Jump to content

ttonon

Members
  • Content Count

    254
  • Joined

  • Last visited

About ttonon

  • Rank
    Chatty concertinist

Contact Methods

  • Website URL
    http://www.bluesbox.biz
  • ICQ
    0

Profile Information

  • Gender
    Male
  • Interests
    Many
  • Location
    Princeton Junction, New Jersey, USA

Recent Profile Visitors

484 profile views
  1. Hi Chris and Alex, a little more on what powers the vibration. Until very recently, it was pretty much unknown just how the free reed worked, or how energy was extracted from the steady bellows pressure in order to maintain vibration. I never saw any description of it in the scientific literature. I wanted to apply principles of fluid mechanics to this problem and for that, I needed a physical model to describe the forces acting on the vibrating tongue, and it finally dawned on me that there is an important asymmetry in the up and down motion, and it’s that asymmetry that explains the crux of the issue. The tongue covers the top surface of the slot in traveling both upwards and downwards. The important difference is that there’s a violent stoppage of airflow through the slot only during the downward motion. In the upward motion, the tongue gradually shuts off air flow. In downward motion, the airflow through the slot just before cutoff is high, up around a few hundred feet per second, several times higher than the tongue velocity in that location. You can calculate this very simply from the steady flow Bernoulli equation for incompressible flow, and let me state a diversion: yes, I said “incompressible.” It’s a well-known fact that air behaves as an incompressible fluid in flow situations when the Mach number squared is negligible with respect to unity, which is the case here. So when I use the term “water hammer,” I’m being precise. Water, in normal flow situations, can be considered incompressible, and for the air velocities associated with the free reed, air behaves like any other “incompressible” fluid. Getting back, in downward motion, the tongue suddenly stops the airflow, and as the air piles on, the kinetic energy of the air is converted to static pressure acting on the top of the tongue. This static pressure is about equal to the pressure that cause the air flow to begin with, which is the bellows pressure, and that pressure adds to the bellows pressure which is already there. We thus come to the important conclusion that the static pressure above the tongue at the moment of stoppage is about twice the bellows pressure. How long does that pressure act on the tongue? It would be difficult to determine that precisely, however, we know that it decreases as the tongue moves further downward into the slot and as pressure waves move outward from the region. The region of high pressure above the tongue expands and the pressure therefore drops. We know the velocity of the tongue and we know that the pressure wave moving away from the region travels at the speed of sound. But most importantly, the excessive pressure is acting in the same direction as the tongue motion. It adds energy to the motion. From all that, we can model the water hammer effect on the downward motion of the tongue. What about when the tongue is moving upward? In this case, 1) the stoppage of air flow is more gradual, thus any excessive pressure due to a water hammer effect will be less than in the case for downward movement of the tongue, and 2) any excess pressure caused by a stoppage of air is directed against the tongue motion. We thus uncover the secret for how the tongue vibrates, the mechanism by which a net bellows pressure energy is put into the motion. In the downward tongue motion, the water hammer effect helps, or adds more energy to the tongue motion than during the tongue upward motion, when a smaller water hammer effect works against, or subtracts energy from the tongue motion. Using experimental data, I have been able to verify this description, which I have assembled into a physical model, and with which I have formulated a mathematical description. I however do not have final numerical results to report here, and I’d prefer to first put it in publishable form. For me, this is an ongoing project, and I’m able to spend time on it in a hit or miss fashion. But I’ll keep you informed as I progress. Best regards, Tom
  2. Hi Chris, I believe the method you chose to suppress high frequencies from the sound spectrum has theoretical backing. As the tongue moves through its swing cycle, the pressure and aerodynamic forces on the tongue change very abruptly in both magnitude and character. This is because the slot provides a very different environment from the free space above the slot. In cases where the tongue passes completely through the slot, we have an additional region of free space, which is also significant. The velocity of the tongue is zero at the extremes of motion, and it is maximum just about where it downwardly enters that slot. Such a rapid change in forces is in mathematical terms a step function, and the representation of a step function by a Fourier series contains many higher harmonics. So if you reduce the "suddenness" of the step function, you reduce the contribution by higher harmonics. I thus believe that your approach would result in less higher harmonics. However, I think there's a price to pay. You see, the