Big Reed Performance

[d.elliott]

Some people may rememember my posting and ensuing comments about trying to get an even playing volume and responses on a newly aquired Lachenal contra bass instrument.

 

The symptoms were that a number of notes on this steel reeded, single action, english system bass sounded muted and slow compared with comparable notes within the instrument, and against other (even) brass reeded bass instruments.

 

All the reeds are surface mounted by being screwed (with two wood screws) onto the top surface of individual chamber pipes which are built onto the underside of the pad board. The pad board providing the 'floor' of each chamber and the pad hole feeding direct into each chamber.

 

The problems are now all resolved, I have a strong instrument sounding consistent accross its full compass.

 

For reference the problems encountered were:

 

1. some of the wood screws had shanks equal or slightly longer than the thickness of the reed frame, this resulted in a reduced clamping force onto the chamber pipe.

 

2. one or two of the chamber top plates had partial glue failure beween the underside of the chamber top plate (onto which the reed is fixed) and the chamber walls, resulting in the loss of some air (minimal) and a good vibration transmission between the reed attachment and the rest of the chamber/ instrument.

 

3. Shrinkage/ glue failure between the underside of the chamber walls and the pad board which forms the base of each chamber. again resulting in (more extensive) air losses and a poor resonant connection to the pad board.

 

From this experience I would conclude that mechanical connections are important to the resonant characteristics of the reeds, and that cumulative minor air leaks can have the effect of signficantly reducing a reed's sound volumes.

 

I hope others may be able to short cut my learning curve. most of the glue issues were not discenable to the eye, and were only evident when feeler gauges were used. Repairs included dismembering the accoustic chambers/pipes and pad board chamber assemblies and then a process of re-construction and bedding in joints.

 

Dave Elliott

[dbowers]

With respect to mechanical connection, I don't think its as much a resonance as a damping effect. If your "clamping" system allows the reed frame to vibrate, a fraction of the energy imparted to the reed by the moving air will go to moving the mass of the reed frame.

 

The exact effect will depend on the amount of play in the mount, whether one or both ends are free to vibrate, etc. It could, potentially, affect pitch as well as volume, but I don't remember enough of differential equations (after 40 years of disuse) to present a mathematical disquisition.

[d.elliott]

You may well be right, I have heard the argument about energy losses in frame displacement before. The key point that I wanted to make to people who have these big reed instruments are that if they have an apparent muting of the reed sound and no obvious cause, then and irrespective of mechanism, there are a series of things to check and do that can make a very significant difference to the 'power' of the instrument and or specific reeds.

 

Some people dispute the effect of resonant chambers all together, some say that the chambers only affect the speed of reed starting. I do not have the equipment, nor perhaps the patience to analysis or calc through the equations and transforms to try and bottom this. What I have found, empirically, is that relatively minor glue failures, and a slight loss of reed securing/ clamping force can have a very marked effect on the reed's performance.

 

Dave

[goran rahm]

Once again Dave....as long as we do not have any evidence that there is any resonance detectable I suggest we avoid speaking of possible resonance since it remains a speculation and we do have enough other factors involved which are known and demonstrable anyway to keep us busy....

The reed pan, action board, end plate and other parts certainly ARE *vibrating* and the frequence of vibrations likely will have relations to sound frequences but these vibrations probably are not producing significant sound in their turn and thus do not influence the audible 'tone'. There is no reason to compare squeezeboxes in this respect with instruments having a *resonator* .

Goran

[d.elliott]

Goran,

 

I know your views on this topic, and I have tried to steer away from claiming 'resonance' in its technical sense, what I said was:

 

'From this experience I would conclude that mechanical connections are important to the resonant characteristics of the reeds, and that cumulative minor air leaks can have the effect of signficantly reducing a reed's sound volumes.'

 

Tis makes two points, air losses may have an effect on reduce reed amplitued, and the mechanical connection transmitting vibrational energy to the chamber pipe and the pad board.

 

I have tried to make public the experience gained over several months of trying to get my 'new to me' Bass up to playing standard. I am very pleased with my efforts, and I wanted to share what I had learned with others who may venture into the world of the big reed.

 

As a thought, single acting big reed instruments to not have reedpans sat in chamois lined wells. the vibration is tangible as well as audible.

