Chamber Design

[Chris Ghent]

Anyone have any thoughts about the relevance of chamber volume (or shape?)? I am interested that people say chamber size is important, and suggest it may have been tuned to the reed, or the note, according to a harmonic basis, perhaps, but the examples are hard to find. And Jeffries concertinas seem to run contrary to any formula. By this I mean that I understand there is no difference between a Jeffries G/D and a C/G except the note the reed is tuned to. The same sized pieces of steel are tuned a fourth apart, and both sound good. This suggests chamber size, reed shoe size, and reed length are not magic elements, or even fussy, in design.

 

Any thoughts anyone..?

 

Chris

[d.elliott]

Hi Chris!

 

you may have seen my comments on another post elsewhere on this new fangled forum (NFF) about chamber influences.

 

I am unsure just how the chamber design works, I have seen opinions along the lines that its absoluely critical, C. W. applied himself to masses of research on the topic, so it must be. The best concertinas had/ have stopped off chambers to improve performance so it must be true. Chamber tuning, combined with long series reeds defined the orinal aelos c.f. the bog standard concertina................

 

What is true is that, once mounted in the chamber the pitch of a reed changes by a few cents (compared with being sounded in free air), and the tone it produces is improved. Something is happening.

 

Other opinions seem to run along the lines of: zero real influence at one extreme, to the chamber only influences reed starting speed.

 

I looked at the mathematics to see if I could find any correlations, but is the chamber comparable to an open tube or a closed tube? A bit of both, with the pad opening at one end and the signal generator at the other, but both on the side of the tube wall, and both part way down the chamber length. In the case of the reed (signal generator), often in the middle of the tube length. The best way to visualise what is happening is to look at th big reed instruments (sorry to be a bore on this topic, but its only a passing phase, I will grow out of it when I move out of the male meopause).

 

I say big reeds because the chambers are so much bigger relative to the reeds, see the photos of the Lachenal bass you once displayed for me, and of the damaged chambers on the 'Big Reed Performance' posying on this NFF.

 

On the basis of observation, I believe that the chambers do influence harmonics, as much as primary frequency projection, but that their design is not so critical.

 

My basis for this is:

 

Big bass, I have found evidence of reeds being moved to standardise the fingering from some form of obscure brass band equivalent transposed fingering, this does not appear to have affected reed performance

 

Anglophiles change the pitch (key) of instruments by moving reeds around

 

On repairs/ service jobs I have inserted false end walls into chambers, to try to smooth out unpleasant harmonics, sometimes with success.

 

As usual, no hard facts, and a lot of semi-educated guesswork, what my Professor used to call SWAG, Sophisticated Wild-Assed Guessing.

 

Dave

[ttonon]

Recent posts in this forum (Chris Ghent and d.elliot) and other posts elsewhere have queried on methods that might enable the free reed designer to more easily match the chamber geometry to the associated reed, in order to produce "more pleasant" sounding instruments. As d.elliot notes here, this is not a simple task, and there appears no known method by which this can be accomplished. In thinking about this problem, my first impulse would be to apply the simple principle of a Helmholtz resonator to the design, though in carrying out the details, certain difficulties become apparent.

 

Before stating off hand what these difficulties are, let me outline the approach, which still may prove useful in certain special cases.

 

If we look upon the reed chamber as a simple Helmholtz resonator, then perhaps we can fit its design into a simple mathematical framework. A Helmholtz resonator is, in its original form, a volume with a single, small aperture, and the resonant frequency of such a geometry is given by f = (c/2/pi)*sqrt(A/L/V), where c is the speed of sound, pi the ratio of the circle's circumference to its diameter, A the cross sectional area of the port, L the "effective length" of the vibrating air mass, and V the resonator volume.

 

It's good to keep in mind the assumptions behind this simple formula, which is derived assuming a "lumped parameter" system; i.e., all dimensions of the resonator are much smaller than the wavelength of oscillation under question. Necessarily with this assumption, the air within the resonator will vibrate uniformly, defined by one pressure, temperature, etc., and the only significant air motion associated with the vibration will be concentrated in and about the aperture. This lumped parameter assumption is the crux of the problem that often will arise when one follows this simple design approach. (One must always check this assumption with the above formula for resonant frequency.) In order to point this out, we proceed as follows.

