Free Reed Tongue Tip Offset Theory

[ttonon]

My thoughts have swung to free reeds again and it occurred to me that there may be an interesting way to theoretically determine an optimum static offset distance for the quiescent tongue.  As most of us here know, the offset is the distance the tip of the motionless tongue stands away from the plane of entrance to the slot below.  I’m sure some of the makers here ask, what’s the need for such a theory?  There isn’t really; makers get along well without it.  But for those of us fascinated by the operational details of the free reed, such a theory can illustrate more of the intricate physical principles by which our musical source works. 

 

My interest in this topic was piqued during a discussion on this forum when several of us were postulating just how the tongue starts vibrating.  Someone posted slow motion videos of the starting tongue and what struck me was that the first sign of motion of the tongue tip was a minute vibration that slowly grew in amplitude, until the tip entered the plane of the slot opening.  At that instant, the amplitude of vibration increased very rapidly. 

 

The western free reed has a notoriously slow start transient – the time it takes from the application of pressure difference to the moment in which the musical tone can be considered to be fully developed.  In fact, there was a period during the 20^th Century when free reed organ pipes were in disfavor, precisely because of that.  Typically it takes many tens of milliseconds for the transient.  I myself think this feature is a hindrance to the bellows driven version in some musical settings.  Perhaps it can be improved by somehow linking the key to an arm that flips the tongue the moment the key is pressed.  Such a mechanism might eliminate the sluggish start period of time in which the tip is building up the amplitude of its vibration outside the slot.  Apart from the time delay, a short, sudden start transient usually adds color and character to the entire musical tone.  When we hear the fully developed tones of many musical instruments, when the start transient has been digitally removed, we can’t distinguish them, for instance a violin from a free reed, or even a beating reed, such as a saxophone, or a string sound from a guitar or piano.  A crisp start transient also helps distinguish a musical instrument from other instruments in an ensemble, and it helps distinguish one note from the instrument from another note from the same instrument.  The piano is a marvelous instrument with a wonderful, percussive start transit.  In my opinion, it’s why it works so well in Jazz, hammering out complicated chords in which individual notes can be well distinguished, much unlike the accordion, or English concertina.  This is an interesting, though large topic, and let’s get back to nudging the tongue in order to start speaking. 

 

The fact is, when the tongue is in that entrance plane, the static pressure force on the tongue is maximum.  Assuming that we have a perfectly made reed with a tongue that perfectly fits its slot, this maximum pressure force is equal to the (static) bellows pressure difference times the footprint area of the tongue.  There are no other static pressure forces in the system that can be larger than this, because before that moment, with the tongue still vibrating outside the slot, the static pressure in the air flowing around the sides of the tongue, under the tongue and into the slot is everywhere a little above the static pressure on the underside of the slot.  It’s when the static pressure under the tongue equals the static pressure below the plate that the maximum bellows static pressure force is experienced by the tongue, and that occurs when the tongue completely covers the slot, blocking all air flow.  This explains the observed very rapid increase in vibration amplitude.  At that moment, the mechanism for nudging the tongue changes from eddy induced vibration (explained below) to a more direct and much larger pressure force that acts uniformly over the total top area of the tongue.  At that moment of tongue coverage, we can write, during push of the bellows and a rectangular tongue, J = (Pb – Pa)*L*W, where J is total pressure force distributed uniformly over the tongue, Pb is bellows pressure, Pa is atmospheric pressure, L is tongue length, and W is tongue width.

 

The above static pressure description is really only secondary to the offset theory I’d like to present here.  However, it prepares ground for good visualization of the physics. 

 

My other key observation of the slow motion video start of tongue vibration was that the initial, small amplitude vibration of the tongue – before being forced into the slot – was obviously (to me) the result of vortex induced vibration (VIV), or at least periodic eddy induced vibration.  VIV is an extremely well studied phenomenon, being of interest to architects and aeronautical engineers concerned for the potential damage such a process can cause to large, expensive manmade structures such as bridges, buildings, transmission towers and lines, etc. and aircraft, rockets, and the like.  Most of us know about the collapse of the Tacoma Narrows bridge, only four months after it was built over Puget Sound, Washington, in which self-induced vibrations shook the bridge to pieces in only a 40 mph wind, causing the death of a pet dog in a car, both of which were never recovered in the 200 foot deep water below.  For many such structures, VIV is bad; for free reeds, it’s good, because your concertina can’t speak without it (I think). 