 

I don't rule out some form of resonant factor, prefering to keep an open mind.

 

Dave

[JimLucas]

There's resonance and resonance. One needed postulate a resonant cavity or sympathetic resonance of the wood in the instrument to consider resonance in a concertina. The vibrating reed is a *resonant* system; that's why it vibrates at a given frequency and produces sound at a given pitch.

 

What I believe Dave has suggested is that certain external factors can *directly* affect this *primary* resonance. In particular, if the reed frame is not held rigidly but can move slightly (ever so slightly is enough), this additional motion will bleed energy away from the primary resonance of the reed.

[ttonon]

I don't rule out some form of resonant factor, prefering to keep an open mind.

There’s another physical effect involving acoustic waves that may help shed light on the existence, or non existence, of so-called “resonance.” When a plane wave strikes a wall, the wave rebounds, reproducing the same waveform, only now, the reproduced waveform is moving in the opposite direction as the original wave - in the same medium as the original. Let’s say the original wave is moving from right to left. It strikes a wall, and the new wave moves from left to right. Consistent with the linear formulation of musical acoustics, any such wave modifies the pressure of the medium through which it passes by (algebraically) adding its “delta p” to that of the medium. Thus, when the right moving wave passes into the medium which is also carrying the left moving wave, delta p’s are algebraically added. The result is that, at fixed positions in the medium, the amplitude of the pressure fluctuations is twice the amplitude of the original wave. This makes sense simply because where we once had one wave, we now have two, producing twice the amplitude as the single wave.

 

Thus, whenever a cavity is constructed about a sound source, such as a free reed, the pressure fluctuations, because of multiple rebounds from the cavity walls, within the cavity will be significantly larger than otherwise. Notice that this pressure amplification occurs whether or not the cavity is “tuned for resonance.”

 

This effect can be easily observed by playing your concertina into a closed corner of a room and noticing how the sound is much more “resonant” than when the instrument is played out in the open, away from walls. This effect is, I believe, largely responsible for the “cassoto” (tone chamber) effect in many accordions. Have you ever wondered why a single, fixed geometry like a tone chamber can affect the sound of reed with pitch spanning that of the entire instrument? With the concertina played in a corner, or indeed, very near a single wall, the rebounding pressure waves add to the waves emanating directly from the instrument. Moving away from the wall greatly diminishes the effect because now, the waves are no longer plane, or one-dimensional, and are dispersing into 3-D space. Of course there’ll be an optimum range of frequencies for which one can clearly hear the effect, determined by how close one can get one’s ear to the wall and by how much dispersion of the wave has occurred before striking one’s ear.

 

We have a choice here: whether or not to consider this effect “resonance.” I don’t think this is an easy question to answer, and this difficulty, I believe, may often cause confusion (and arguments) on the existence of resonance in free reed cavities. Perhaps the best way to look at it is that resonance is more complex an issue than most people recognize. It’s much easier to define “resonance” as that phenomenon which occurs with more familiar geometries, such as the Helmholtz resonator and quarter wave tubes, with clear analogies to mechanical spring-mass-damper systems. Confusion can then occur when other mechanisms associated with other geometries produce similar audible effects.

 

I think this effect is present in most all free reed cavities, and that its magnitude is influenced by the ratio of oscillatory wavelength to certain cavity dimensions. I’d be pleasantly surprised if the effect can be made as large as that produced by a properly tuned geometry, though I’d also expect that the effect is significant with many cavities. Because of this, I can understand very well what David has experienced with the deleterious effects of leaks and poorly mounted reeds in so-called "non-resonant" cavities, all which would tend to allow sound pressure to escape in dissipative ways (through small openings fraught with frictional effects). I also commend him on his open mindedness, which I take as recognition of complexities that are not yet fully understood.

 

Best regards,

Tom

www.bluesbox.biz

Edited September 13, 2003 by ttonon

[d.elliott]

Thanks Tom,

 

I had not considered the reflection and additive factors of wave forms, but there must be an element of randomness in the physical phenominon applied to un-tuned chambers, even the existence of a white -noise effect making the reed sound softer.

