Jump to content

ttonon

Members
  • Posts

    309
  • Joined

  • Last visited

1 Follower

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

1,036 profile views

ttonon's Achievements

Chatty concertinist

Chatty concertinist (4/6)

  1. Lukasz, from my calculations, I see no possible resonance interference with the tongue vibration. This applies to both Helmholtz and quarter-wave tube resonance. I thus cannot make a suggestion here, only I'm still not clear on what the basic problem is. Are you saying that all the F3 reeds you put into the "bad" cavity show the same problem? Also, that when you put those same reeds into a different cavity but with the same dimensions of the "bad" cavity, there is no problem? Best regards, Tom
  2. Hi Lukasz, I just now heard your wav file and I agree, the 1st and 3rd sounds seem restricted, and the 2nd and 4th more open or free. In my experience, such a restricted sound occurs when the mounting of the reed - most often the geometry of the cavity - begins to acoustically interfere with tongue vibration. For instance, if the port is too small, or even if the leather valve isn't opening completely enough. Alternatively, there could be some defect in the construction of the reed, such as a burr, or other way to interfere with tongue vibration. Your guess that the problem lies in a restricted second partial seems to be borne out by the spectrum. In the bad note, that partial is about 13 dB below the fundamental, and in the good note, it's only about 5 dB below the fundamental. I'm not sure there's evidence in the spectrum for your claim that the energy of that second partial is fed into the higher partials, which from my experience would be an odd occurrence, knowing what I know about the conversion of pressure pulses to audible sound. We can notice that the bad note has also a greatly reduced 8th partial, compared to that of the good note, but I doubt that has anything to do with what we hear, since the pitch of that partial is about 22,000 Hz, far above our normal hearing range. However, your discovery of this reduced second partial may give us a clue. If you'd go through the trouble of presenting here all the exact 3-D dimensions of the cavity and the size of the port, I'd be glad to take a look at it, possibly finding an identification of the second partial frequency with some way the cavity could resonate. If it's a simple rectangular cavity, it should be easy to do. Best regards, Tom
  3. Hi Lukasz, Audacity enables the calculation of a frequency spectrum showing a rectilinear graph, with sound intensity on the vertical axis and frequency on the horizontal axis. I believe that such a representation is much more useful than the kind of spectrum you posted here. If you can't figure out how to get that graph on Audacity, let me know and I can perhaps lead you through it. More basically, I don't know what your "problem" is. Apparently there's a problem with the sound of two different F3 reeds when alternatively mounted in the same cavity. Is this correct? If so, could you please explain in detail the problem in the sound? Do you think you see the "problem" in the spectrum you posted? If so, please explain. In the "Bellows pressure and musical pitch" thread, if I recall, you thought that the poor sound was because of a pronounced harmonic and I suggested you do a spectrum analysis. Do you still think the problem is with a pronounced harmonic? If so, the kind of spectral graph I describe will be most useful. Regards, Tom Tonon www.bluesbox.biz
  4. Fifer, I'm intrigued by your planned journey, and if appropriate, could you please explain some things about the mission? It's main strokes, purpose, and the organizations behind it? Are there chiefly scientific goals? Methinks it's a great opportunity for experiences that you will never forget. Best regards, Tom
