ttonon
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Posts posted by ttonon
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14 hours ago, Dana Johnson said:
Sorry, I haven’t mastered responding to quoted sections.
Very simple. Just highlight the text in my post that you want to quote and a little window will appear immediately with ""quote selection." Click on it.
14 hours ago, Dana Johnson said:i still get caught making incorrect analogies between the slugs of air in woodwind finger holes and the flow through the ports, though with reeds vibrating hundreds of times per second, sound waves kinda have to result somewhere in the process.
Here's more. Impedance is defined as oscillatory pressure divided by oscillatory velocity, and the definition is sometimes useful, sometimes not. Bellows pressure is not oscillatory, and it can be considered steady, or constant, for the purpose of examining its affects on tongue vibration. The slugs of air going in and out of a finger hole in a woodwind is caused by the wave motion inside the body of the instrument. This wave motion is characterized by standing waves - waves that are the additive result of forward and backward moving traveling waves - producing pressure and velocity oscillations that are a function of time, but not position (hence, "standing"). Thus a given finger hole, being at a fixed position ejects an air slug when the pressure just inside the hole, caused by the standing wave, rises (above atmospheric), and the motion there is periodic. This is entirely a wave phenomenon and you can define an impedance for the air oscillations in and out of the hole (with time average flow being zero). On the bottom of the vent in a free reed, there are pulses of air, but they are not caused by wave flow, but rather by the opening and closing of the tongue/vent valve. In order to understand the nature of this oscillatory flow, we need to look at the equations of fluid mechanics, coupled with the governing equation for the tongue vibration. The simplest equation for tongue vibration is the Euler-Bernoulli equation, which can also be called the tongue's wave equation. Once the motion of the air slugs passing through the vent is calculated by this approach, you can now define an impedance for that oscillatory air motion, the same way as for a finger hole. At this point, the oscillation does not care how it is produced. In fact, we can use that definition of impedance for the underside of the free reed to couple it to the equations governing the air vibration in the cavity, or for resonators. I was maybe too restrictive to say that impedance is not helpful here. My point is that it's not helpful in understanding how the pulses are formed and the effect of the reed geometry on those pulses. Once we understand those pulses, we can define their impedance in order to do further study. Trying to understand how those pulses are formed by invoking impedance can give the erroneous view that there is a sound wave passing through the vent and encountering a change of impedance because of the sudden area change. That's not what happens. But it does happen with the woodwind, when the forward traveling wave meets the end of the tube, or bell, and out to the atmosphere, where there is a large change of impedance. There, impedance is a very useful concept. The free reed is its own animal. Very unique when it comes to musical instrument sound sources.
14 hours ago, Dana Johnson said:4” is quite loud for my concertinas. I do measure it directly through a port in my tuning bellows, though I just use the bellows for mounting my reed pans and use a coaxial blower to generate the vacuum.
So you're not measuring bellows pressure generated by human muscle during playing, and my guess is that your judgement on the magnitude of driving pressure is made by the perceived volume you hear. I wonder. As I said, I measured the bellows pressure in an actual instrument, while playing. The bellows I used measured roughly 18 x 6 inches, inside measurement. Pushing on that "piston" with 20 lbs of force produces about 5 inches w.c. My guess is that accordion players could produce such a playing force, though that's probably around the highest. A concertina cross section is, my guess, around 6 x 6 inches, so that the same force produces three times the bellows pressure. I doubt concertina players push anywhere near 20 lbs, but these figures to me suggest that 4 inches is well below the capability for the instrument. All this may only prove that concertina players play for reasonable volume, and the difference from accordion players is interesting. But all in all, the free reed can sound extremely loud with your ear next to it, and that perception is quite different from what we get when it's being played inside an instrument by a musician wanting to be heard above other instruments in a session. It would certainly be an interesting experiment to fix an instrument with a pressure transducer and record the data during a session.