 

With concertina reed chambers, there's a walled volume with a hole controlled by a valve, and a free reed mounted elsewhere in the wall. The approach is to consider the volume, V, of the resonator to be the volume of the chamber, and the area A and length L to be associated with the hole. The idea is that any overtone of the reed that matches the resonant frequency of the resonator will be enhanced. My guess is that the "most mellow" sound of the reed will be achieved when the resonator frequency matches the fundamental pitch of the reed. I don't believe the simple resonator has overtones. This is simply because, if it did, the assumption of lumped parameters would be violated, and other analyses would have to be performed in order to calculate the pitch of these overtones. In such a case, the object would not be functioning as the classic Helmholtz resonator.

 

We address one detail here; i.e., there's a kind of hole in the wall associated with the reed, and the question arises, should this hole be considered in the determination of A, as used in the above formula. I think not, simply because the reed is a driving force for the resonator behavior and is thus not greatly affected by the resonator. If the pitches of the reed, including its fundamental and overtones, are not matched to the pitch of the resonator, the resonator is off design. With proper design, the independant pitches of both the reed and the resonator will not change when both are coupled into a single system.

 

A word on L. As mentioned, all air vibration associated with the resonator is confined to, and about, the aperture. Thus, one can visualize a small slug of air vibrating in and out of the aperture. If one were to contain this vibrating air mass into a cylinder with cross section A, its length woul be L. For a very thin walled resonator then, L would be considerably larger than the wall thickness. In fact most people write, L = t + c, where t is the wall thickness, and c is a correction that accounts for the amount of air vibrating immediately inside and outside the hole. The correction c has been calculated fairly accurately for special cases, and in our case, c should be a little less than the diameter (or major dimension) of the hole. Since there's an end correction on both sides of the aperture, one might approximate: L = t + 1.5d, where d is the diameter of the hole. (We are assuming that the valve opening the aperture is moved far enough away from the aperture so that it does not interfere with the vibrating air mass. )

 

Before getting into numerical calculations, we can easily see what the effects of various chamber dimensions have on tuning the chamber to the pitch of the reed, as long as the lumped parameter assumption is valid. Thus, the resonator pitch decreases with increasing wall thickness and increasing resonator volume, and larger apertures produce higher resonator pitches.

 

As an example, consider chamber dimensions which are typical of existing concertinas: a chamber 2" long, 1/4" deep, and 3/8" wide, with an aperture 1/4" in diameter and a wall thickness of 1/4". The resonator pitch calculates to 1360 hz, with a corresponding wavelength of 9.7". Changing only the aperture diameter to 1/2" gives a pitch of 2150 hz, with a wavelength of 6.1". Notice two things: Firstly, the pitches of these resonances are rather high, and secondly, the lumped parameter assumptions are breaking down. Herein lies the difficulty of this approach and why producing mellow sounding reed chambers is a difficult chore.

 

For current constraints on concertina dimensions, we conclude, 1) the reed cavities cannot be made large enough to resonate in a simple way with many of the pitches played by the instrument, and 2) simple lumped parameter methods of design cannot be applied.

 

Both these issues would, in principle, disappear if larger reed cavities can be made to fit into the instrument. For instance, if we take the following (unrealistic) dimensions: a chamber 4" long, 1" deep, and 3/4" wide, with an aperture diameter 3/8" and an aperture extension of length 1/2", we get a resonant frequency of 391 hz, and a corresponding wavelength of 33.7". The pitch here is close to "G" above middle "C", and the lumped parameter assumption is reasonable.

 

Of course, with more realistic dimensions, there can be favorable resonances, but in these cases, standing waves exist within the reed cavity, and calculations in these cases become exceedingly more difficult than the ones presented here.