 

Two most notable individuals associated with understanding vortex formation are Vincenc Strouhal, a Czech, and Theodore Von Karman, a Hungarian-American. Strouhal studied the inherently unsteady process of vortex formation in the wake of bluff bodies, arriving at well-known correlations between Strouhal Number and Reynolds Number.  Von Karman shed (no pun) much light on what are called “vortex streets,” which are periodic formations of vortices in the wake of a blunt body in a fluid flow stream.  Depending upon the Reynold’s Number (Re), or for a given geometry and fluid, the fluid velocity, turbulent eddys in the wake of the body form various patterns in space and time.  A large regime for Re produces von Karman vortex streets, wherein vortices appear alternatively on both sides of the object, becoming regularly spaced and periodic in time.  These periodic structures in turn cause periodic forces on the structures, and when these periodic forces couple to the natural vibration modes of the structures, large amplitudes of periodic structural motions can occur, called “galloping,” or “flutter.”  Such vortex streets encompass a tremendous range of scale, ranging from geological scales observable from satellites in the wake of Eastern Atlantic Ocean islands, down to individual blades of grass, and down to our tiniest free reed tongues.  They probably also occur in outer space.  Notice here that the wake of the free reed tongue doesn’t extend very far (taking “far” to mean in comparison to W) before any turbulent eddies approach/hit the slot.  I’m assuming the eddy interaction about the immediate region of the tongue surface is key and that perhaps the slot allows eddies to pass through easily enough not to greatly disturb the picture. 

 

The attached figure shows the St vs Re plot attained by Strouhal, around the year 1878.  This is a cleaned up plot, eliminating the large number of data points and error spread bars.  We make use of this information in our Free Reed Tongue Tip Offset theory. 

 

The Strouhal Number is given by St = F*W/V, where F is the frequency of eddy formation, W is tongue tip width, and V is air velocity past the tip.  Notice that it’s the ratio of two times, the time an air particle moving past the tip remains in the vicinity of the tip (W/V) and the period time of vortex formation, since F = 1/T, where T is the period.  This ratio has physical meaning.  For instance, if the time an air particle spends near the tip is small compared to the period, its view, or experience of a forming vortex will be as though the vortex is stationary.  Unless it’s trapped in the whirl of the vortex.  The vortex, being a whirlpool just like a hurricane, experiences very fast tangentially moving air, while the entire structure moves at a relatively slow rate (e.g., the “eye”).  Such a picture explains why St values in the figure are not much above 0.2, even though the motion is unsteady.  Normally, such low time ratios in periodic fluid motion would lead to a conclusion that individual fluid packets experience very little unsteady (time dependent) changes, allowing one to view the overall motion as “quasi-steady.”  But in this case, the washing machine regurgitation of the packets in the vortex - like Hurricane Harvey did to Houston - keeps the packets in the region of unsteadiness, leading to the conclusion that vortex formation is inherently unsteady.  Without vortices, air flow produced by constant pressure difference would cause steady flow.  Hence, vortices convert a steady flow into a periodic unsteady flow, which is necessary to start the unsteady vibration of the tongue. 

 

The Reynold’s Number is given by Re = W*V/nu, where nu is the kinematic viscosity of the fluid.  As I understand it, this expression was first introduced by George Stokes, an Irishman, but it was made popular by Osborne Reynolds, another Irishman.  The Reynolds Number is an extremely important parameter in fluid flow, showing up in all kinds of disparate flow conditions.  It is usually interpreted as a ratio of forces, inertial forces to viscous forces, and I recommend that interested people check out the wiki page (https://en.wikipedia.org/wiki/Reynolds_number) that explains the progression from these forces to the expression above.  This page is useful also because it shows a cartoon video on the vortex street behind a bluff body in fluid flow.  It's a beautiful play produced by Nature, and from it, one cannot but help not to make the connection between vortices and periodic structural forces.  (This is an example of the www at its finest, and incidentally, wiki asks all of its users for a measly $3 donation per year.) 