14 hours ago, Dana Johnson said:I do find a lot of variation in pitch with the degree the bellows is extended, Maybe as much as 5-6 cents, even at constant playing pressure.
That's surprising. Do you see higher or lower pitch with extended bellows?
14 hours ago, Dana Johnson said:I don’t find having the higher reed ports closely match the reed widths makes much difference, so I avoid an extra tool change.
You're saying that the precision with which the tongue fits the vent doesn't make much difference? Intuitively, I would've guessed that a tighter fit would lead to more higher harmonics, but maybe they're just not noticeable.
14 hours ago, Dana Johnson said:I do increase the window draft angle from 5 degrees per side at C3 to 20 degrees per side at G6. For my reeds I find this increasing draft angle is required to get a similar volume at a similar pressure over the entire note range.
This is most interesting, because I'm in the process of extending my model to include the effect of a draft angle on the sides of the slot, as well as the effect of a gap in the fit between the tongue and the vent. Do you put the draft on all sides, including the tongue tip side? Of course, there, the tip naturally pulls away from a non-drafted side as it travels through the vent, and this can be calculated. But that effect is much smaller than that the draft angles you quote. Incidentally, I find that the smallest fitting gaps are a couple thousandths of an inch. Does that check with your fits? Except for the tip corners, which can go down to a thousandth.
14 hours ago, Dana Johnson said:I find this increasing draft angle is required to get a similar volume at a similar pressure over the entire note range.
Interesting. You're saying that a draft angle increases volume, something that I'll look for in calculations.
14 hours ago, Dana Johnson said:I hypothesize that thinner reed sets may need less draft angle because they deflect more at a given pressure than heavier reed sets.
Confusion here. If "thinner reed sets" means thinner tongues, they might bend with more curvature than thicker tongues. Is that what you mean? The higher curvature would mean that the tips pull away more from the tip side wall, suggesting that they would need less draft than thicker tongues, and I agree. But your results seem counter intuitive, because as I mentioned above, tongue travel through the vent is prime time for which static bellows pressure can move the tongue. When you cause more leakage between the tongue and vent wall, that pressure effect decreases. Of course, some of the effects that happen during down-travel of the tongue through the vent are compensated for by up-travel a half cycle away. This is something I need to ponder.
14 hours ago, Dana Johnson said:I don’t really know why varying the draft angle has the effect I find, just that it answered my question about keeping the high reeds volume comparable to the low at the same playing pressures.
It's an interesting feature to explore. My experience with accordions is very limited, but I've never noticed any draft angle with accordion reeds. Maybe someone else here can add to that.
Best regards,
Tom
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Many thanks Dana for what you've done here: a plethora of information that will be of much help. To be sure, the notation f# 5 invokes the piano keyboard, so this reed has a fundamental vibration frequency of about 740 Hz, correct? What I understand is that this seems to be about the dividing line between reeds that have their tongues that most often travel through the shoe, and those above that do not, generally, but not absolutely. Since you make the vent for this f# reed slightly longer than the reed window (the hole in the wooden chamber wall), my guess is that you're not too concerned about the tongue hitting the wood during normal playing. Is that right?
On 1/23/2020 at 9:09 AM, Dana Johnson said:Also, I find that reeds in overly thin shoes can lack power, and the long reeds benefit from the added time the reed spend in the confines of the shoe vent.
Yes, in my physical model, the time spent while the tongue moves through the vent is when the major portion of energy is transferred from bellows pressure to the vibration. With analogy, this is the moment the father pushes on the swing his boy is riding. There is a small contribution when a tongue exits through the vent, but still moving downward, due to the air jet passing through the vent and hitting the tongue. But that small contribution is then largely cancelled when the tongue comes back up, before again entering the vent.
On 1/23/2020 at 9:09 AM, Dana Johnson said:Nor do I have any idea about the small slug of air in the port that goes from tight confines immediately to complete openness. I wonder what might be reflected at this change in impedance.