 

In conclusion, a simple Helmholtz approach to concertina reed cavity construction appears not generally useful for existing instruments. A suggestion that arises from these ashes would perhaps be to find a way to install resonant tubes of small diameter inside the instrument, but such an approach may also require a larger instrument.

 

Best regards,

Tom

www.bluesbox.biz

[d.elliott]

wow!!

 

well done , I even understood some of it! The comment that the instrument size is a limiting factor is quite important. Its worth remembering that CW would have worked empirically, without the aid of even the basic analysis equipment and science.

 

Dave

[Daniel Bradbury]

Goran???......

[Chris Ghent]

Recent posts in this forum (Chris Ghent and d.elliot) and other posts elsewhere have queried on methods that might enable the free reed designer to more easily match the chamber geometry to the associated reed, in order to produce "more pleasant" sounding instruments.

Tom,

 

thanks for the reply, it took me a while to digest it.

 

I take it that your thrust is that tone is specific to chamber design, but I wonder, reiterating my final question, how fussy are we talking here..? A lot of what you talked about involved shape but I am talking volume.

 

To use another example, when you look at older and beloved designs, while efforts have been made to limit the size of chambers, they are not finely tuned. Typically, partitions have been placed immediately next to the tip of the reed shoe to block off the chamber, and of course some adjustment is made by limiting the partition height, but if the 19thC designers had been making small or fine adjustments to the chamber size surely we would have been seeing the equivalent of wheel balancing weights placed (don't get hung up on composition, I just mean a small adjustment device) in the chambers to change the volume. .?

 

If I recall you might have experimented with chamber size in developing your pitch bender, and if so are probably one of the few who has heard a note sounded and then listened as the chamber volume was changed. If so, what difference does it make..?

 

regs

 

Chris

[chris vonderborch]

I have just completed a radial-pan Anglo, having made a Jeffries-style one previously. I designed the wedge-shaped chambers to be as small as possible in volume, by placing the partitions as close as possible to the reed ends. The result is a bright instrument with a rapid response, and one that sounds (to my ear) very sweet when chords are played. However, all I can truthfully say is that I cannot discern any real difference between my radial pan and the Jeffries type. I have not used any acoustical devices to test my observations, by the way!!

Some time ago, I began to experiment with reed tone in mock-up chambers...using a piston type of slide, I could vary chamber volume as I sounded a reed, a little like the way a harmonica's tone can be varied by mouth position, hand placing, etc. I confess that I didn't get too far with the experiment. However, someone with the acoustic frequency-response gear might like to try it!!

[goran rahm]

First, many thanks to Tom for the substantial survey of what I have said dozens of times in the simple form that there is NO theoretical reason to assume that chamber resonance is significant for concertina tone.

 

In practise I believe there is no ground either to assume that the chambers have been designed for acoustical reasons despite there has been a lot of hearsay around in this direction.

 

The sizes of reed chambers evidently have been choosen purely for mechanical purposes (length and movement requirements of the reeds), to provide the necessary separation of reeds valves and other parts and to optimize the onset (which varies with chamber volume). The later factor also is the main reason for the introduction of the said partititions of the chambers. As Dave pointed out these are usually located as near to the reed top as possible and have NOT been located to provide resonance conditions!! I have a guess that the hearsay regarding this may be based on some kind of early misunderstanding.

 

There are however acoustical effects related to the variants but these are likely related to change of absorption, varying with frequencies.

 

There have been laboratory studies in these phenomena carried out with accordions and they support the above. Furthermore my own simple pilot type spectrometric studies some 15 years ago revealed NO evidence of resonance phenomena present in the concertina spectrograms AT ALL....

[Richard Morse]

Anyone have any thoughts about the relevance of chamber volume (or shape?)? ... [replacement reeding] suggests chamber size, reed shoe size, and reed length are not magic elements, or even fussy, in design.

My experience is that the air travel path is far the more accoustically influential than is chamber volume or shape.

 

Take the universally accepted standard that the free end of the reed is located at the opposite end of the chamber as the outlet pad is and wonder why? The tone and response of the reed changes considerably to less robust and slower acting when you move the outlet hole closer to the free end of the reed - and when directly over it - will sometimes render the reed inable to speak at all.