  5. Hi Fifer, You'd want to be aware that the reeds might buzz when you try to play the instrument when it hasn't warmed up all the way to room temperature after a cold transport outdoors. That's because the vibrating tongues might rub against the end walls of their slots. The different coefficients of linear thermal expansion (CTE) in the reed plate material and the tongue material will cause the tongues to close the small air gap between them when at the lower temperatures. The extent of this rubbing will be affected by the kind of material used in the reed construction. The CTE for brass is about 10E-6 per degree F; for Aluminum, about 12.5E-6, and for steel about 6.5E-6. It's the difference between these values that matters. Thus for steel tongues in brass plates, the difference is 6E-6 and for aluminum plates with steel tongues it's 3.5E-6. Let's say the outside temperature is - 40 F, and let's consider one of the longer reeds, with tongues about 2 inches long. The reeds were designed (gaps were set) for room temperature, say 70 F. The delta T to consider is thus 40 + 70 = 110 F, so multiplying the delta CTE's, the delta T's, and the nominal lengths, we calculate the difference between how much the tongue lengths expand/contract and how much the slot openings expand/contract, as about 0.00076 inch for the brass/steel combination and about 0.0012 for the aluminum/steel combination. I believe I recall Dana saying that his gaps are roughly a half thousands, or 0.0005 inch, on average. We can thus conclude that for some concertina reeds, and for the lowest temperatures, there will be closure of gaps. This is especially true considering expected variations in workmanship, and for the tight condition at the corners of the slot/tongue gaps. In fact, I've noticed that with my Baldoni accordion with handmade reeds, I'd get buzzing when I brought it inside from winter temperatures only as low as about freezing. I would often press on the keyboard and gently draw air in and out of the box, until its interior warmed. I concluded that the variation in workmanship plays a large role here. Best regards, Tom www.bluesbox.biz
  6. Lukasz, I recall some of those discussions, and my conclusions are that - as a video showed - there needs to be minute tongue vibration while in its standoff position. The only physical mechanism I can think of for that vibration is the same one observed in many other systems. It's called, "vortex induced vibration." With it, vortices are shed periodically, forming "von Karman vortex streets" downstream. This shedding causes air pressure oscillations that excite the vibrator, and they couple with the vibrator, forming at a frequency close to the natural vibration frequency of the tongue. Eventually, the amplitude of tongue vibration becomes large enough to allow the tongue to enter the slot. Once that happens, the normal self-excitation mechanism takes over and the tongue vibration amplitude rapidly builds up to its normal level. I say "rapidly" only in a relative sense, since compared to other sound sources, the start transient for free reeds is rather long. Best regards, Tom
  7. Hi Dana, Perhaps we need to coin a different word for when those high frequency reeds fail to start because of a resonance conflict with the air of the cavity. Let me propose something like "dead," but I'm not too creative at the moment and maybe someone else has a better term. Of course there could be difficulty in distinguishing whether that reed is really dead or if it fails to start for other reasons, such as for instance, too small a quiescent standoff distance. I agree that the 15th harmonic is virtually always negligible and it's highly unlikely that a hindrance to its expression would prevent tongue vibration. I think at the time I illustrated the magnitudes only to give the reader a better idea of the scope and relative comparisons of the ideas I was trying to communicate. But how about for the second partial? As you well know, a chamber has many characteristics, and many involve specific dimensions, as you observe with the brightness effect of cavity height. The Helmholtz conflict I describe is only one, and being a universal property, it involves only the volume and port of the cavity. Another is the quarter wave tube vibration that is treated in the columns on the right in the Table. I don't know how much clearer I can be. I'm pointing out a simple way in which two resonant systems - the tongue and the air in a resonator - can interfere. I have not proved it rigorously, but I think that the known observation that a dead reed with a particular cavity can be brought to life through a minor tweak to a (any?) cavity dimension or a drilled hole, strongly points to resonance conflict, particularly since calculation proves that the Helmholtz frequencies coincide with the fundamental pitch of the tone in those cases. There certainly are electrical circuit analogies, and all that adds a broad base of evidence for my suggestion. Let me try this. With a standard Helmholtz geometry, during resonance, air pulses in and out of the aperture and pressure in the cavity oscillates. If