 

With that background, the gist of the Free Reed Tongue Tip Offset Theory is to first make two important assumptions.  1) the minimum bellows pressure to start the eddies is that static pressure that will hold the tongue tip into the entrance plane of the slot.  2) the dissipation (friction) in the system is small enough to allow a very sharp and pronounced resonance when the tongue is excited by an external periodic force very close to its natural frequency.  That last complicated sentence can be greatly simplified by stating that a “high Q” is assumed.  Anyone who has studied vibrations or electric circuits should know what that means. 

 

Physically speaking, what we are doing here is to assume that, in order to start speaking, the tongue needs to be excited by a periodic fluid force that is near its (first mode) resonance frequency, and that periodic force is the force supplied by formation of periodic eddies in the wake of the air flow passing over the tongue tip.  When those frequencies match, we have excitation.  Strictly speaking, we are not even assuming a vortex street.  We are only using the data from Strouhal, in which he has correlated discernible periodic wake forces, as expressed by F in his expression for St (above).  We know that, for Re from about 47 to many thousands, as in the case with free reeds, vortex streets are the most likely outcome, but that’s an added understanding to the starting process details. 

 

 Continuing, one then looks up the Young’s Modulus (E) and bulk density (rho) for the tongue material, decides on the geometry of the tongue, using W, L, and t, the tongue thickness, and assumes a starting value for a, the tip offset.  The rectangular cross area moment of inertia is calculated as I = W*t^3/12.

 

Assumption (1) then allows calculation of the start pressure difference Ps (push or pull of the bellows is inconsequential) in terms of the assumed a and geometry, using a well-known beam formula for a cantilever, fixed at one end, free at the other (where the tip is), and uniformly loaded (as in the case here, with a constant pressure difference).  The formula is Ps = a*E*I/(W*L^4).

 

From the calculated start pressure, Ps, one calculates the resulting air velocity for the air moving about the tongue tip, as follows: V = (2*Ps/rho)^0.5.  This air flow of course occurs before the tongue covers the slot, and it’s obtained from energy considerations (Bernoulli), not dependent on the particular geometries, passages, and streamlines. 

Next, calculate the Reynolds Number: Re = W*V/nu

 

Here, we use Strouhal’s correlation in the attached figure, between Strouhal Number, St, and Reynolds Number, Re.  The fact that it’s in graphical form breaks complete automation of the calculation.  I did make curve fits for this correlation, but I used Excel, and there I’m limited in the number of functional forms.  Notice that the graph is a semi-log plot, so the polynomial fits offered cannot cover the entire range of Re.  I thus split the graph into three fits, all using a 6^th order polynomial.  I will return to this shortly.

 

Once we find the appropriate Strouhal Number value for the Reynolds Number calculated above, we can calculate the frequency of periodic eddy formation from: F = St*V/W, which is derived from the definition of St, above. 

 

With F now calculated, one compares this value to the actual vibration frequency of the tongue.  If the calculated F and the actual frequency differ too much, we then assume another offset, a, and redo the calculation until we get reasonable agreement.  Physically, this comparison completes the application of the model, which states that the starting eddy frequency is very near the actual vibration frequency. 

 

Returning to the graph issue, I have put in an Excel spreadsheet the entire calculation for a.  There’s an input section (material properties, geometry, etc.), a calculated parameter section (I, Ps, V, Re, etc.), a logic (decision) section in which the user selects the proper curve fit for the calculated Re with a simple click, and a final comparative section for the two frequencies. 

I would gladly email this spreadsheet to anyone who’d like it.  A small request I make is that anyone who receives it to please not share it with others, because I’d like to know who the interested people are.  Or, if you do send it to someone, please let me know who it is.  For that, I thank you.  Though regardless, I won’t contact my lawyer over it. 

 

As a finer point of discussion, the two assumptions listed above are perhaps a bit contradictory.  The second requires a large resonance response, yet the first chooses a static start pressure, Ps, able to statically deflect the tip an amount equal to the offset, a.  Considering the fact that any mechanical resonant system acted upon by a periodic force produces vibration amplitudes much larger than the amplitude of deflection that force would produce in a strictly static process, one might suspect that the calculated Ps is far larger than the true Ps.  But I think it’s more complicated than that.  First off, there is no real Ps operating on the system.  It’s a fictitious quantity necessary to estimate a start air flow velocity, and since pressure is the only mechanism here to cause air flow, we look for the minimum pressure that can be defined by the given parameters.  The actual external periodic force on the tongue that causes motion is that due to the dynamics of eddy formation.  This force is different from a Ps pressure force.  The eddy dynamics are of course ultimately the result of the applied static bellows pressure, Pb, but there isn’t a one-to-one identification.  For me, at least now, I think the idea may give decent results.  Of course, we can add complexity to the model by defining a modified start pressure, Ps’ = k*Ps, where Ps is as before and k is some number less than unity that would have to be inputted.  I’d be surprised if anyone would be interested enough in this modification to try it, because it means for the maker to keep track of how well the model works for different assumed values of k.  I clarify this for purposes of completeness.