 

Tnere are other parameters influencing this air path too: the length of the air path; the size of the outlet hole and how far off it the pad lifts up (which seems should be added to/included with the length of the air travel path); the "ends" of the path (how far beyond the reed tip and pad hole the chamber extends); and the proprtions of the path (ratio of average grith to length)....

 

Anyone have any thoughts/corroboration with this?

[A.D. Homan]

Take the universally accepted standard that the free end of the reed is located at the opposite end of the chamber as the outlet pad is and wonder why? The tone and response of the reed changes considerably to less robust and slower acting when you move the outlet hole closer to the free end of the reed - and when directly over it - will sometimes render the reed inable to speak at all.

I've wondered about this, also, Rich.

Take an example from melodeon reed setup: often the highest reeds are "flipped" so that the free tip of the reed is closest to the aperture where air enters the chamber.

I was always under the impression that this was to speed up the response of these extremely short reeds, but there was no basis for that.

[Richard Morse]

Flipping reeds on an accordion does seem to be a way makers get those short/small reeds to be more responsive (primarily in enabling them to start at all). Another way is to introduce some bleed air (usually by inserting an awl in the wax near the corner of a reedplate in order to make a small tunnel for pressure vent/release).

 

I wonder if this flipping works only with exit holes perpendicular to the reedplate and only if the reed has no valves.

[goran rahm]

Rich, I agree basically regarding the 'air travel path' or rather 'sound travel path' as being a lot more influencial than the chamber parameters but not much concerning its length a such,only the influence by materials,spaces and edges meeting the sound on the passage.

 

Lab studies with accordions have confirmed the little effects by the chamber measures as well as materials used for the chambers. Surprisingly little (or no) difference has been noticed when comparing wood/plastic/metal (BTW - Chris Vonderborch, You did make some trials with this too didn't You?)

On the other hand striking effects have been noticed (tonespectrometrically) from covers and filters and the cassotto effect is particularly interesting.

Despite we can exclude specific chamber resonance related to the the fundamental notes there may be 'formant' resonance present that may have influence on tone and maybe this could be a factor involved in subjectively noticed effects by different endplate constructions and their influence on the space between action board and endplate.

[ttonon]

The above conclusions in my previous post concerning the limited usefulness in treating the chamber geometry as a Helmholtz resonator does not rule out the possibility that the chamber can still resonate with the reed motion, resulting in an enhancement of fundamental and/or overtones. If resonance does occur, it may be because of physical interactions other than those described by the simple Helmholtz geometry. Some of the recent posts in this thread have touched upon these other mechanisms.

 

If I recall you might have experimented with chamber size in developing your pitch bender, and if so are probably one of the few who has heard a note sounded and then listened as the chamber volume was changed. If so, what difference does it make..?

 

I know from my experience that the sound of a free reed can be greatly influenced by the presence of a resonator. This influence is most often an increase in loudness with a more “hollow” or “mellow” tone. The asian free reeds are striking examples of such effects.

 

Some time ago, I began to experiment with reed tone in mock-up chambers...using a piston type of slide, I could vary chamber volume as I sounded a reed, a little like the way a harmonica's tone can be varied by mouth position, hand placing, etc. I confess that I didn't get too far with the experiment.

 

Bravo, I appreciate your experimental energy. For this to work, and considering the dimensions of interest, I’d guess that you need a very tightly fitting piston. If there were even small air leaks, the acoustic coupling between the reed and the air mass in the resonator would be destroyed. This point is taken up in reference to Richard’s post, below.

 

However, all I can truthfully say is that I cannot discern any real difference between my radial pan and the Jeffries type. I have not used any acoustical devices to test my observations, by the way!!

 

My guess is that neither your cavities nor the Jeffries cavities are of dimensions that exploit acoustic coupling with their associated reed.

 

First, many thanks to Tom for the substantial survey of what I have said dozens of times in the simple form that there is NO theoretical reason to assume that chamber resonance is significant for concertina tone.