 

When I was at University in the late 1960's (it even feels like a millenium ago) one senior lecturer told us that 'understanding in enlightenment, but knowing what works is a gift, always listen to practical experience'. Unfortunately I don't always do this.

 

Taking Jim's point there is 'resonance' and 'resonance', and we don't all understand all the physics, but shared experience will help us develop understanding, I appreciate your input, as well as Goran's and Jim's.

 

Dave

[goran rahm]

Jim first, he said:

"The vibrating reed is a *resonant* system; that's why it vibrates at a given frequency and produces sound at a given pitch.

 

What I believe Dave has suggested is that certain external factors can *directly* affect this *primary* resonance. "

 

Goran now:I don't understand what you mean by the above:"The vibrating reed...a resonant system"

There is no resonance in this system making the reed "produce sound at a given pitch"

The cause of a certain pitch with the free reed is only the regular clipping of the air flow and consequent pressure changes. No "resonance" !!

 

Comment to Tom:

Yes I'm with you concerning the chamber effects described but (with reservations for linquistic shortcomings...) I doubt that you should call this *resonance* according to the common use of the term in musical acoustics. The phenomena you mention are rather enhancement related to reflexion and phase effects and not related to a *resonator* by common definition....or?

Back to the thoretical presence of *true* resonance in the reed chambers but theoretically this resonance is expected to be without significance and as long as practise shows we still lack any support for it being audibly significant either as long as the subjectively perceived tonal characteristics can be explained by known factors.

Proper lab studies could shed some more light on the matter but keep in mind that there has been some work done on the subject. Anyone else reeding/speaking german? Or someone reading/speaking russian? You could have some research tasks to fulfill.....:-)

[GBlake]

I enjoyed picking up on this discussion. I've been studying acoustically tuned chambers as an application to a sister instrument to the concertina, the portable (or Indian) harmonium. Re. the discussion of the Lachenal chamber pipes....be careful of the use of the term "pipes", as the resonance effect due to a pipe or tube operates on a different set of equations than for a resonance chamber or box.......the lack of a strong reinforcement of tone for a few reeds (now fixed) was due to the leakage noted....in effect, the walls were loose....not due to damping of vibrations. A golden rule of resonance chambers is that the walls be rigid....clearly a leaking wall doesn't comply......you fixed the leak, the wall became rigid and the chamber again became resonant. This has been found to be true in the making of resonance chambers for marimba and similar instruments.

 

Wheatstone, and after him, Lachenal, clearly used resonant chambers to amplify and modify the tone of their concertina reeds....the resonance effect improved the speaking of the reeds as well as the tone...these deep reeds in the contrabass produce many many harmonics that can be picked up by the ear, and without these chambers the effect is pretty rough. Wheatstone was one of the great scientists of the 1800s and knew the basis for designing the chambers, which is now included in acoustics texts by Kinsler-Frey, Rayleigh, Helmholtz, Olson and others. The trick is that the chamber volume is rarely (only for the highest reeds) tuned to the actual fundamental tone of the reed....through the law of consonant tones, the chamber is sized to a higher octave in the series of overtones (1, 2 or 3 octaves higher, generally)...the reinforcement of that higher tone through the resonance effect in the chamber produces a strengthening of the reed's fundamental tone....Rayleigh discovered this effect in the 1800s although Wheatstone likely discovered it at about the same time. The advantage is that chamber size (through the formula given in a previous entry) is considerably smaller for the higher consonant tone, allowing the chamber to be "fitted" in the concertina. Note that by moving up to the fourth higher octave for consonance incorporates many nearby harmonics (at the fourth octave, the reed produces overtones barely a half-step lower and higher)....tuning the resonator to this octave would be a bear....that's why the chambers are still fairly long in the Lachenal instrument and in another I've seen by Wheatstone.

 

Standing waves can be a problem in the chambers when the walls are parallel (the "singing in the shower" reverberant effect)....note that Wheatstone designed his chambers with walls that aren't parallel or are parallel across the short width of the channel, avoiding standing waves......the Lachenal cavaties may experience some standing wave harmonics along with the reinforced fundamental tone, since the opposite long ends of the channels are parallel.