 

From calculations, it seems this model predicts at least realistic values for a, though I plan to investigate it experimentally.  Trouble is that my workshops are a mess now, in the middle of major clean up.  My guess is that makers – like myself – are so familiar with setting a value for a that it’s done almost by second nature, with simple start trials to check out the setting.  But I’m not a maker, and if there does appear that such a theory can be useful in any aspect of this chore, I’d appreciate knowing about it.  For me, it was just fun enough putting the pieces together.  It gave me a better physical feel for underlying physics, and as far as I know, it’s entirely my own.  For instance, I have not come across any hint of it in academic literature on acoustics and vibration. 

 

Best regards,

Tom

www.bluesbox.biz

Edited October 21, 2018 by ttonon
To separate paragraphs. Why wasn't I given the "preview" option?

[Ken_Coles]

Tom et al.,

 

While only a few follks here are qualified to take up Tom's discussion (and I'm sure they'll turn up), I have to note that these are all terms from my childhood. My late fater was a professor of aeronautics and mentioned some of these issues, particularly turbulent flow and Karman vortex streets, all the time I knew him. (von Karman was a character; my mother, now 95, remembers him but that's another story.) In the 1970s my father did an experiment in a ten-foot wind tunnel with a graduate student where they mounted a metal tube perhaps 4 inches/10 cm in diameter transverse to the flow and measured the vortices shedding off it with a hot wire (a thermocouple whose temperature varies with the speed of the air passing and thus cooling it). My father said. "The shedding of vortices in this way is what makes harp strings sing in the wind."

 

I'll dig up an example of a vortex street and add an image here later for folks who don't have time to check wiki right away.

 

Ken

 

[PS: Tom, I see preview as a little magnifying glass icon near the top right of the "Add reply" window.]

[Ken_Coles]

Here is a von Karmán vortex street (including the cylinder from which they are shedding), courtesy of Sanjay Kumar and George Laughlin, Dept. of Engineering, U. Texas Brownsville, who posted it at https://www.aps.org/units/dfd/pressroom/gallery/2009/kumar09.cfm.

 

Ken

 

[ttonon]

I'd like to comment further on the topic I discussed in the next to last paragraph of my post above, concerning the need to choose a (minimum) start pressure, Ps.

 

Minimum and maximum start pressures, Ps:

 

It's possible for the tip offset, a, to be so large that the tongue will not speak.  This would occur if the eddy induced vibration amplitude of the tongue tip cannot grow as large as a.  With any vibration, at resonance, the vibration amplitude is fixed by the dissipation in the system, the energy supplied by the external periodic force being then balanced by the dissipation in the system.  In my own experience, it's also possible for the offset to be too small for the tongue to speak.  I've seen the tip trapped in the slot entrance, in the presence of bellows pressure.  Such a fact gives credence to the basic assumption here; i.e., that an unsteady turbulence in the wake of the air flow passing over the tip is required to initiate the tongue to vibrate, then speak.  Because of this fact a maximum  start pressure Ps should be that required to statically deflect tongue so that the tip lies just within the slot entrance.  Experimentally, we can find a minimum  start pressure by pushing the tip into the slot with a sharp edge, at a given bellows pressure.  Then let go of the tip and see what happens, keeping the same pressure applied.  Try this for different pressures, starting from very small.  With the minimum pressure, I envision that upon release the tip will come out of the slot (because the pressure is less than the Ps max), but then start vibrating because of turbulence, and quickly start speaking.  We have thus found the minimum start pressure that induces speaking, for that particular value of tip offset.  This is a simple experiment, which I plan to do, and I'm interested in the difference between the minimum and maximum values for Ps.  My guess is that the difference should increase as the tip offset increases, up to the point where the tip offset is too large to allow speaking, as explained above.   