My explanation above about the jet addresses this. "Impedance" is not a good description of the underlying physics. Impedance is a useful concept for understanding the behavior of wave motion. Here, we're dealing with fluid mechanics, no waves.
On 1/23/2020 at 9:09 AM, Dana Johnson said:What happens once the reed enters the port in the reed pan I don’t really have a good idea of since the gap at the reed tip becomes quite large in comparison to being within the shoe window.
Yes. This sudden widening of the gap is what ends the (static and dynamic) bellows pressure force on the tongue. Without the vent, the action on the tongue from the bellows pressure is indirect. When the tongue is below the vent, the air jet caused by the bellows pressure acts on the tongue. And as I said, this aspect of energy transfer is both positive and negative, with nil net result. When the tongue is above the slot, there is nil direct influence from bellows pressure. There are small effects caused by air motion into the slot just before the tongue gets there.
On 1/23/2020 at 9:09 AM, Dana Johnson said:I tune my reeds at around 1” of water column, though bellows playing pressure can certainly go as high as 4”.
Have you measured bellows pressure directly? Years ago I attached a manometer to a full size accordion bellows and concluded that the larger pressures should be around 6". That's for those bellows with a push area much larger than concertina bellows. Aso, I didn't push with all my might, and I do know that some gorilla accordion players can be exceedingly loud. I thus concluded that concertina maximums should be around about twice that. Of course it depends on how strong the player is, and maybe the limiting pressures are determined by what happens to the reed. What are your thoughts on this? Do you think the reed can be damaged at such high pressures? What happens to the sound of your reeds when you push or pull on your bellows with all your might?
On 1/23/2020 at 9:09 AM, Dana Johnson said:My target for starting to sound is .2” of water.
I'm impressed with how low that is. If you recall, I came up with a theory on how to predict the minimum starting pressure for a reed and published it here a few years ago. I'll have to go back and look see that post.
On 1/23/2020 at 9:09 AM, Dana Johnson said:Travel at those pressures is really minimal, just requiring the reed tip to enter the window.
I'm very curious now, concerning pitch, or vibration frequency. Can I trouble you, or anyone else when you get a chance, to observe a real time reading on a pitch meter (tuner) to see how the pitch changes as you slowly increase bellows pressure (either plus or minus), from the very lowest pressures to the maximum pressure? Just qualitative results, such as "higher" or "lower" would be very interesting from a theoretica point of view. One complication here is the presence of the leather valve, which moves as pressure changes, and that might have an effect on pitch, so ideally there shouldn't be a valve.
On 1/23/2020 at 9:09 AM, Dana Johnson said:If they were in the open you could use a strobe and make a feeler jig to gage the physical travel, but absent the chamber interaction, it would hardly be representative of real world data.
Yes, in my workshop, which currently is in disarray, I have an electric powered testing rig, with the reed mounted over a window. The electric blower can suck steady air through the reed, with the tongue vibration freely visible and accessible. The strobe is a nice touch. The "real world" data I'm after does NOT contain a chamber, or any confined space. I'm interested in the reed vibration by itself, without any other physical phenomenon going on. Once I understand this, I can formulate the case when the reed is mounted on a cavity, or resonator, which is an entirely different problem. I'm sure you'd agree that I cannot hope to analyze all real world effects, only to give an accurate-enough description.
Best regards,
Tom
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On 12/16/2019 at 12:27 PM, paulbrennan said:
Thank you all for these interesting opinions! It sounds like consistency might be an issue with twanging, which is interesting in itself, does anybody know technically why this would be the case? I'm thinking of the violin played pizzicato on the high notes, it's a brief note but stable and presumably tunable (though of course a different pitch from the bowed note at the same position). If you think of the "jaw harp" there is harmonic instability from the twanging but is there pitch instability too? My background is not with reeds so these are new questions.
