Adjustable Reed Tuning And Test Cell

[ragtimer]

Here are some shots of my just-now completed adjustable reed test cell. The upper part is what I had built two weeks ago and reported it wouldn't work, so I added a much larger expansiion chamber box beneath it.

 

The jig is built for accordeon reeds but could be adapted to trad concertina reed shoes.

It's adjustable in length by pulling hte center section up or down.

Width is changed by replacing the top and bottom center sections with pieces of the desired width (which must be rip-sawn from the same plank as the two sides).

What you can't see is that the two side pieces have enlarged holes where the bolts go thru, so they can be slid sideways to fit whatever center pieces you put in.

 

Here's a perspective view of the complete test box:

And this view shows the holes in the bottom of the small cell to couple to the big air space in the lower box:

Hard to see are the two brass nipples and rubber tubes out the left side of the box. The large tube is to suck/blow on, the small tube goes to the gauge. You can see these in the original small jig, but I have sealed off those holes and now the tubes connect to the large box.

 

All this was built with just a table saw and a drill press. You can build your own version

Edited November 5, 2007 by ragtimer

[ttonon]

Hi ragtimer,

 

Having the movable side makes it easy to accommodate reeds of different lengths. The way you did it, though, adds some cavity volume, and this can cause some problems with impedance matching. True, the movable wall affects the resonant frequency of the cavity, but I can see some possible conflicts in where the wall should be placed; i.e., to accommodate a reed of certain length, or to eliminate an unwanted resonance.

 

I see the need for a cavity when you want to control the airflow through the reed, as with a pallet in a musical instrument. But for some tuning boxes such as yours, where you blow or suck on the box with your lungs, it may be more trouble than what it's worth.

 

Best regards,

Tom

[ragtimer]

Having the movable side makes it easy to accommodate reeds of different lengths. The way you did it, though, adds some cavity volume, and this can cause some problems with impedance matching. True, the movable wall affects the resonant frequency of the cavity, but I can see some possible conflicts in where the wall should be placed; i.e., to accommodate a reed of certain length, or to eliminate an unwanted resonance.

Actually I do have some leeway. All my reed plates are 1.5" (37mm) long, but the higher-pitched reeds do not go all the way to the end. So I can pull the top end out to the full 37mm, or slide it in to just clear the reed tip, or somewhere in between. Thus I can tune out any resonances in the small upper cavity.

 

But I really don't worry much about resonances in the small top adjustable section, since it has three large holes in the bottom that connect to the big box. I trust the big box not to resonate with any of my right-side reeds (down to Middle C). (I could be wrong about that, but so far, so good).

I see the need for a cavity when you want to control the airflow through the reed, as with a pallet in a musical instrument. But for some tuning boxes such as yours, where you blow or suck on the box with your lungs, it may be more trouble than what it's worth.

Well, when I built the big box, my original paln was to just cut a rectangular opening in its top piece of MDF, exactly 12mm by 32 mm, and set my reed over that hole. But cutting such a rectangle precisely is not easy, requiring a lot of drilling and filing (and MDF dust particles are a little bit toxic, so I hear).

 

Since I already had my little, adjustable tuning cavity, I just drilled three big holes where that rectangle would have gone, and mounted my original device on top of the big box.

 

Does this answer your concerns? Thanks, Mike K.

Edited November 9, 2007 by ragtimer

[ttonon]

Does this answer your concerns? Thanks, Mike K.

 

Hi Mike,

A small concern is that the moisture in your breath might cause some rusting on the reed tongues.

 

In any event, good luck with the rig, and I hope it serves you well.

 

Best regards,

Tom

[ragtimer]

Does this answer your concerns? Thanks, Mike K.

Hi Mike,

A small concern is that the moisture in your breath might cause some rusting on the reed tongues.

"Officially" I use it only in suction mode, where the reed you can see and work on is the one being tested.

You can test either reed in a plate simply by flipping the plate over,

 

In practice, I often blow into the tube to compare the sound and action of the opposite reed. But only for a moment, being aware of the damage that moisture might cause.

In any event, good luck with the rig, and I hope it serves you well.

Best regards,

Tom

Thanks, Tom. I'm about to get disgusted enough with certain notes on my big 67 to start "taking names" and doing some serious reed-set tweaks with the new box.

 

So far, I've found reeds that will speak on 1/4" of water gauge pressure, and others that need a full inch to get going. --Mike K.

[Chris Ghent]

So far, I've found reeds that will speak on 1/4" of water gauge pressure, and others that need a full inch to get going. --Mike K.

 

Reeds need to be set to take into account playing style, and if you set for starting alone you may cross over into choking territory. There are a number of factors other than reed set that will affect the relevance of magnahelic readings; pitch in particular (though this is predictable and less true in quality reed sets), but also variations in anything handmade; profile, reed slot shape, leaks such as reed/frame clearance, and any other leaks in the box.

 

Chris

Edited to add "the relevance of"

Edited November 13, 2007 by Chris Ghent