Reed Test Cell Doesn't Work

[ragtimer]

Yesterday I built an adjustable reed test cell jig, to aid in tuning and "setting" the acordion-style reeds in my Bastari Hayden Duet 67-key. But the reeds I tried in it would not speak their fundamental pitches -- with some 6" of wind pressure (way too much), they grudgingly squeaked out a pitiful octave sound.

 

My guess is that the cell cavitiy under the reed is too small. Length and width are a little less than the reed plate itself (12 x 37mm), and depth about 3/4" or 20mm. Air feed is thru a 5/16" brass nipple connected to a foot or so of rubber tubing into my mouth.

 

I figure that the air feed hole wiht its long tube does not properly simulate the pad opening to free atmosphere as found in the concertina. Even tho the reed cells in my concertina are shallower than my test jig, they do enjoy access to free air via the pad opening.

 

Yes, a reed under test sits on top of the test jig, so the top is exposed. FWIW, the reeds act the same (poorly) on blow or suction.

 

I am now building a much larger box, about half the size of a shoebox, to act as a much larger chamber udner the reed, hopign the extra air space will let reeds speak freely.

 

Meanwhile, the Bastari seems to be "playing in" or maybe I'm just getting used to it.

 

Any suggestions or experience would be most welcome! Thanks, Mike K.

[Chris Ghent]

A chamber is needed when creating vacuum via a tube, don't know technically why, perhaps someone will elucidate. Mine is 300mm x 200 x 200 for no other reason than that was a box I had. Unfortunately mouth suction is not adequate with a chamber. I think there is enough suck but not enough volume...

 

Concertinas reed can start as low as .1" for a low reed and .3" for the highest. Don't know about accordion reeds.

 

Chris

Edited October 27, 2007 by Chris Ghent

[ragtimer]

A chamber is needed when creating vacuum via a tube, don't know technically why, perhaps someone will elucidate. Mine is 300mm x 200 x 200 for no other reason than that was a box I had. Unfortunately mouth suction is not adequate with a chamber. I think there is enough suck but not enough volume...

Thanks. My new chamber, not yet finished, is about half of those dimensions. Yours sounds about the size of a shoebox. Like you, I had scrap lumber of a certain size available, so that determined the size.

 

About mouth suction -- I think that if you have no leaks, the chamber size should not matter.

 

I did try some of my highest-pitched reeds in my first, small chamber, and they sounded fine. But just a few steps down the scale from teh topmost notes, the reeds began to "choke". I take this to mean that chamber size does matter, relative to the reed pitch, so my bigger chamber should work down thru the treble-clef reeds, at least.

 

I also was able to blow/suck my cocnertina's reed cells from my little chamber, and that worked fine down to almost the bottom of the range.

Concertinas reed can start as low as .1" for a low reed and .3" for the highest. Don't know about accordion reeds.

Chris

Yes, with my Magnehelic gauge, I observed the high pitched reeds starting at 0.3" H2O. At 2.0", they were screaming for mercy. I figure the lower pitches should start on even less, and I will "set" the reeds until they do.

Thanks, Mike K.

[ttonon]

Hi Ragtimer,

 

The phenomenon you experience is due to acoustic coupling between the reed and the air column that the reed is in acoustic communication with. This column includes the cavity, the tube, and the nasal/larynx/lung cavity of your own body. Thus, the reed is only part of an acoustic (vibrating) system with many parts. The opening of the system that the reed looks into requires a particular relationship between oscillatory pressure and air velocity; i.e., that relationship that the system – minus the reed - provides when air within it is set to vibrating. In acoustic terms, the reed sees an impedance when looking into the rest of the system. If this relationship (impedance) is not compatible with that which the reed requires in order to vibrate, the reed chokes; i.e., it will not vibrate.

 

Thus, the reed and cavity are only part of a dynamically (acoustically) coupled system, and changing any part of this system may allow compatibility (vibration). What people usually do is what you have done yourself – change the dimensions of the cavity. In order that one cavity enable vibration for a variety of reed pitches, the safest thing to do is to simply make it large enough. In this case, pressure oscillations within the cavity become small enough so that they cannot interfere with the self-excitation mechanism that makes reed vibration possible. In other words, the reed can operate more or less independently; the large cavity isolates, or de-couples, the reed from the rest of the system.

 

For more explanation on this type of coupling, and on many other issues regarding vibrating reeds, you can go to an article I wrote for PICA (Papers of the International Concertina Association), available for free on the web, at http://www.concertina.org/pica/

 

 

Best regards,

Tom

www.bluesbox.biz

[Dana Johnson]

Hi Ragtimer,

 

The phenomenon you experience is due to acoustic coupling between the reed and the air column that the reed is in acoustic communication with. This column includes the cavity, the tube, and the nasal/larynx/lung cavity of your own body. Thus, the reed is only part of an acoustic (vibrating) system with many parts. The opening of the system that the reed looks into requires a particular relationship between oscillatory pressure and air velocity; i.e., that relationship that the system – minus the reed - provides when air within it is set to vibrating. In acoustic terms, the reed sees an impedance when looking into the rest of the system. If this relationship (impedance) is not compatible with that which the reed requires in order to vibrate, the reed chokes; i.e., it will not vibrate.

 

Thus, the reed and cavity are only part of a dynamically (acoustically) coupled system, and changing any part of this system may allow compatibility (vibration). What people usually do is what you have done yourself – change the dimensions of the cavity. In order that one cavity enable vibration for a variety of reed pitches, the safest thing to do is to simply make it large enough. In this case, pressure oscillations within the cavity become small enough so that they cannot interfere with the self-excitation mechanism that makes reed vibration possible. In other words, the reed can operate more or less independently; the large cavity isolates, or de-couples, the reed from the rest of the system.