you put a hole in the wall of the cavity, small compared to the aperture opening, you'd sense air pushing in and out because it's driven by those pressure oscillations. This airflow is what the Helmholtz resonator wants to do. Put a reed in the wall of the cavity and stop it from vibrating, say by gluing it's tip to the plate. During Helmholtz resonance, you would probably sense air fluctuating in and out through the minute leakage gaps between tongue and slot wall. Now release the tongue, and you can reason that the Helmholtz resonator will want to impart vibration in the tongue. But the tongue has it's own way of exciting it's own motion (not explained here), and why assume that what the resonator wants to do is the same kind of airflow the tongue needs to vibrate? You can reason by considering the phase relationship between cavity pressure oscillation and the oscillating flows through both the aperture and the reed. Each system demands its own phase relation between displacement and pressure oscillation. Why should they be the same? That's why I'm suggesting that there's a conflict. Before I looked into this, I naively thought - as many others did - that perhaps Helmholtz resonance can be exploited in helping and amplifying tongue vibration, by designing tuned cavities for each reed. But when I did the experiments, I found the opposite. Coincidence with Helmholtz or quarter wave tube resonance killed the reed! That's when it seemed obvious to me that Helmholtz resonance may indeed be a key reason for "dead" reeds. Best regards, Tom
  8. Auldfellowmelad, thanks for your suggestions and encouragement. I can find no record in the scientific literature that deals with the physics of tongue vibration in the free reed. There are articles on more general topics, and several very good articles on the acoustic sound field produced by the reed. But the latter regard the reed vibration itself only as an input sinusoidal vibrating valve. They don't "solve" for the tongue vibration on its own. Thus if any reed manufacturer in Italy made such attempts, it's largely buried to the public. From my own experience though, I doubt it. The accordion/reed industry in Italy and probably elsewhere is dominated by fixed ideas that were developed over a hundred years ago, and the field is populated by closed minds. Many people in this industry didn't think it possible that my acoustic technology for pitch bending in the free reed was even possible, and some claimed that they already knew all about it. Humorous, to say the least. Your recommendation to visit a maker is a very good one. I should mention that Dana and I have had extensive discussions via email, which probably took us to the extent of our communicable knowledge on the subject. In addition, Dana has provided me with a couple expertly-made reeds to my specifications and with which I plan to use in my own experimentation. More future plans that I hope to accomplish. Best regards, Tom
  9. Hi Lukasz, it's not clear. You say you measured the spectrum when the reed speaks on your table. Did you also measure the spectrum when it's speaking in the instrument? Regards, Tom
  10. Ales, yes, two completely different phenomenon, involving different physics, with vastly different remedies. Thanks for pointing it out. Regards, Tom
  11. Lukasz, you take me from "beating" to "buzzing," and I don't understand either. But your claim that a higher partial is overly emphasized intrigues me because such an event is rare, in my experience. Sometimes with free reeds the second partial (one above the fundamental) can have a little greater amplitude than the fundamental, but I don't know if it's really noticeable by ear. I suggest you first verify that what you say is true, and that's easy to do. Make a digital recording of the tone with an audio recorder and feed the data in a spectrum analyzer, such as Audacity, free software available on the web. You can then see very clearly whether there's a partial with an abnormally high amplitude. If that's the case, we'd know the frequency of that partial and that could be a clue in determining what cavity dimension is involved, if any. I may be wrong but I think you totally misunderstand the purpose of the table I presented. I'm not trying to design reed cavities using Helmholtz theory. I'm suggesting that the choking we observe, especially with the higher frequencies, is when the air resonance in the cavity matches close enough the fundamental of the musical tone. Air