 

Some concertinas may have smaller apertures or openings to the resonance cavaties in the higher reeds. This is likely an attempt to offset the resonance effect known as quality, which makes a larger (lower toned) resonance chamber produce a greater sound level gain (due to resonance effect) than a chamber with less volume (higher reeds). The imbalance of output between the bass and treble notes of a concertina was a major problem which Wheatstone and others sought to address in lots of ways.......by making the aperture of a small chamber just a bit smaller (say 1/32nd" in diameter), and also reducing the volume a bit, the quality of resonance could be made sharper and the sound level gain could be increased to balance the sound level gain from the larger chambers.

 

Looking for resonance chambers in accordions is pretty futile...while the reed is housed in a chamber that appears to be sized to meet the reed scale, in fact there is relatively little accord between the volume of these chambers and the required volume to achieve resonance.....close, but not exact and to be effective the chamber has to be tuned to within at least one-half step of the reed tone (or an octave of that tone). In accordions, the smaller reeds have smaller chambers and vice versa for the larger reeds, but that's about the end of the attempt at resonance.......recently, a renowned Conandian accordion player recognized this acoustical deficiency and set about designing an accordion that achieves more of its musical potential....what is achieved in the cells of a typical accordion reedblock is a standing wave effect with cancelling and reinforcing of reed harmonics that produces a certain timbre, which when passed to a cassotto (tone chamber) is further modified, then modified again when passing through the fretwork to the listener.....this can be a nice effect but is different from the effect achieved by a resonance chamber. Note that reed organs used a similar effect, although Cottingham (who's leading some neat research on reed acoustics) has shown that much of the achieved effect for various reed stops (violin sound, cello sound, horn sound, etc) is due to the voicing (bending) of the reed, not the reed cell or chamber.

[GBlake]

Dave Elliott, do you have the dimensions for the 3 reed chambers that are flipped over in your photo? Also, what's the opening diameter? Do you know the type of wood for the reddish piece, at the tongue-end of the chamber?

 

Gene

[goran rahm]

Gene, I've read your comment with interest, but now do tell me what your evidence actually is for thuis claim:

 

"Wheatstone, and after him, Lachenal, clearly used resonant chambers to amplify and modify the tone of their concertina reeds...."

 

I've heard and seen this statement many times before but so far nobody has come up with any documentation to support it apart from references to Wheatstone's general experiments on these acoustic matters which of course says nothing about practising any of it in the specific /'true' instruments.

 

When I made some simple tonespectrometric tests many years ago I did not detect any signs of resonance at all. This does not exclude the possible existence of the resonance talked about before related to (high) overtones since the test method may not have been sensitive enough ...but considering known relations between other audible overtones the musical significance of such diminutive resonance frequences ought to be negligable.

 

If the 'concept' you describe had been used consequently you would expect the progression of reed chamber sizes to correspond with the progression of note frequences .... they do NOT. The sizes of reed chambers in Wheatstone's concertina progress only according to reed sizes, mechanical demands and the practical construction of the box.

 

Further:the 'tone' of concertinas does vary quite a bit but this variation seems not to be related to specific variation of arrangements of radial/ parallel orientation,

irregular/regular walls of the chambers but often to other factors....or??

 

So, again Gene....

HOW do you know that Wheatstone constructed the instrument according to these claims (if he intended to..)?...

WHAT do you actually mean with "resonant chambers"? (there are many squeezeboxmakers who do use the term, or similar ones....but I have not seen any technical support for it...the chambers in fact seem to be designed without resonance intentions and the said "resonance" being a pure speculation) and

WHERE do you mean to find the evidence in the construction ?...and at last

IS THERE any lab study that has objectified presence of significant resonance frequences in concertinas?

 

Goran

 

Today spectrometric analysis could be made much easier than 20 years ago and there is software to find on the Net. Anyone tempted to set up some lab tests to investigate the matter?

[d.elliott]

Dave Elliott, do you have the dimensions for the 3 reed chambers that are flipped over in your photo?  Also, what's the opening diameter?  Do you know the type of wood for the reddish piece, at the tongue-end of the chamber?

 

Gene

Gene,

 

Not without stripping down the instrument, I will gladly do this if it helps, but before I do, is there any oither information you wish me to gather?

 

The white(ish) wood is sycamore, the reddish wood is mahogany. I believe that this is one of the reasons why the glue failed, as a result of wood shrinkage/ movement at different rates.