There are a couple details I didn't see mentioned yet in this interesting thread that might be of interest. One is that our perception of pitch is very complicated. It's influenced by the number and nature of the overtones we hear, becoming more difficult as the number and relative intensity of overtones increases. It's also influenced by the duration of the tone, becoming more difficult as the duration shortens. Also, loudness of the tone also affects our perception of pitch. In some sense, it's a wonder we can make sense out of most sounds.
In psychological experiments on tone perception with varying duration and loudness, researchers usually use a pure tone, a single sinusoidal wave, because that eliminates complications due to overtones. So plucking a free reed with its attendant overtones is not such an easy job for the brain/ear system.
When you pluck a tongue, you cause "transient" sound, meaning that it's not a steady, periodic sound in the main. However, the frequency of vibration can be much quicker than the rate at which the overall sound (its amplitude and overtone structure) changes, and thus, you can often pick up enough cycles of tongue vibration to sense a pitch. But not always.
As some have pointed out here, the musical tone of a free reed is an air driven phenomenon and most musical notes have relatively long enough duration so that the vibration can be called "steady," or periodic. But the physics of steady vibration is much different from that of plucking. With plucking, if it has long enough duration, the vibration frequency is called the natural frequency of vibration, whereas with air driving, there are enough physical phenomenon at play to produce a calculable - and often perceptual - difference.
The physical phenomenon I refer to involve so-called viscous and aerodynamic forces and are due to the air motion instigated by the vibration. These forces are also at play during plucking, but not precisely in the same way. In other words, if you would pluck a tongue by some mechanical means while the reed is in an evacuated bell jar, those air effects would not be present and there, the tongue vibrates truly at its natural frequency, a much simpler arrangement. It would be very interesting to do an experiment measuring the true natural vibration frequency of a reed in a vacuum chamber and comparing it both with the frequencies obtained by plucking and by steady air flow. The three are different. You can very accurately measure the true natural frequency by exciting the vibration in an evacuated bell jar by say a periodically changing magnetic field, acting on a steel tongue.
With my own calculations, based on a fluid mechanics study of free reed physics, I find that there's a discontinuity in the frequency of vibration as the tip vibration amplitude becomes large enough to cause it to emerge below the slot (vent). While vibrating within the slot, the vibrational frequency is extremely close to the true natural frequency - even with all that air motion -, but at the transition, the frequency suddenly drops a small amount, perhaps too little for the ear to detect. Then, as bellows pressure is increased, the frequency slightly rises.
From practice, I believe that the vibrational frequency with steady air flow drops when bellows pressure increases to a certain level. I'm not talking about any effect cause by a continual receding of the leather valve as air flow increases. I'm currently trying to understand all this from a theoretical point of view.
Regards,
Tom
www.bluesbox.biz
PS. Chris, I'm disheartened to learn of your difficulties because of the Australian fires, a world class disaster. I fear things will get worse most everywhere because of Global Warming, and we as a world community will slowly adjust to it, but not without much death and destruction, with some people more unlucky than others. Here in Eastern USA, our weather last Summer was unusual in that we had over all much less sunny days. A lot of overcast and cooler weather, but I don't think much more rain. So far this Winter, we haven't had much cold nor precipitation. Very mild and pleasant.
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Greetings,
I have a question for the kind folk here, particularly the makers, or any one else very familiar with the details of tongue vibration with some kind of documentation lying around.
When the vibration amplitude is large enough, the tip of the tongue passes completely through the slot, or vent. Whether that happens depends on the playing volume as well as the length of the tongue in comparison to the slot thickness. I'm interested in knowing how common such an occurrence is in normal playing. Is there a tongue length above which this event occurs for most all playing volumes? Is there a tongue length below which this event doesn't usually occur for normal volumes and maybe only the most loudest volumes would cause the event? It would also be nice to know in any resulting discussion something about what portion of the length of the tongue makes it below the slot and for concertinas, what would be a normal range of bellows playing pressure, say in centimeters (or inches) of water column?