resonance happens in most all musical instruments, although the free reed is exceptional in that it occurs only sporadically, without a well understood benefit to the musical tone. I also suggest that a match with one of the lower partials can also cause problems. Once you understand the self-excited vibration of the tongue you will see the sense to this suggestion. For a builder like you, there's no need to even think about this or do any calculations. If you experience reed choking, all you do is try one of your trusted remedies and it will work, whether or not you understand why. Being human, you come up with your own theory on why, without any need to compare it with established acoustical principles. For me, there's value in identifying the underlying physics, but you don't really care about that and there's no reason you need to care. I offer this information only for those curious enough in this way. Best regards, Tom
  12. Auldfellowmelad, I think I made my aim clear. I'm intending to understand the physics behind the tongue vibration in the free reed. As far as I know, no one has done this before. It's a challenge, and it brings me great pleasure in going about the process. I hope I succeed to some degree, at least to be able to publish it for the world to see. Is that enough?
  13. Lukasz, you confuse me greatly. Why do you assume that the Helmholtz model requires orthogonal cavities? The Helmholtz model assumes that all acoustic wavelengths are much larger than any of the resonator dimensions. This allows us to treat the three separate components, Volume, Aperture Area, and Aperture (effective) Length as parts of a "lumped parameter" system. That means the shape of the volume doesn't matter. Spherical, cubical, rectangular, cylindrical, etc. doesn't matter. Only the Volume of the cavity matters. I mention "orthogonal" in my article only to make the volume calculation simple. Thus you're assumption that my model for choking reeds requires orthogonal cavities is erroneous. Concerning your F3 reed problem, what do you mean by "beating?" In acoustic terminology, beating usually refers to the interference of two sounds that are separated by small frequency differences. What is that one F3 reed beating against? It's also difficult for me to visualize the construction, and maybe a picture would help. I have some ideas, but I want to be sure I'm understanding the issue. Best regards Tom
  14. Hi Alex, my guess is that the valve enables a larger body of air to vibrate with the tongue, thus causing a higher inertial load to the vibration. It "cups" the tongue with a larger air mass, forcing more air to participate in the vibration. In addition it adds its own mass to the vibration. With any single parameter spring-mass vibration, larger mass results in lower natural frequency. One reason for the absence of leather valves on the high pitched reeds is to lessen the tendency to choke. At least with accordions, when two tongues share the same cavity, the lack of a leather on the one reed provides air leakage as the other speaks. This leakage lessens the effect of the Helmholtz vibration on the speaking reed. For my work, I sometimes need valves on all reeds and when I installed valves for the highest reeds, they tended to choke. As I mention in my PICA article, a similar solution is to drill a small hole through the cavity wall or the reed plate. Thus, the air vibration caused by Helmholtz is not concentrated solely on the playing reed. In other words, leakage through the hole lessens the acoustic coupling to the cavity. Incidentally, this is another phenomenon that you cannot explain with Lukasz's "length theory." Best regards, Tom
  15. Lukasz, thanks for your comments, which indicate a considerable amount of experience in trying to understand the idiosyncrasies of the free reed, which I believe is among the most complicated sources for musical tone. I think I can explain the observation you mention in your above quote by means of the following table, which is in the PICA article I mentioned above. (If you can't take the horse to water, bring the water to the horse 😋) - - - - - - - - - - - - - - Table 1: Cavity Geometry from Resonant Acoustic Model Calculations -------- Helmholtz Calculation ------- --- Tube Calculations --- Note Partial ν λ Vadj W L d t H S max H fixed L-QW L-FW G1 1 49 277 -0.26 1.00 3.00 0.40 0.25 166 41.5 0.8 68.69 138.12 G1 9 441 30.7 -0.26 1.10 3.00 0.32 0.38 1.11 4.61 0.8 7.12 15.09 G1 11 539 25.2 -0.26 