 

I have another much much bigger contrabass (thought to be one of only two suviving of an original ten made) for repair to chambers, again the woods have shrunk at different rates causing significant air losses. Here the chambers are built into a sandwich between a reed plattern and the pad board. There are then additional chambers for some of the VERY big reeds that are mounted above the main reed plattern.

 

In this case the reeds attachment security is not an issue but air losses are.

 

In reviewing big instruments its worth remembering that wood movement, and the inevitable resultant cracks, glue failures and gaps all become a significant factor in an instruments performance characteristics.

 

Dave

[ttonon]

Standing waves can be a problem in the chambers when the walls are parallel (the "singing in the shower" reverberant effect)....note that Wheatstone designed his chambers with walls that aren't parallel or are parallel across the short width of the channel, avoiding standing waves......the Lachenal cavaties may experience some standing wave harmonics along with the reinforced fundamental tone, since the opposite long ends of the channels are parallel.

Gene,

 

What problems with standing waves are you referring to? When standing waves exist in an air column, this is called resonance. The whole of your article seems to extoll the benefits of resonance, so what is the problem with resonance? Also, do you mean to say that when the walls of the chamber are not parallel, standing waves do not form? In truth, the angle of the walls only affects the kinds of overtones, or the kinds of standing wave modes, that can occur. For instance, in a cylinder (parallel walls), with one end open and the other end closed, only odd harmonics can form standing waves, but for a conical shape, all harmonics can form standing waves. An example of the first of these is the clarinet, and of the second, the saxophone. I believe the walls of the cavity are angled to one another simply in order to allow the reed plate to wedge into place.

 

Best regards,

Tom

www.bluesbox.biz

[ttonon]

Goran now:I don't understand what you mean by the above:"The vibrating reed...a resonant system"

There is no resonance in this system making the reed "produce sound at a given pitch"

The cause of a certain pitch with the free reed is only the regular clipping of the air  flow and consequent pressure changes. No "resonance" !!

Goran, you may be confusing a simple concept here. The reed tongue vibrates in resonance. The sound that emanates is merely a result of this motion, and as you point out, is due to pressure pulses occuring during the motion.

 

In more detail, the reed tongue vibrates in resonance to the forcing function causing its motion. The forcing function is the pressure force vs. time trace that acts on the tongue. The resulting motion is a form of self excited vibration, because the position of the tongue determines the time at which the forcing function acts. Consequent to this, the forcing function maintains the same phase relationship to the displacement of the tongue. In many other systems, this phase can vary, becoming zero at resonance. In all systems, the part of the forcing function in phase with the displacement is what powers the dissipation in the system, whether or not resonance is occuring. When the forcing function is in phase with the displacement, all the energy of the forcing function goes into dissipation, and this serves as a definition of resonance, because then, there is no imaginary component to the impedance. With the free reed then, since the forcing function is always in phase with the motion of the reed tongue, the motion is always resonant, and the energy from the forcing function is dissipated by sound production, friction, and heat.

 

Best regards,

Tom

www.bluesbox.biz

[goran rahm]

Tom,

Thanks...I fear I have some linguistic problems fully to get the meaning of some expressions you present but never mind, we don't have to go into that.

Hasn't there however been some confusion in regard to the use of 1)"resonance" like you describe it here (a mechanical function) and 2)*resonance* as it has been used on and off in the discussion as an acoustic function. Different matters but having physically some elements in common.

In the chamber discussion we have merely (2) in the reed functional system we have nothing of (2).

Goran

[GBlake]

First, Tom, don't tear anything apart, please....by the way, that hole in the photo, is that the air release hole, not a chamber opening, right? I thought you might have measurements on the chambers, but it's possible to estimate things from your photo.

 

Goran, thanks for the comments and questions....well, this all may be semantics....I believe Wheatstone and Lachenal used resonant chambers because I've seen chambers in a variety of photos for concertinas under repair and their sizes correspond to the formulae Tom has given, with some slack for "the art of tuning"....Wheatstone also describes resonant chambers in his patents (no formula there, of course !).