 

Goran, I appreciate your comment, though I myself cannot say there is NO theoretical basis for chamber resonance. It may be true that few instruments exploit chamber resonance, and if so, perhaps only by happenstance, but, as I said above, there can, at least theoretically, be resonance that is not explained by the simple Helmholtz resonator. You may be arguing that any theoretical basis cannot be put completely into practice. On that, you are perhaps right, though, considering the full pitch range of the concertina, there will be some pitches whose resonant geometries have realistic dimensions, whether or not this resonance can be easily explained. There's more on this below.

 

My experience is that the air travel path is far the more accoustically influential than is chamber volume or shape.

Take the universally accepted standard that the free end of the reed is located at the opposite end of the chamber as the outlet pad is and wonder why? The tone and response of the reed changes considerably to less robust and slower acting when you move the outlet hole closer to the free end of the reed - and when directly over it - will sometimes render the reed inable to speak at all.

 

Richard, you do have a point with “air travel path,” though, for completeness – and for acoustic meaning – the length of this “air travel path” must be related to the wavelength of oscillation, as follows. As with many physical phenomenon, quantities have meaning only in relation to other quantities of the same dimension. So-called non-dimensional groups are of great value in physics and engineering.

 

Consider a simple tube, which provides a very useful model for acoustic phenomenon (and particularly those involved with musical instruments) when the wavelength of oscillation is comparable to, or less than, the air masses defined by the geometry in question. In the ideal case, the length of this tube and its “end conditions” are the only meaningful acoustic parameters. Thus, if one makes a tube of length equal to one quarter (or ¾, or 5/4, etc.) wavelength of a musical tone, with one end closed and the other end open, one has a sound dampener for that wavelength, when a sound source is placed near the opening. With a source of oscillating pressure placed next to the closed end, inside the tube, sound is amplified. The reason for this is really simple, though it will take a bit to explain, as follows.

 

Take a tube of quarter wavelength, closed at one end, open at the other and apply a source of oscillating pressure to the open end. When a (+) pressure pulse is made at the open end, it moves inside the tube and reflects from the closed end (as a +) and arrives again at the open end in a half period of time (total travel distance is a half wave) - the same time that it takes for the sound source at the open end to go negative. The exiting (+) pulse thus “cancels” the newly generated (-) pulse.

 

With the oscillating pressure source placed near the closed end inside the tube, the sound from the source will be amplified, as follows: A (+) pulse is made at the wall (or hits the wall and bounces back as a +) and travels towards the open end. When it arrives at the open end, it bounces off the opened end (returns) as a negative pressure pulse. That’s important: the boundary condition provided by the open end requires that a positive pulse be bounced back as a negative pulse. (This is mathematically and physically necessary in order to maintain the open end at constant pressure; i.e., open to infinity.) So, the (-) pulse now comes back a half period later just when the source is also (-). Sound is thus enhanced. (I should mention that not all of the energy is bounced back from the open end. Some of the energy escapes the tube and is heard as the musical tone.)

 

What about three-quarter wavelengths and five-quarter wavelengths, etc.? One can see the reasoning here by picturing a sine wave. At the origin, the signal is zero. Every three-multiple of a quarter wave away, the signal is a maximum. One can see graphically that the result in these cases is the same as for the quarter-wavelength tube with one end open, one end closed.

 

There are similar lines of reasoning for a tube opened at both ends, but this is not the case here.

 

Richard, I suppose that drilling a hole in the side of the tube will, as you say, release pressure, and destroy the acoustic coupling between the reed and the tube, when the reed is mounted near the opening, thus allowing the reed to speak. It would be interesting if you could measure the length of the cavity in those instances when you notice stifling and compare it to the wavelength of oscillation. My guess is that the presence of leather valves would greatly enhance the stifling influence of the tube, working just the opposite as drilling a hole.

 

(Digression, mainly for completeness: Is the reed a pressure source of sound, or a velocity source? The answer to this question is important in understanding how cavities affect reed sound. Without thinking much on it, my guess is, as I assumed above, that the reed is a pressure source, because of the fact that a free reed can be blown and be made to sound without any cavity around it. It is thus not like an edge tone, which is, in reality, a flow-control valve.)