key feature that allows the tongue to vibrate is the "water hammer" effect that occurs by virtue of the sudden stoppage of air flow down through the slot at the moment the tongue first enters the slot. The air "piles up" on the tongue as the air velocity rapidly drops from a high value down to the velocity of the tongue. Without the tongue in the slot, the air velocity through the slot is several times greater than the maximum tongue velocity. But importantly, if it wasn't for that sudden build up of (dynamic) pressure on the top surface of the tongue, in which the kinetic energy of the airflow is converted to potential (pressure) energy, the tongue could not vibrate. So when you reduce the suddenness by which pressure forces act on the tongue, you also lessen the water hammer effect. My guess is that such a tongue will not sound with as much volume, or energy, as with the conventional set up. In addition, I don't think your novel design will allow the tongue to vibrate with both directions of air flow. To understand that, we need to understand how the conventional tongue starts its motion. I explained this in a previous post, but I'll summarize it here. Picture the quiescent tongue situated just above the slot. With the slightest bellows pressure, air flows around the tongue and becomes turbulent under the tongue. Characteristic of such conditions, this turbulence consists of vortices that are shed in a periodic fashion. It's a well studied fluid mechanics problem under the heading of "bluff bodies in an air flow," and the phenomenon is sometimes described as von Karman vortex streets, and more specifically, "vortex induced vibration of a cantilever." These vortices, much like miniature tornadoes, have reduced pressure at their interior, and the fact that they form periodically and pass on means that the tongue experiences a slight, oscillating pressure difference. This causes the tongue to vibrate at its natural frequency, and when that vibration amplitude equals the tongue offset, the tongue blocks the slot and the total bellows pressure immediately acts on the tongue. This is an enormous increase of pressure forcing the tongue further into the slot. There's too much detail to explain how the steady-state vibration builds from there, and I won't try it here, but this is all we need to know in order to answer whether your tongue can be bi-directional. I think not, and the bottom line is, if the tongue can vibrate in one direction of airflow, it can't in the other. Here's why. If the tongue is starting in one direction, it lies at a position sufficiently removed from the slot for the changes in pressure caused by vortices to be larger than any contribution due to the bellows pressure (bellows pressure multiplied by slot cross sectional area). With such a situation, pressure introduced from the other side of the tongue cannot reproduce the necessary starting conditions; i.e., allowing vortices to amplify tongue vibration to the point where the tongue enters the slot. In the latter case, there is no slot for the tongue to enter, from the direction that the starting air flow wants it to go. Incidentally, we can also now understand why a tongue can't start if it's rest position lies in the slot, because then, the total bellows pressure always acts on the tongue, and any pressure fluctuations caused by shedding vortices will be much smaller than that. As a result, the tongue could only stay in the slot, with at most a very small quivering. I agree that all that might be a little confusing, but feel free to dig up that reed and let us know if my guess is right. I'd love to be proven wrong. Best regards, Tom
  3. Very interesting. How do you seal from air leakage around the keys? It seems this is an instrument that will sound only on the push of the bellows. Does that restrict any of the music that you play with it? I'd love to hear a sound file. Thanks. Regards, Tom www.bluesbox.biz
  4. Very interesting. How do you seal from air leakage around the keys? It seems this is an instrument that will sound only on the push of the bellows. Does that restrict any of the music that you play with it? I'd love to hear a sound file. Thanks. Regards, Tom www.bluesbox.biz
  5. Hi Ted, yes, we can distinguish between average pressure on the reed and pressure difference across reed. Let's stick with practical numbers. A typical pressure difference is about 5 inches Water Column, so for that same pressure difference, the push reed experiences 5 inches WC higher average pressure than the draw reed (2.5 + 2.5 in WC). I fail to see though how that is significant concerning the playing of the reed. One atmosphere is about 14.7 psi absolute, or about 408 in WC, and so the average pressure difference is only 5/408 = 0.0122 parts in atmospheric pressure. How can that slight average pressure affect the operation of our friend the free reed? The only way I see is through density, but still, only a 1.2% difference in density is very small. If you're curious enough, you can try to discern such an effect of density by playing your concertina at sea level and compare the sound to playing at about 500 feet above sea level. Keeping the temperature the same, that will correspond to about a 1.2% difference in atmospheric density.