 

For more explanation on this type of coupling, and on many other issues regarding vibrating reeds, you can go to an article I wrote for PICA (Papers of the International Concertina Association), available for free on the web, at http://www.concertina.org/pica/

 

 

Best regards,

Tom

www.bluesbox.biz

Ditto all that. Remember though that the reeds themselves work best with a small chamber mounted with a connecting hole about the size of the normal pad hole on top of a larger box, then the breathing tube of bellows (which can be the box ) connected to the box not the reed chamber. The impedance of the box needs to be subsatntially bigger than the impedance of the reed chamber. The reed chambers are about the size of your reed plate and can vary in depth from 1/4" or less on high reeds to much deeper on bass reds with a lot of swing. Make the "pad" hole across from the root of the reed, not the tip. Too much pressure can choke a reed as well if it it is set to low for it's surroundings. Occasional resonances in the whole system can interfere or weaken the sound of reeds pitched at that resonance as well. The bellows is less subject to that than a box since it's volume (soft walls and all) is constantly varying.

Dana

[ragtimer]

Hi Ragtimer,

 

The phenomenon you experience is due to acoustic coupling between the reed and the air column that the reed is in acoustic communication with. Best regards,

Tom

www.bluesbox.biz

Ditto all that. Remember though that the reeds themselves work best with a small chamber mounted with a connecting hole about the size of the normal pad hole on top of a larger box, then the breathing tube of bellows (which can be the box ) connected to the box not the reed chamber.

 

Make the "pad" hole across from the root of the reed, not the tip. Too much pressure can choke a reed as well if it it is set to low for it's surroundings.

Thanks you, Tom and Dana, for your information. I know about acoustic coupling from pipe organs, but wasn't sure how much it affected frree reeds.

 

I had indeed noticed that pad holes tend to be at root end, not tip end, of reeds, at least in accordion-reeded instruments, and was wondering if that was for acoustic purposes or just convenience in accessing and observing the reeds. Apparently it is better to feed the wind from the root end.

The bellows is less subject to that than a box since it's volume (soft walls and all) is constantly varying.

Dana

I suspect the bellwos are "acoustically transparent" to some degree, and as far as hte vibrations are concerned, are one with the open air.

 

In fact, I have an idea to make one side of my new reed tuning box cut open and ocvered with airtight cloth or rubber sheet, to make the box one with the atmosphere.

Thanks again, Mike K.

[ttonon]

The impedance of the box needs to be subsatntially bigger than the impedance of the reed chamber.

 

Hi Dana,

 

I think you mean the opposite; i.e., that the impedance of the box needs to be small. Since impedance is defined as (complex) pressure divided by (complex) velocity, the idea here is that you want pressure oscillations in the box, and thus at the opening between the box and the cavity, small. In this way, the cavity (and reed) doesn't know the box is there.

 

I'm also not so sure about the need - in general - for a cavity in a tuning rig, which needs to accommodate reeds of all pitch. If the cavity and everything else can be made the same as the acoustic geometries that the reed experiences in the actual instrument, then this would be ideal, and there would be no need to check the tuning once the reed is put back into the instrument. But such an arrangement is usually not practical, and tuning needs to be accomplished in an iterative fashion. With accordions, one can tune the reeds while mounted in their working cavities by fitting the entire reed block to the tuning rig. But even here, iterative tuning is hard to avoid because of the complex geometries associated with ports, the proximity of the pallets, the fact that bellows surround the blocks, etc. I understand that for your concertina construction a cavity may work better than no cavity, but it may not be true for all applications. Perhaps others can share their experiences on these points.

 

Best regards,

Tom

[chris]

Hi

not that I know anything about the acoustics etc but I wonder if a variable cavity (which seems to be the desire) could be constructed using the same principle as a 'swanee whistle' i.e. a piston in a 'tube'

do feel free to shoot this down as I really haven't a clue what I'm talking about

chris

[ragtimer]

Hi

not that I know anything about the acoustics etc but I wonder if a variable cavity (which seems to be the desire) could be constructed using the same principle as a 'swanee whistle' i.e. a piston in a 'tube'

do feel free to shoot this down as I really haven't a clue what I'm talking about

chris

Yes, having one side moveable to vary the cubic content miht be a good idea.

 

I suspect that what's important is not so much achieving a resonance for each pitch fo reed tested, but rather *avoiding* anti-resonance (high impedance) at each pitch.

 

That is, for a box of fixed size, most reeds will sound just fine, but there will be a couple of pitches for which the box will resonate the wrong way, and stifle the tone. Then having a movable side might save the day.

 

Another approach, if you can supply enough wind, is to just perforate the box sides with holes, to spoil any kind of resonance from developing. But I still like my idea of one flexible (rubber or cloth sheet) side.

--Mike K.

[Chris Ghent]

I'm not sure the situation is quite that fussy. Try the box without a moveable side first. If you use rubber for the side you may end up with the vacuum fighting the rubber to get pressure down.

 

Chris

[ragtimer]

I'm not sure the situation is quite that fussy. Try the box without a moveable side first. If you use rubber for the side you may end up with the vacuum fighting the rubber to get pressure down.

Chris

RIght. I just finished building my box today, but have yet to dig into my concertina yet again to try out some reeds. I expect no trouble on the RH (treble) side of the tina.

 

The smaller reed cell (my oriignal, that choked all but the tiniest reeds) is now bolted to the top of the box, and is adjustable in length to fit different reeds, and can be adapted to wider or narrower reed plates just by rip-sawing one new piece of hardwood.

 

I'll upload some photos in a few days. --Mike K.

Edited November 4, 2007 by ragtimer