This information would help me to decide on what kinds of calculations to make on the tongue vibration analysis I'm doing from a fluid mechanics approach. Whether the tongue passes completely through the slot determines the kind of mathematical formulation to use, and it would be interesting to know how accurately the model describes this event.
Now, may I be bold enough to also request whether anyone could share data - numbers - on the details of tongue vibration, say amplitude vs bellows pressure? I doubt that, but if you do, I'd have to get downright greedy by asking for detailed tongue and slot geometry information to accompany it. Yes, I know I should be doing that myself and I will, but I thought I'd get lucky by asking.
Best regards,
Tom
www.bluesbox.biz
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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
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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
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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
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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
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1 hour ago, Tradewinds Ted said:
But the average pressure of the air that reed is surrounded by on the draw is lower than on the push, maybe by as much as 1/4 of a psi (1/60th of an atmosphere)?
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.
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11 hours ago, wunks said:
Whoever invents a bi-directional single reed assembly will revolutionize the instrument.
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.
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3 hours ago, Tradewinds Ted said:
With a bellows instrument, the push is the stronger direction ergonomically. Physics also assists, as the push provides a positive pressure inside the bellows to start the air movement, limited, while the pull can only create a relative vacuum compared to the atmospheric pressure outside the bellows, so that limits the pressure differential produced. (Practically, we never approach that limit, or the bellows would collapse!) So starting on the push for the downbeat would seem the preferred direction.
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
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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
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1 hour ago, d.elliott said:
Another factor may well be that for a constant bellows pressure the velocity of air flow will be lower the larger the reeds
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
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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
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5 hours ago, d.elliott said:
when I make brass reeds I buy rolled strip ten beat it down to section.
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
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6 hours ago, Johann said:
My theoretical influence is strongly influenced from may former profession working and teaching in the field telecommunication. I never would say i can't be wrong if someone comes up with other explanations that fit better. The usual models and explanations are for me always a bit to much simplified.
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
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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
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14 minutes ago, Johann said:
No Tom see the index on the right. I did not change anything just the lins are now drown ble on top of orange.
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
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2 minutes ago, Johann said:
I have attached the same chart but this time with blue line on top.
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
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8 hours ago, Dave Leggett said:
Clay has been removed to a depth of 1.5mm from the drag reed-slot using the 'hag's tooth' tool (7, below) before the cope is applied and re-compacted.
Hi Dave, thanks for the interesting pictures and info. What's the reason to take that sand out of the slot, is it so that metal flows more easily to fill all portions? I assume you then have to remove the metal hymen. I'd opt to see pics of the freshly molded piece. Do you have an idea of the surface roughness, particularly on the sides of the slot?
Best regards,
Tom
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Hi Johann, you're right that the picture shows a tapered tongue, and I don't see much filing, so I'd guess that the tongue thickness is constant (no profiling) and that guesses about modal frequencies are probably reasonable. I apologize for not looking at that picture more closely.
The biggest question I have about the spectral data is the prominence of the 5th harmonic. Since there's no red color on the bump - or at the base of the bump - I assume that particular harmonic shape is duplicated by steel and titanium, since all other harmonics show a dark color whenever the values for steel and titanium overlap. I think you'd agree that this puts into question the accuracy of that harmonic measurement, because it's unlikely that both steel and titanium would have an identical 5th harmonic response. It's probably caused by an artifact somewhere in the recording. Perhaps a resonance in the microphone? Do you have a copy of the frequency response for your microphone? Maybe it's in the electronic circuit or some feedback to the microphone common to both metals. But it's very curious that it occurs precisely at the 5th harmonic. If we truly hear that harmonic, for any reason, and it's not something hidden in the electronics, you'd agree that such a large harmonic could dominate the sound of both reeds. If that's the case, a hearing test would be invalid. The art of experimentation is an art; it's usual very difficult to obtain good data, and as you know, bad data is worse than no data.