 

A relevant question here is, can one come up with a quarter-wave design approach to produce resonant cavities in concertinas? Let’s explore this question and pay particular attention to the pitch range of the instrument. There are always complicating issues when attempting to apply idealized models to reality. In the simple tube, the air is assumed to be an ideal gas with no friction, but the most important assumption relevant to us is that the air motion within the tube is considered one-dimensional, and equivalently, the tube diameter (or cross section) is much smaller than the wavelength of oscillation. These assumptions are okay for quarter wave tubes with lengths at least several times larger than the cross section, but in present concertinas, this may not be the case. There are also other issues; e.g., the reed is mounted on the side of the tube, and so the pressure pulses made by the reed occur over some length of the tube, on one side of the cross-section, with perhaps much of it removed from the closed end. I also agree with Richard that the size of the opening hole and the proximity of the pad may have a significant effect.

 

Middle “C” (262 hz) has a quarter wavelength of about one inch. A three-quarter wavelength is about 3.2 inches. So if someone could make a reed with a plate of between one inch and 3.2 inches and mount it in a cavity with cross section less than about one quarter inch, there may be a place to start experimenting. There are similar results for lower pitches and bass instruments.

 

At 1,000 hz, the wavelength is 1.1", with quarter wavelength 0.3", three-quarter wavelength 0.8", and five-quarter wavelength 1.4". Here again we can perhaps find accommodation in a realistic instrument, but the tube cross section should be less than about an eighth of an inch. Things are becoming very tight.

 

At 2,000 hz, wavelength is 0.55", quarter wave is 0.14", and seven-quarter is 0.96". A 2,000 hz reed can probably be made about an inch long, though the cross section of the tube would need to be no larger than about 1/16" in order that the cross section be much smaller than the wavelength. Is this realistic? My concern here is that, even if the cross section can be made so small, would there be enough air mass in the tube to achieve the proper effect? One would have to experiment here.

 

For frequencies above 2,000 hz, the wavelength is becoming so small that the cross section of any practical tube will be larger than the wavelength. The simple tube assumption thus breaks down and one must devise another model by which to tune the cavities. Incidentally, the "C" three octaves above middle "C" has a pitch of 2,093 hz.

 

As with the Helmholtz approach, the simple tube approach also presents challenges - because of the small wavelengths involved.

 

A question arises: Would it be objectionable to have an instrument with some, or even most, of the pitches mounted with resonant cavities and others not? Perhaps the reeds without a resonant cavity would sound very weak, in comparison to the others, as if they belong to another instrument. Is this an all or nothing endeavor?

 

Perhaps not. If all the non resonant chambers are confined to one end of the pitch range, perhaps the mismatch would not be overly noticeable. Other instruments have similar issues and through the years, makers find ways to minimize tonal nonuniformities. There’s no "perfect" instrument.

 

How important is tonal quality? I suppose this gets back to Chris’s recent question. How possibly better is the tone and how much weight and expense would people be willing to trade for that better tone?

 

If I were an instrument maker, I’d perhaps put more effort into finding out how feasible it would be to build an entire instrument with resonant chambers, even bearing in mind the risks. I say this because I'm aware of the extent of tonal enhancement, though I'm sure this issue has been present with the concertina all the way back to Wheatstone’s time. I've seen papers on the web describing Wheatstone's experiments with (large) tubes as resonators for free reeds, though I don't believe these tubes were associated with cavities. Nevertheless, here’s a suggestion. Someone – a maker with extra time and enthusiasm, or an interested experimenter – can make a modified instrument with at least one (but better, a few) resonant cavities. He/she can take it to one of the concertina gatherings, or mail it to selected people to hear what it sounds like. In this way, a consensus can perhaps be obtained on how valuable such a sound is. Looking at the size of the chambers can help one estimate the size of a completely resonant instrument, and so an estimate on weight and expense might then be also estimated.