 

Now there are chambers and chambers....I'm talking about chambers that attempt to strengthen the fundamental tone of the reed so that the tone is heard as more pure and round. There are other chambers, reed cells if you will, that were used in the American Reed Organ to emphasize certain reed overtones to correspond with the wave spectra of a violin, horn, etc. The effect achieved was, well, it needed your imagination....it needs mine, anyway.

 

My calculations (using the formula Tom has given) appear to correlate the expected size of Wheatstone and Lachenal's chambers with their physical data...at least close enough so that it appears that they were trying to construct these chambers......I don't know about your prior tests and you're right, technology is advancing so perhaps another round of tests is in order. A Prof. Cottingham at Coe University (Iowa?) is doing this kind of research today (he has a web site.)

 

It appears that Wheatstone's chambers were, as Tom says, tuned an octave or more higher than the reed fundamental tone (for deeper reeds) and to the fundamental tone for higher reeds. It's interesting to me that the Lachenal contrabass in the Elliott photo shows chambers about 1/2 the depth of the Wheatstone contrabass chambers (which are about 1 in deep)...it appears that Lachenal may have tuned the deep bass reeds to one higher octave than Wheatstone, thereby needing a smaller chamber volume. Both, I believe, were trying to strengthen the reed's fundamental tone by building a chamber with a volume that corresponded to the Helmholtz resonant V for an octave overtone in the reed's harmonic series. A reed without a chamber has a wave spectrum diagram where the fundamental and overtones have essentially the same amplitude (see Olson's "Musical Engineering"). A reed with a chamber (properly tuned) should have higher amplitude in the fundamental tone, at least, you can hear that. Rayleigh explains this effect of fundamental tone strengthening by sounding a consonant tone an octave higher. Rayleigh noted that the effect "tends toward obiliteration" the farther the selected octave gets from the fundamental tone. What an experiment should measure is an increase in the amplitude of the fundamental tone....and that's about all that could be measured, I guess, because the other overtones are still there, just relatively smaller in amplitude.

 

With respect to your question about the size of the chambers not uniformly correlating with the reed size or tone as one moves from Bass to Treble, that troubled me for a time (I have an Indian Harmonium where the chambers are sized to correlate with reed size...and accordions follow the same trend)...but my measurements show that these chambers are not sized to the fundamental tone or an octave consonant tone...at least, I can't reconcile the measurements with any formula. So when I reviewed Rayleigh's anlysis of consonant tones, it became apparent that the chamber can be sized to an octave above the reed's fundamental tone which will strengthen this tone...the benefit is the ability to use a smaller chamber V....but the choice of the consonant octave tone necessarily must change as you move up the scale from Bass to treble, for sizing purposes (note Tom's caution, if you pick a higher consonant octave tone, the chamber you build must have dimensions that are much smaller than the consonant tone's wavelength, which is considerably shorter than the fundamental's wavelength)....so as you move up the scale from Bass to Treble, at some point for sizing purposes you have to flip out of the consonant tone octave you're using and flip to a higher tone. Thus, the chamber V will not smoothly progress across the playing scale. On a Wheatstone Contrabass which shows up at one web site, I was able to determine the notes where the chamber size changed based on this approach, and correlate them to the photo and the formula.

[GBlake]

Dear Tom:

 

Thanks for the comments on standing waves, I'm learning....you are right....I was trying to distinguish between the kind of resonance you get in a "shower stall" chamber (lots of overtones get reinforced) from Helmholtz resonance (one tone primarily is reinforced).

 

I was interested in your recent comments (another string) about chamber length...from my earlier studies, I thought that parallel walls strengthened resonance from wave reflection, which reinforces certain overtones, and therefore a resonant chamber based on the idea of strengthening just the fundamental reed tone should avoid parallel walls that are more than a little bit apart....here's a case where resonator shape is important, but it all gets back to the point you made about chamber dimensions and the wavelength. I saw Wheatstone Treble concertinas, disassembled, where opposite walls were intentionally set askew...I don't think it was to fit in the reed plate, but maybe.....I looked at the dimensions and it appeared to me that certain (unwanted) overtones in the reed would be strengthened by wave reflection if those walls were parallel.....when I saw the Lachenal chambers and its opposite parallel ends, I wondered.....looks like that's not a problem, or perhaps some reinforcement of other overtones is a good thing.

 

Gene