In my previous post, I forgot to point out that the first harmonic in my plots dominate the others by only a couple/three orders of magnitude - for the higher pressures, only one. Johann's data, and with other measurements I've seen, it dominates by about four - excluding the spurious 5th harmonic. This is a huge difference in relative amplitude, and perhaps lends more credence to my hypothesis, that higher harmonics from the reed displacement can perhaps contribute to the overall sound.
Best regards,
Tom
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I should clarify. When I hypothesized that higher harmonics in the first mode vibration of the tongue might translate to musical tone, I didn't mean to imply that the musical tone would be the result of only those harmonics. Harmonics associated with sudden changes in pressures and velocities of air motion due to the vibration are indeed a major cause for what we hear, as acoustic models and experiments verify. My hypothesis is based on the fact that different materials seem to sound differently to many listeners, and if that's the case, how else can they be heard? In more detail, it's possible tongue motion harmonics do not directly contribute, but only indirectly, as for instance if the minute motion of the harmonics superimposed on the main sinusoidal motion first affect the chopping of air flow through the slot. So rather than sound emanating directly from the quivering surface of the tongue (which is my hypothesis), that quivering mostly affects the time dependence of the chopping phenomenon, which is really what we do hear. However, whether indirect or direct, these higher harmonics need only make a contribution to the sound, enough so that we can discern it in the musical timbre. For anyone who claims that these tongue harmonics do not affect the musical tone, I ask by what mechanism can different materials affect the sound? I mean between two metals that can be fashioned into geometries having equal levels of preciseness.
So Dana, I never disagreed with Benade's explanation, and I'm assuming here his explanation has to do with the mainstream idea that pressure wave harmonics are associated with chopping airflow. My hypothesis simply adds to it, in the growing comprehension of how complicated Nature really is. As an aside, I'm curious whether Benade attempted any explanation for how different materials can be distinguished.
When I chastised Johann for spreading false concepts, I didn't mention that higher harmonics of the acoustic reed sound can be coupled to the air in the cavity, affecting the harmonics, and thinking back on it, that may be all he was referring to. But this is a very weak form of coupling, and it's far different from the "acoustic coupling to an air column" usually referred to with such terminology.
Dana, I appreciate your explanation of the practical issues involved with different tongue materials and find it fascinating. The scope of any theoretical contribution I can make to the issue is very small, and it's the maker who carries the real burden. In fact, I wouldn't object to anyone describing my suggestion as a flippant remark from a theoretician.
11 hours ago, Dana Johnson said:I am not sure how you can control for variations in stiffness in these models so all reeds are equally stiff at the designated pitch.
Can you please explain further what you mean by "equally stiff at the designated pitch." I recall you explaining that you used a scale to measure spring force of tongues. Do you use that in this process?
11 hours ago, Dana Johnson said:I don’t know how you can create mathematical reeds of these materials that differ so much in density and modulus of elasticity so that at a set pitch, stiffness and swing amplitude you can make a comparison of harmonic amplitude.
Again, can you please elaborate?
I do hear a difference between Johann's steel and titanium tongues, and to me surprisingly the titanium sounds brighter. This is emphasized while wearing headphones, as Johann suggests. There is a definite preponderance of the higher frequencies from titanium, and I notice that the steel first sounds much more "mellow," until about 2.5 seconds (where you can actually see a slight increase of amplitude on the graphics) higher frequencies ensue, but don't seem to dominate as much as with titanium. I agree that my perception seems to contradict what the frequency spectrum suggests.
As interesting as Johann's data is, it cannot be decisive. Important information is left out. Most importantly, what kind of a microphone was used, where was it placed in relation to the reeds, is that relationship the same for both, was the same microphone used for both, what is the pressure level, what are the thicknesses and lengths of the tongues, what is the width and is it constant with tongue length, is there any profiling, etc. All these can have important influences. I'm struck by the very large 5th harmonic. In my experience, it seems exceptional. I notice that around the 6th harmonic, there's a blip that might suggest a slight contribution from the second bending mode of the beam. There's also a blip around the 16th and 17th harmonic, suggesting a third bending mode, however, we can't say for sure because we don't know if the geometry conforms to that of a simple constant area cantilever, for which such conclusions can be made. Additional blips might be contributions to torsional modes, though that's not too likely, but if pressures are high enough, maybe so. Then we have to wonder about the quality of the recording equipment and the software used to calculate the Fourier coefficients.