  6. Ha! I invented such an animal and got a patent for it just for fun, and of course there was no revolution. Bi-directionality is only a small part of the many issues involved in making such basic changes to an old hand-crafted technology. I'd rather not bring up this issue again, but you relative new comers can do a search for old threads on this issue.
  7. Hi Ted, I don't think we can look to Physics to help solve this riddle in the way you say. From a physics point of view, the push and pull are entirely symmetric. The reed cannot tell whether the pressure difference it experiences is either a push or pull. Neither can the bellows. A bellows can collapse from too much vacuum, but it can also blow outwards because of too much compression. I do agree that we can probably exert more force on the bellows with a push, mainly because of the way our muscles and skeleton are constructed. I'm sure an anatomist can provide solid reasoning, but simply, the push is accomplished by some arm muscles and chest muscles, whereas the pull is accomplished by different arm muscles and back muscles. Perhaps the main difference lies in stronger chest muscles than back muscles. Best regards, Tom www.bluesbox.biz
  8. Thanks Dave, interesting process, looks like fun. In your first photos, it looks like you don't use runners, or passages to direct the molten metal to more than one area, and to let out air. Interesting that you don't have a problem with getting air out of the way with the gravity feed. Regards, Tom
  9. This is incorrect. The air flow velocity is a function of only bellows pressure, not flow area. The Bernoulli Equation, V = (2*Pb/Rho)^0.5, shows it, with V velocity, Pb bellows pressure, Rho air density. Physically, it means that specific potential energy (pressure) converts to specific kinetic energy (V^2), intrinsic properties of the fluid, nothing to do with geometry. Remarkably simple. Best regards, Tom
  10. Hi Dave, I understand. You illustrate the difference between "hand-made" and "machine-made," and I don't mean those labels in the way used by Italian and Chech reed makers, but in the more general sense. Before my questioning, I looked at it from an industrial perspective, but you have many more variables available "at hand," treating the fabrication more like an art. Your cost is time, but hopefully artisan work such as yours can continue to be something people are willing to pay for, of course because of the superior result. Thanks for the detailed info. Best regards, Tom
  11. Hi Dave, thanks for the descriptions. Why do you cold work it? I understand that brass usually comes in annealed, half hard, and full hard, sometimes with quarter hard increments, depending on how much cold working the material has in manufacturing. To me, the manufacturer did a useful thing to provide material with definite properties that are probably uniform and consistent. What's the advantage of doing your own work on it? Can you make a full hard material stronger by cold work? Won't it start producing cracks? Why buy a grade less than full hard? Beating implies hammering, which suggests a rather crude method. I suppose that if you have your own precision roller, you can avoid that, though still, why not use material with strict specs? Best regards, Tom
  12. Johann, I've mentioned it several times in this forum that James Cottingham has done extensive experimental investigation of free reeds. You can see a list of his publications at: https://www.researchgate.net/scientific-contributions/58932239_James_P_Cottingham Jim is joined by many others on these investigations, both theoretical and experimental, and I think you would be interested in the many details and the precision with which this research is done, using the scientific method, and proceeding far beyond anything we have discussed here. We often lead ourselves in circles, and unfortunately don't include the deep insights uncovered in the literature. This forum interests me mainly because of the interesting information provided by makers and doers and the resulting personal contacts. It still can't come close to an adequate stage to present all the knowledge about free reeds currently available. Best regards, Tom
  13. Johann, yes, our hearing response comes into it and it's entirely possible you hear more of the highest steel frequencies than I do. I know my hearing is compromised, thanks to my time as a carpenter and the circular and radial arm saws. Tom
  14. Okay, I misunderstood what you mean by "on top." I thought you meant "above." But the important thing is what I described, the titanium dominance of the 5th harmonic can very easily make the titanium sound brighter than steel. Do you agree? Best regards, Tom
  15. Hi Johann, don't you mean with the red line on top? I can see that you simply reversed the colors for steel and titanium, and now, titanium is red. Before, the red line in the 5th didn't show, so this is a better choice for color. But I think this indeed explains why titanium to me sounds brighter than steel. It's that fifth harmonic, which dominates all except the first, by at least a factor of 50 in amplitude. It's often very difficult to make guesses about how different spectrums will sound unless there's clear dominance among enough harmonic amplitudes, but in this case, and you may not agree, if I saw that spectrum, I'd be confident to guess that titanium would sound brighter. Best regards, Tom
×
×
  • Create New...