[alex_holden]

Hi Tom, I'm afraid most of the maths goes over my head. Something I've noticed is that reeds with a lot of clearance between the tongue and frame seem to be less prone to choking at high starting pressure than reeds with tight clearances, conversely though a reed with a lot of clearance will tend to have a higher minimum starting pressure.

[ttonon]

Hi Ken, thanks for the interesting comments on your early experiences.  If you're from Indiana, I'd guess that your dad was stationed at Purdue University, a very well known center for aerospace/aeronautical education.  My best wishes for your mom, and thanks much for the dramatic picture of a von Karman vortex street. 

 

Alex, thanks for your comments.  I believe the theory does support an interpretation consistent with your observations.  Larger offset means that the spring of the tongue can resist being trapped inside the slot with higher bellows pressure, up to a limit, I suppose.  Also, with larger offset, the theory predicts higher starting pressure, again because of the springiness in the tongue.  

 

Have you observed for small offsets that when the reed is choked, the tongue sits motionless in the slot?  (I guess so, since this is what we mean by choking.)  

 

Best regards,

Tom

[Don Taylor]

Tom:

 

Just to be clear before I start messing with some audio samples.  

 

Are you saying that the start transient, the first few milliseconds, defines most of what we perceive as the timbre of a musical instrument?  (I have always assumed that it was the relative amplitudes of the harmonics that performed this magic trick for us).

 

I could extract Phil Taylor's concertina samples from his sound font, edit out the beginning of each sample, make a new sound font from the chopped samples and make sample recordings from both sound fonts. Would this prove anything?

 

Don.

 

 

Edited October 24, 2018 by Don Taylor

[alex_holden]

45 minutes ago, ttonon said:

Alex, thanks for your comments.  I believe the theory does support an interpretation consistent with your observations.  Larger offset means that the spring of the tongue can resist being trapped inside the slot with higher bellows pressure, up to a limit, I suppose.  Also, with larger offset, the theory predicts higher starting pressure, again because of the springiness in the tongue.  

 

What I meant was that, in my experience, reeds with a lot of leakage between the tongue and frame seem to be less prone to choking, so you can get away with setting the tongue lower than you would need to with a reed made to tighter tolerances. Leaky reeds struggle to start at very low pressure, so it can be helpful to set them lower to help them out.

 

48 minutes ago, ttonon said:

Have you observed for small offsets that when the reed is choked, the tongue sits motionless in the slot?  (I guess so, since this is what we mean by choking.) 

 

I just did a quick experiment on the tuning bench, both with a Lachenal reed with poor clearance and a Holden reed with fairly tight clearance. I set them too low, then applied a sudden high starting pressure. In both cases the tongue pulled down into the opening and appeared to be motionless. The Lachenal reed produced a significant hiss/whistle from air leaking around the tongue while choked; the Holden reed made a very quiet hiss. In both cases, when I kept the choking pressure applied long enough, after several seconds I heard a very very quiet sound at the reed's pitch for a second or two (though the tongue still appeared to be motionless) before the reed jumped almost instantaneously to sounding at full volume.

[ttonon]

Hi Don,

 

Thanks for your interesting comments and suggestions.  I never thought to omit the start transient from all the notes of a musical recording as you suggest and it would be a very interesting experiment.  If you’re willing to do the work, I guess you will find some interesting results. 

 

Other researchers have done experiments in order to evaluate the importance of start transients (although in accepted terminology we should replace that term with “attack”).  For instance, playing a musical tone from a piano or guitar backwards in time produces incomprehensible sounds.  We rarely experience musical tones without the associated attack, which is remarkably important to our entire perception of tone.  In fact, as humans are likely to constantly find complexity in Nature, in the literature you find the terms “perceptual attack time,” as opposed to “perceptual onset time,” PAT and POT.  PAT is the perceived time it takes for the attack to be completed, and POT is the perceived time when the onset of the tone begins.  Of course, these are different from the time of the actual physical evidence for the start of the tone; i.e., a pressure pulse, or the time it takes for the actual physical duration of the transient, as measured by Fourier wave forms.  Usually, the fundamental and low overtones are established first, with higher overtones establishing later.  At some moment our brains tell us that the tone started, even though the higher harmonics of some musical tones are still building, sometimes taking up to a second or so to complete.  And then, in some tones, transients never stop; i.e., there’s a distinction between a steady tone and a constant tone.  We don’t experience constant tones.  We call a tone steady when it doesn’t appear to change in time; however, it does change with most any musical instrument, in minute ways by which various overtones keep adjusting. 