It's clear that theoretically, experimentally, and practically, a full understanding of tongue material would require considerable effort.
Best regards,
Tom
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Greetings again. Here we compare steel to a titanium/tantalum alloy, Ti/Ta 70/30 Beta, which is used in surgical equipment and prosthetics. It interests me, not so much to propose it as tongue material, but because the tantalum increases the density well above the neat material. The modulus is only a third of that for steel, but its density is a little above that for steel, resulting in a modulus-to-density ratio about 0.3 that of steel.
We can compare these results to those comparing Titanium to steel and find that the higher harmonic amplitudes in comparison to steel are very much less in the Ti/Ta alloy. The behavior with increasing bellows pressure shows the harmonic amplitudes making up the difference – just what happens in the Titanium/steel comparison; however, since the higher harmonics of the alloy start with such low values, they don’t catch up to those of steel nearly as quickly or as completely as in the case of the neat material.
Comparing these plots with the plots for brass/steel, we see an enormous difference. If we can discern an audible difference in timbre between brass and steel, according to these plots, and if our basic assumptions are correct, there should be an enormous difference in timbre between either Ti or its Ti/Ta alloy and either brass or steel.
Another interesting outcome in the case of the alloy is that I couldn’t get a convergence with a solution for bellows pressure less than about 2.2 inch w.c. On the other hand, solutions continue further on for the higher bellows pressure than they do for steel and other materials. This might indicate a material that may not be able to play at very low bellows pressures, but may perform better than other materials at the highest bellows pressures. The last plot in these calculations show that the trend of increasing harmonic amplitude with bellows pressure continues for even the highest bellows pressures, for those above solutions for steel become impossible.
We can make such plots for most any feasible tongue material, though at some point, the usefulness tapers off, at least if we are in search of a practical tongue material. The theoretical interest continues, as a way of getting a good intuitive feel for the effect of material properties on musical tone. At this point, I think we need wait for experimentalists to compare information they have from work already accomplished or from new work. There might be other materials I'd like to do calculations on, such as glass, and other geometric factors, such as tongue thickness and plate thickness.
A docx file showing these plots are at: https://app.box.com/folder/79305691686
Best regards,
Tom
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Hi Johann, I was maybe not clear enough. I respect your conclusions on observation and I'm eager to hear about them. It's when you try to explain the physics behind them that I sometimes cannot follow, and it's in the latter area where documentation would help.
Best regards,
Tom
For what scale reeds does the tongue pass completely through plate?
in Instrument Construction & Repair
Posted
Hi Chris, so good to hear from you! Yes, something seems amiss and I guess it's back to the drawing board. A simpler experiment is to put a digital scale between one of your palms and the concertina and have someone read off the forces as you play steady notes. My guess is that the pressure calculation from bellows cross section and force should be pretty accurate, but that should also be checked. It should also be said that we need to use the same unit system, because far more disastrous results have occurred because of a mix up in unit system. I'll look for your post.
There are differences in construction between accordion reeds mounted in the common way using a reed block and the traditional concertina method. With the accordion method, the reeds are situated with one end near the air hole (port) and the other end removed, at some angle not far from 90 degrees. The concertina method places the reeds flat (zero degrees). In my experience, the flat mounting produces louder tones, because some of the melodian-type accordions, such as the Louisianan Cajun type, place them flat, and I play both. The question is, do the accordion-reeded hybrid concertinas with reed blocks sound at lower volume than the traditional concertinas (for same pressure)? But I suspect any difference would be too small to explain these differences in normal playing pressure.
Interesting comment about draft angles in accordion reeds.
Best regards,
Tom