1.20 3.50 0.40 0.38 0.84 3.77 0.8 5.67 12.27 G1 15 735 18.4 -0.26 1.20 3.50 0.40 0.38 0.48 2.77 0.8 4.00 8.92 G2 1 98 138 -0.19 1.00 2.50 0.40 0.25 49.9 20.8 0.75 34.10 68.94 G2 6 588 23.1 -0.19 1.10 2.50 0.32 0.31 0.85 3.46 0.75 5.26 11.28 G2 7 686 19.8 -0.19 1.20 2.70 0.32 0.30 0.56 2.97 0.75 4.45 9.64 G3 1 196 69.2 -0.19 1.00 2.00 0.30 0.25 10.1 10.4 0.50 16.87 34.38 G3 4 784 17.3 -0.19 1.10 2.00 0.32 0.30 0.65 2.59 0.50 3.83 8.40 G3 5 980 13.8 -0.19 1.20 2.70 0.40 0.30 0.41 2.08 0.50 2.92 6.65 G4 1 392 34.6 -0.13 1.00 1.80 0.30 0.25 2.84 5.19 0.50 8.22 17.08 G4 3 1176 11.5 -0.13 1.10 1.80 0.30 0.25 0.34 1.73 0.50 2.45 5.55 G4 4 1568 8.65 -0.13 1.20 1.80 0.32 0.20 0.24 1.30 0.50 1.77 4.13 C5 1 523 25.9 -0.12 1.00 1.70 0.30 0.25 1.72 3.89 0.38 6.05 12.75 C5 3 1569 8.64 -0.12 1.10 1.70 0.32 0.20 0.27 1.30 0.38 1.77 4.13 C6 1 1046 13.0 -0.07 1.00 1.50 0.32 0.20 0.63 1.94 0.20 2.85 6.29 C6 2 2092 6.48 -0.07 1.00 1.50 0.32 0.20 0.19 0.97 0.20 1.23 3.04 C7 1 2093 6.48 -0.08 1.00 1.25 0.32 0.18 0.24 0.97 0.15 1.25 3.05 C8 1 4186 3.24 -0.02 0.80 1.00 0.32 0.13 0.11 0.49 0.15 0.49 1.46 Notes: All lengths in inches Note is nomenclature for piano keyboard, typically found in concertinas Partial is partial number ν is frequency of corresponding partial (Hz) λ is wavelength of corresponding partial Vadj is volume adjustment to orthogonal cavity structure, to account for reed volume (cubic inches) W is orthogonal cavity width in Helmholtz model L is orthogonal cavity length in Helmholtz model d is aperture diameter t is aperture length (Action Board thickness) H is calculated orthogonal cavity height for Helmholtz resonance, Equation 2 & 3, k = 0.6 S max = 0.15λ is about maximum size of any component for Helmholtz model to remain accurate H fixed is cavity height used for tube calculations in next two columns L-QW is length of cavity for quarter-wave model, Equations 4 & 5, k = 0.6 L/2-FW is length of cavity for partitioned full-wave model, Equations 6 & 7, k = 0.6 BOLD numbers indicate regions on the musical scale where resonance occurs and/or where reed choking may occur (when Partial = 1) - - - - - - - - - - - - - - - The notes after the table should be adequate explanation. For the calculations in this table, I assumed what I think are typical cavity dimensions (L, W, H). The bold numbers indicate the cavity dimensions that form resonant geometries with the reed frequencies shown. Most often the involved resonators are Helmholtz or quarter-wave, but mostly Helmholtz. Notice that as the reed frequency increases, there is a greater chance that the cavity will produce a Helmholtz frequency that is close to the reed frequency (in bold). In these cases, the air vibration induced by the Helmholtz resonator disrupts the air oscillations that are required in the tongue's self-induced vibration. Hence, these are areas where the reed is likely to choke. For the longer reeds, the frequencies are much larger than what a Helmholtz resonator can supply, unless you consider the frequencies of the higher partials. Since in some cases second and third partials can compete with the fundamental, there remain possibilities for choking even with these reeds. The phenomenon of choking is not new, but explanations vary. As far as I can tell, you simply assert that the cavity length is the culprit, without any physical explanation on why that could happen. In the light of the Helmholtz calculation, it's misleading to focus only on the length. You say that you change the length and it does away with choking. Why does that work? My explanation is that when you change the length you change the cavity volume, and thus you change the natural vibration frequency of the Helmholtz resonator, moving it away from its conflict to tongue vibration. It's very simple. Many times in technology development, people come up with intuitive ideas to explain what they observe. Then along comes a theory that explains things very well, and this theory is based on a much broader body of knowledge, adding confidence to the theory. I think this is the case here. Despite it's complexities, the free reed is still subject to Fluid Mechanics and Acoustics, not magic. As an acoustic source, it can couple to other acoustic sources, something that you cannot understand unless you invoke those valuable and proven physical principles. That's why theory is so strong, when it works. Especially very simple theories, like this one. Best wishes and stay well, Tom www.bluesbox.biz
×
×
  • Create New...