 

I hope you don’t think I’m digressing too far here.  I sense that many members here are interested in these perhaps esoteric subjects, and if anything else, it all describes the enormous complexity behind the operation and perception of the free reed tone, even though it’s often assumed to be a simple minded thing. 

 

But more direct to your comments, I quote the abstract of the paper, “Attacks and Releases as Factors in Instrument Identification,” Charles A. Elliott, Journal of Research in Music Education, Vol. 23, No. 1 (Spring, 1975), pp. 35-40, below.

 

Abstract

Identification of musical instruments according to their individual timbres was the focus of this study. It was theorized that the attack and release of a tone could be a factor in identifying specific instruments. For testing purposes, a two-part master tape recording was prepared--part A containing 18 randomized instrumental tones with attacks and releases spliced out, part B containing 18 unaltered, randomized tones sounded by the same instruments as in part A. A total of 57 graduate music students served as subjects. Results showed that in part A (attacks and releases removed), only three instruments--Bb clarinet, oboe, and trumpet--were correctly identified a significant number of times; in part B (unaltered tones), all instruments except the cello were correctly identified a significant number of times. For all participants, the mean score was significantly higher on part B than on part A. Thus, it was concluded that attacks and releases may well be influential factors in differentiating between and identifying specific instrumental tones.

 

Notice in the above that the “release” of the note can be an important musical clue, since the manner in which musical tones terminate can be complex, especially when considering perception.  The experiment doesn’t seem to separate attack from release, though other experiments yield similar results only for attacks. 

 

The experiment you suggest I think carries this idea to the ultimate.  Usually, experiments like the above involve the presentation of long duration, single note audio to a listener, and there, the mind is less cluttered with all the other stuff in music performance.  There are of course other experiments, and one I recall concluded that with many fast, short notes being played, the listener is relying virtually only on the attack, and not the steady tone for identification of different instruments. 

 

Your suggestion for a magnetically enhanced attack is interesting, though such initiation of vibration cannot be as fast as a direct mechanical stimulus.  Unless maybe if you borrow one of those superconductive magnets they use at LHC, but such an apparatus could be dangerous for the player and anyone else in the room! 

 

Best regards,

Tom

[ttonon]

 

Hi Alex,

 

Sorry I misinterpreted.  That’s an interesting observation and consistent I think with what I’m proposing.  The leakage means a couple things.  First, it reduces the maximum static pressure that the tongue could experience, because the leakage flow causes a slightly larger pressure under the tongue than the static pressure under the reed.  The max pressure difference is thus slightly less than the full bellows pressure (either + or -).  It's thus harder for static bellows pressure to hold the tip in the slot (to choke). A smaller offset therefore increases the effect of the reduced static pressure.   Second, leakage probably aids the formation, or persistence, of turbulent eddies in the wake of the tongue because of enhanced air flow.  The tip can now rest closer to the slot before the valve effect cuts off the airflow necessary for the eddies. 

 

I should correct myself here.  According to the usual rules in the Physics community, I have an hypothesis with this eddy idea, not a theory.  It’s not yet a theory until substantial experimental evidence has supported it.  But since hypotheses that satisfactorily explain heretofore unexplained phenomena, maybe we’re on the road to a theory!

 

Thanks for your diligent reed experiment, for which I commend you, and I think it demonstrates remarkable behavior.  According to my hypothesis, the explanation why a small bellows pressure can start a tongue with small offset, yet a large bellows pressure will choke the tongue in the slot is that, with small pressure the static deflection it produces in the tongue is less (linearly so with pressure), and a wake is allowed to persist with the formation of periodic eddies.  Then, yada yada, the tongue initiates vibration by the resulting periodic forces.  With high pressure, the tongue is quickly bent down into the slot, not allowing enough time for resonant response to build to sufficient amplitude in order to initiate vibration.  It always takes time for resonant amplitude to build, because of inertia in the structure and in the surrounding air. 

