
  8. Nice color. One way to think about a finish coat is as a protective coat to guard against wear and fading of the underlying colored varnish. Consider something that dries clear and hard in a thin coat and can be easily touched up and polished to a range of finishes, satin to gloss, to give yourself a lot of flexibility. If it provides some UV protection, that is a plus. You want a finish with a refractive index close to the colored varnish so maximize optical transparency. Although if the colored layer is very smooth, then a finish with a higher index can intensify the underlying colors. Rosin based oil varnishes and high quality shellacs can all be formulated to fit the above requirements. Like most things violin, you need to experiment with test samples first to figure out what works for you.
  9. Beautiful work. I love the scroll. Thanks for sharing.
  10. This stuff is about 75% solvents: ethanol and ethyl acetate. Although ethyl acetate occurs in small amounts in may foods and drinks, it is a highly volatile solvent and can induce a variety of adverse reactions in living creatures. In terms of French polish prep, the essential ingredients are the shellac and oil, in this case, linseed oil. There really is no reason not to use just ethanol as the solvent. Maybe 5% to 10% by weight of shellac to ethanol to replicate the formula. Linseed oil is slightly soluble in ethanol at room temperature. Start with an amount of high purity ethanol and weigh it. Then apportion about 5% of that weight of artist grade linseed oil. Add a little at a time and shake until no more will dissolve. Add your shellac and you are good to go. Typically what happens is the shellac dries very rapidly as the ethanol evaporates. The linseed oil remains liquid on the dried surface and is easily wiped off. Further applications of shellac compounds will make any remaining oil float to the top of the finish as the shellac hardens.
  11. Colin Gough's work is basically an impossible read for anyone without significant education and experience in physics and higher mathematics. However, a lot of those formulas can be simplified if one considers the constraints on violin geometry due to player requirements, and violin material due to tradition. For example, one finds that the core frequency of A0 (air mode) for violins, violas and cellos is mostly dictated by the length of the body. f(A0) ~= c/(4L) c = 343,000 mm/s L(4/4 violin) = 356mm, A0 = 241hz L(16in viola) = 406mm, A0 = 211hz L(4/4 cello) = 755mm, A0 = 114hz These base values are modified a bit by two main factors: the volume cavity and the area of the f-holes. Also, the flexibility of the plates play a factor but this is a complicated effect and can be harder to control. For a maker, changing the rib height or the bout widths can be used to adjust the A0 frequency. Bigger will lower the frequency. You can estimate the reductions as follows: If V1 is the volume of an existing model, then changing the volume to V2 will change the A0 frequency by a factor of: SquareRoot(V1/V2). Changing the f-hole area will increase the frequency. The effect is a little more involved but a similar estimate can be used: SquareRoot(A2/A1). Because plate mechanics are so complicated, it is not easy to extract simplified design rules for body modes like B1- using the theoretical equations in Gough's work.
  12. A0 and B1- are coupled by the way energy is input to the violin. The air in the cavity for the A0 mode needs to be "moved". This is done by the movement of the plates. The plates are moved by the bridge. So in effect, the energy input to the A0 mode is through the body modes, like B1-. If you excite A0 independently of the body by blowing across an f-hole, then the movement of the air will have a certain "shape" and "frequency". If you excite the B1- mode independently of the air mode by playing in a vacuum, then the movement of the body will have a certain "shape" and "frequency". When you play the violin in a normal atmosphere, the wo modes are now "coupled" as described above. You actually get two new modes, each of which is some combination of the independent A0 and B1- modes. The "shape" of each mode is some synchronized combination of independent air and body modes, as is the frequency of each new mode. If the modes are weakly coupled, then each of the new modes will strongly resemble one of the uncoupled modes. If the modes are strongly coupled, the new modes can look quite different than the original independent modes. Saying the "tail" of the B1- mode drives A0 is not an accurate way of looking at it. The mode shape of tail is no different than the mode shape at the peak. The difference is in the amplitude of the deformation. So for notes near A0 peak frequency, B1- contribution is not much, but what it is doing is being strongly amplified by A0. Similarly, at a note near the B1- peak, even though A0 may not be very resonant, it is offset by the larger volume of air B1- is moving.
  13. A mathematical "circuit" is created for the A0 effects and numerically simulated, i.e., as if there is no B1-. That is the "air" line in the diagram. A similar thing is done for the B1- effects. That is the "body" line. The two circuits are then coupled and the new circuit simulated. Because these are two complex, COUPLED motions acting simultaneously, there will be small time segments where the effects do cancel, and other time segments when they reinforce. For any driving frequency, the combined plot represents the TIME AVERAGE of these effects. Phase variations certainly enter into the results, especially at driving frequencies between the natural frequencies of the two modes, but the dominant effect is the rapid decrease in the amplitude of each mode that drives the sound output that is observed in the uncoupled results. It is not a simple case of "Oh, they are completely out of phase. That is why it drops off so rapidly."
  14. The width of the "skirt" about each peak, typically taken at -3dB from the peak, correlates directly with internal energy loss (damping) for that mode of vibration. The horizontal axis represents the frequency of the note relative to the natural frequency of the mode. So when the note frequency lines up with the mode natural frequency, the peak is the highest. The dip between A0 and B1- is mostly due to the note frequency being "far" from the natural frequency of either mode, rather than any significant cancelling of air motion between the two modes. In terms of the ability to generate raw acoustic power across the playing spectrum, you want a lot of natural modes crammed across the playing spectrum. In other words, figure out how to create more natural modes in that frequency range. Simply switching the position of the A0 and B1- modes might not do much in terms of power or tone, assuming one could figure out how to do such a thing without a significant redesign of the basic shape of the violin.
  15. If you make a violin out of infinitely stiff material so there is, in effect, no B1- mode, then A0 will have a certain frequency. If you put a typical violin in a vacuum so there is, in effect, no A0 mode, then B1- will have a certain frequency. Once you have a violin made out of regular material in an atmosphere, the modes become physically coupled to each other and their characteristic frequencies change based on that coupling. There is no inherent cancelling or reinforcing based strictly on observed frequency of vibration. Cancelling or reinforcing of air movement is a rather complicated function of the driving frequency (note being played) relative to the frequency of each mode and the internal sound damping of the air for the air mode and the wood for the body mode. When playing notes on the G string, I would expect the A0 mode to be moving most of the air for the lower harmonics and the B1- mode doing almost nothing. For the higher harmonics, the A0 would see very little excitement while the B1- would spring into action. For notes played on the A string, the reverse would be true.