Jump to content

Bellows pressure and musical pitch


Recommended Posts

3 minutes ago, ttonon said:

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. 

 

Are we talking about two different phenomena? What I think of as "choking" is when the pressure quickly goes from nothing to 'very high', the reed gets immediately pulled part way into the vent opening, blocking the airflow and preventing the oscillation cycle from starting up. If you bend the reed up slightly so there is a larger gap at rest, you need to use higher pressure to make the reed choke (but the reed doesn't start as easily at low pressure). This effect also happens on my tuning bellows, which doesn't have a chamber between the reed holder and the bellows cavity.

 

It sounds like you are describing the situation where a particular chamber just doesn't work for a particular pitch. The reed mysteriously won't sound, or it's very weak. You change the size of the chamber and it starts working again. I've occasionally seen that happen while experimenting, though luckily not in an instrument I built.

  • Like 1
Link to comment
Share on other sites

11 hours ago, ttonon said:

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?

 

That is a great aim, I wish you all the best in it.

 

I have often wondered if any of the larger or older reed manufacturers from Italy (or the resulting modern versions on the form of mergers) ever tried to perform such research. Or if their approach was something different more akin to a tradition with incremental improvements. Who knows, in either case they might not necessarily want to share those ideas and knowledge, business is business after all. 

 

May I recommend visiting someone who makes reeds and if they are amenable to it just spend a day or so having them explain and make some reeds to directly show the ins and outs of what problems have been encountered here and there, I have found it very hard to describe or understand what someone means by say "buzzing" or "beating" without actually hearing it in person, not just recordings. Perhaps even a collaboration with a reed maker would be possible. In my experience of academia there is sometimes a disjuncture between academic pursuits and the world of business - even to the point of each side not understanding what the other is even trying to say, when really they should be getting on well. Your research could be of real interest to existing reed makers, its great you are talking to some on here but I really would recommend visiting someone as well, you would be very surprised what unexpected approaches and practices you might find, in practice sometimes applied sort of like rules of thumb.

 

Best of luck with the project, I will follow it with great interest and respect.

 

 

Link to comment
Share on other sites

Exactly as Alex wrote, you can choke any reed, over any cavity, if you overblow it during the first half cycle. We have long ago established in discussions on this forum, that initial oscilation happens and has to happen entirely above the reedplate, otherwise the tongue sinks in the plate as you would expect from a simple valve and cavity resonance has nothing to do with it. 

 

Another question about irrelevancy of chamber shapes - aperture location relative to tongue tip makes all the difference for both reed response and strength of the tone. With long, thin chambers, location of the reed along the wall and aperture location also play significant role. In a simplified model of „only volume matters” those should not have any impact.

 

But I think I know why we missunderstand eachother - for me the isolated pitch question is not really all that important or interesting, because I can tune deviations happening due to say valve choices or anything else with a simple file. It is the response problem, the timbre problem and the volume problem that is trully important when building an instrument, and with those, everything matters to lesser or greater degree and simplistic models don’t usually provide meaningfull clues when you encounter a problem.

 

Now about burr/beating - it may help to know in this context that I’m not a native english speaker and finding an adequate translation for sound phenomena is difficult. But when I say that one partial gets amplified, I say that after looking at the spectrum in my tuning app (which sadly has no scale, only graphical representation). The spike is not present when testing without the endplate and is present with the endplate.

Link to comment
Share on other sites

8 hours ago, alex_holden said:

Are we talking about two different phenomena?

Ales, yes, two completely different phenomenon, involving different physics, with vastly different remedies.  Thanks for pointing it out.

 

Regards,

Tom

  • Like 1
Link to comment
Share on other sites

5 hours ago, Łukasz Martynowicz said:

But when I say that one partial gets amplified, I say that after looking at the spectrum in my tuning app (which sadly has no scale, only graphical representation). The spike is not present when testing without the endplate and is present with the endplate.

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

Link to comment
Share on other sites

Lukasz,

  especially for lower reeds (B3 and lower) and especially if the press and draw reeds are close in pitch,  ( and or have a harmonic component near the paired reed ) I often experience an odd coupling where the flap valve adjacent to the active reed slightly opens and closes causing a burr adding to the reed pitch.  This also happens at low to medium pressure when the valve is cupped ( touching at tip but not in center ) even for high reeds, where it opens and closes with the pressure variation, making the burr related in frequency to the active reed.  I can usually solve the low reed condition by using a valve that is thicker but not stiffer so it has a lower ability to vibrate at the activating pitch.  The noise isn’t valve noise, but rather how the driving pressure on the active reed is caused to vary by the slight bypass of the vibrating valve.  I don’t know if this will help your particular case.