 

Your subsequent observation is very enticing.  It’s easy to see why the sloppier fit will cause louder hiss, but the fact that you observe that both reeds spontaneously started while keeping the choking pressure applied after a few seconds puzzles me.  How closely did you observe the very start of speaking?  Did the tongues first come out of the slot, then you heard the low tone, or did you hear the low tone while they were still inside the slot?  Were the tongues well within the slot or very close, just outside it?

 

If you heard the low tone while the tongues were in the slot, then apparently there was some unsteady turbulent effect going on, which is what puzzles me.  The flow through the small gap between tongue and slot wall, with constant Pb, is steady and no doubt laminar, because the Re there will be very small (based on the width of the gap).  Once the flow is through the gap, and since the flow is steady, a jet forms upon exit from the gap.  I would expect this jet to be laminar as well.  After a distance, a laminar jet can turn into a turbulent one, and maybe there is the key.  Such a distance is a certain number of jet widths, and since this gap is in the thousandths of an inch, it’s possible that the jet is turbulent by the time it exits the slot.  Whatever, with turbulence, there’s the possibility of periodic eddies.  The presence of the slot wall on one side of the jet does complicate the mechanism.  But I’m guessing here and I’ll have to think more on it, with maybe more input from you.  The key here may be exactly where the tongue is when you hear the very first instance of tonal sound.

 

Best regards,

Tom

[Ken_Coles]

5 hours ago, ttonon said:

Hi Ken, thanks for the interesting comments on your early experiences.  If you're from Indiana, I'd guess that your dad was stationed at Purdue University, a very well known center for aerospace/aeronautical education.  My best wishes for your mom, and thanks much for the dramatic picture of a von Karman vortex street.  

 

Actually I grew up in Pasadena, California, near a technical university some have heard of. It was a fascinating childhood (JPL nearby during the first exploration of the solar system and so on). One of my father's former graduate students is now on the faculty at Purdue.

 

Back to our regularly scheduled discussion!

 

Ken

[ttonon]

Hi Don,

 

I see I misunderstood your experiment.  You propose to use a synthesized tone that has its attack transient chopped off in a recording that is perhaps made with a digital keyboard?  This is also very interesting.  

 

Related to this is the complexity of perception of musical tones (Perception of Attack Transients in Musical Tones, chrome-extension://oemmndcbldboiebfnladdacbdfmadadm/https://ccrma.stanford.edu/files/papers/stanm17.pdf).  An experiment was done that involved using different musical instruments to play successive notes in a musical composition (klangfarbenmelodie).  The musicians had a difficult time playing in fast passages their required note at the exact "time" as written.  I think most musicians can appreciate how difficult such a task would be.  Yhus, computer music fans decided to take recordings of the separate notes separately, then piece them all together according to the written composition.  An interesting problem occurred upon playback.  According to listeners, the separate notes were not placed into the master recording at the right times.  The rhythm was way off.  It turns out that our perception of attack differs greatly when perceiving notes from different instruments, as I allude to in my other post, regarding the definition of PAT.  Thus, PAT needed to be considered when computerizing the sound file.  As explained in the above reference, a relative PAT (RPAT) can be incorporated, in which any instrument in the orchestra could be used as a reference.  So the time difference between the physical occurrence of a pressure pulse of a violin and its PAT can be referenced to a known PAT of another instrument, and this RPAT can be used for any note of a violin.  In the reference, there are too many other complications presented than I could present here.

 

If your recording contains only the truncated synthesized notes you generate, there should be no complications involving PAT.  However, if you try making your recording with a background of other instruments playing and interlacing with your melody, you may run into problems of rhythm.  Incidentally, it occurs to me that any musician familiar with the sound of his/her instrument must have an intuitive feel for the PAT of the notes played.  here may be significant variation of the PAT for different ranges of the same instrument, and maybe some musicians are aware of that while playing, or the adjustments may be unconscious. 

 

I still encourage you to do the experiment.

 

Best regards,

Tom

[Don Taylor]

Tom

 

My samples are from a real concertina (a baritone EC), there is no obviously audible background noise other than a very low level of ambient noise.  I think they were taken in a studio, or near studio, environment.

 

I will play the samples back through Musecore, a music score program which has midi playback,  so that the basic music will be identical, only the samples will be different.

 

I hope to have something to listen to next week.

 

Don.