 

Best regards,

Tom

[goran rahm]

Quote// Tom:"Goran, I appreciate your comment, though I myself cannot say there is NO theoretical basis for chamber resonance. It may be true that few instruments exploit chamber resonance, and if so, perhaps only by happenstance, but, as I said above, there can, at least theoretically, be resonance that is not explained by the simple Helmholtz resonator. You may be arguing that an theoretical basis cannot be put completely into practice. On that, you are perhaps right, though, considering the full pitch range of the concertina, there will be some pitches whose resonant geometries have realistic dimensions, whether or not this resonance can be easily explained. There's more on this below. !"

 

Goran now:NO *Significant chamber resonance* ....I agree that of course there is physically resonance present related to high overtones but since there is no regular progression of the measures of chambers corresponding to the overtone progression of frequences it will be purely accidental and with the traditional construction impossible to practise. But like you say it could be possible to make a different instrument using resonance related to partials following the progression of say quarter pipe but NEVER related to the fundamental note!! (Well...unless you want a stationary concertina....)

It has been tried with accordions at least at Das institut fur Instrumentenbau.

 

A guess of mine is that such accidental resonance may be a cause that the relative diameter of some pad holes of some notes in the high(treble) octave of at least some Wheatstone englishes are diminished...possible with the aim to reduce the relatve amplitude of these notes and sometimes with other instruments one may find 'unexplained' enhancement of the amplitude of some notes but I have not tried to investigate the matter thoroughly.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

There ought to be accidental resonance related to *some* chamber(s) causing disturbing enhancement however and a guess of mine is that this is the reason that some of the diameters of some of the top (treble) octave pad holes of at least some Wheatstone englishes are diminished...possibly to reduce the relative amplitude of these notes.

[ttonon]

Goran now:NO *Significant chamber resonance* ....I agree that of course there is physically resonance present related to high overtones but since there is no regular progression of the measures of chambers corresponding to the overtone progression of frequences it will be purely accidental and with the traditional construction impossible to practise.

Hi Goran,

 

I apologize for misunderstanding your point. I think you mean that there is no theoretical basis that is or has been used to design chambers with resonance, and on this, I think you're probablly right. Also, as you wrote originally, my calculations seem to support your view.

 

 

 

Best regards,

Tom

[ttonon]

Apart from Helmholtz resonator and quarter wave models, there are other ways to view amplification of sound by free reed cavities. I just posted comments in this regard in this same forum under "Big Reed Performance," and include that reference here because of its implications on cavity design.

 

Best regards,

Tom

www.bluesbox.biz

[GBlake]

Check out my post of 9/16 under Big Reed Performance for more on this topic.

[ttonon]

Thus, if one makes a tube of length equal to one quarter (or ¾, or 5/4, etc.) wavelength of a musical tone, with one end closed and the other end open, one has a sound dampener for that wavelength, when a sound source is placed near the opening. With a source of oscillating pressure placed next to the closed end, inside the tube, sound is amplified. The reason for this is really simple, though it will take a bit to explain, as follows.

 

Take a tube of quarter wavelength, closed at one end, open at the other and apply a source of oscillating pressure to the open end. When a (+) pressure pulse is made at the open end, it moves inside the tube and reflects from the closed end (as a +) and arrives again at the open end in a half period of time (total travel distance is a half wave) - the same time that it takes for the sound source at the open end to go negative. The exiting (+) pulse thus “cancels” the newly generated (-) pulse.

The conclusion here is incorrect, though the logic leading to it is correct. A quarter wave tube, closed at one end and open at the other offers zero impedance while looking into its opening. I hadn't gone through these concepts in many years, and got things confused. The "cancellation" I concluded (which is correct) does not mean that the tube will prevent a reed from speaking, but rather, will result in zero impedance, which is the condition for resonance.

 

On the other hand, a half wave tube, closed at one end and open at the other will produce infinite impedance, and it is this situation that one likely finds in practice when reeds mounted under such conditions fail to speak. Richard, can you verify that non-speakable reeds are mounted with half wave tubes?

 

In a new thread, I hope I make the situation with quarter wave and half wave tubes more clear.

 

Best regards,

Tom