Dana

Link to comment
Share on other sites

Tom, 

   I don’t consider a reed that fails to speak due to chamber length mismatch, “choking”,  what I experience as choking comes from a reed that speaks fine at lower pressure, but stops abruptly if the pressure passes a certain point.  I experience this in two cases.  First is when the reed is set too low, which may decrease the starting pressure  ( seemingly a good thing but limits you to quiet playing ).  Second is when the reed is weak for its length ( incorrectly profiled in the neutral area ).  These reeds will choke regardless of set.  
    I wonder about the hemholtz resonator analogy.  I am not sure how this works since the chamber is open at both ends, though one end changes  as the reed moves.  Is resonance the only characteristic a chamber can have?  Also, in the G1 chart, the bold numbers are for the 15th harmonic which is going to be vanishingly small, and I have a hard time seeing that as killing the vibration.  
    I wonder if electrical analogies like capacitors and inductors might be in play, since both the reed and chamber can be energy storage /modulating  devices. , and not necessarily as a tuned L/C circuit.  Maybe a power factor phase shift ?
   In my experience, varying the height of chamber walls affects the proportions of the harmonic spectrum , with shallow chambers favoring higher harmonics over fundamentals.  Length did not affect this noticeably.  
‘even .032 inches in height difference is very noticeable in tone difference.   

Link to comment
Share on other sites

9 hours ago, Auldfellowmelad said:

Best of luck with the project, I will follow it with great interest and respect.

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

Link to comment
Share on other sites

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

  • Like 1
Link to comment
Share on other sites

8 hours ago, Łukasz Martynowicz said:

We have long ago established in discussions on this forum, that initial oscilation happens and has to happen entirely above the reedplate, otherwise the tongue sinks in the plate as you would expect from a simple valve and cavity resonance has nothing to do with it. 

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

Link to comment
Share on other sites

3 hours ago, Dana Johnson said:

Lukasz,

  especially for lower reeds (B3 and lower) and especially if the press and draw reeds are close in pitch,  ( and or have a harmonic component near the paired reed ) I often experience an odd coupling where the flap valve adjacent to the active reed slightly opens and closes causing a burr adding to the reed pitch.  This also happens at low to medium pressure when the valve is cupped ( touching at tip but not in center ) even for high reeds, where it opens and closes with the pressure variation, making the burr related in frequency to the active reed.  I can usually solve the low reed condition by using a valve that is thicker but not stiffer so it has a lower ability to vibrate at the activating pitch.  The noise isn’t valve noise, but rather how the driving pressure on the active reed is caused to vary by the slight bypass of the vibrating valve.  I don’t know if this will help your particular case.

Dana

Thank you very much for this hint, it might be a part of the problem, as this is a duet concertina, but as I wrote earlier, this problem does not follow the reed when I swap them between sides of the instrument. But maybe it is a significant enough part to it. 
 

What you wrote about wall height influencing the spectrum is in line with what I have observed with my lowest reeds when trying to improve response and pitch stability (this is when I had to and could go creative with chambers in an already built instrument). 
 

@Tom, yes, those were the conclusions on initial tongue oscilation mechanism. Without allowing it to happen by overblowing the reed, tongue acts as a simple pressure valve, exactly as I wrote before. I also second what Dana wrote about too thin/soft/weak tongues being prone to choking. I have a set of russian reeds from a cheap accordion, which are apparently made from a chopped tin can :D. Those are near impossible to set properly, they either choke if you play staccato or are so air hungry that it is pointless mounting them in concertinas.

Link to comment
Share on other sites

Hi Tom,

   I am not sure what different air oscillation modes a chamber can provide, and I am pretty sure hemholtz resonator is one, like the main body resonance of a violin box, as well as a back and forth sloshing mode where the the pressure alternates ends of the box without air entering or leaving the “f” holes.  
   There is also the question of how the reed itself alters the hemholtz frequency.  Calculations for ideal resonators must have to be modified when  for instance, the chamber has flexible walls  ( lowering the frequency. )  
   Your experience of the reed being killed when the resonator matches the fundamental (or any strong harmonic ) is what I expected.  You are right about being able to alter a balky reed’s chamber to get it going properly.  On My Wheatstone Layout G/D there is a pad that opens onto a chamber with 2 reeds that are of substantially dissimilar lengths.  I found the small one of the reed  pair was much quieter than it should have been compared to reeds of similar pitch in other chambers.  I split the chamber into 2 sections and the reed immediately was at its proper response and volume.  Note that the drastic change in chamber volume had no noticeable effect on the either reed’s tone compared with other reeds at those pitches with full width chambers.  ( the reed in the short side is a press reed so is not visible. The split chamber allowed me to dispense with the valve for this small reed where without the partition, it needed one.  
   This idea of resonances interfering with the reed vibration is something I have generalized,  with absolutely no proof, to the notion that the body of the concertina behaves like a filter, suppressing or reducing the various components of the reed’s spectrum.  
    Hopefully you will be able to get as close as possible to an isolated reed without the complications of chamber volume.  You might need a good anechoic ? chamber to do your tests in because normal rooms can have a surprising number of resonances that  can cause trouble.   If you need other reeds, I’ll be glad to help if I can.

Best Wishes,

Dana

DDFAE73A-9489-4B9E-8B06-3F9B8C7DD030.jpeg

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...