Bidirectional Reed Experiments

Recently I was asked to try to come up with a concertina reed that works in both directions, or at least to figure out why it hasn’t been done before. Ordinarily, a free reed only speaks when you suck air down past the tongue, into and through the vent in the frame. Most concertinas have a pair of reeds controlled by each button; one inside the reed chamber that only sounds on the pull stroke and another mounted on the underside of the reed pan that only sounds on the push stroke. Anglo instruments take advantage of this to play different notes on pull and push (this is known as bisonoricity), whereas English and Duet instruments play the same note in both directions so they need a pair of identical reeds for every button. If it was possible to make a bidirectional reed that worked as well as two standard reeds, it could potentially enable unisonoric instruments to be made smaller, lighter, and cheaper.

The way I went about solving the problem was to first build what was pretty much a standard reed in an oversized frame and check that it sounded normally in the suck direction, then screwed on a roughly horseshoe-shaped plate that fit around the tongue. I didn’t expect this to work because there was no way for a significant amount of air to get past the tongue to start the oscillation cycle, and indeed it didn’t.

bidi_reeds_1

I also modified my bellows bench a little to allow me to block up the standard dovetail socket and screw the new oversized frame to it elsewhere, and provided a means to block the one-way valve that normally releases air when I raise the bellows so that the rig only works in the suck direction.

bidi_reeds_2

Next I took the horseshoe back off and started experimenting with filing away various parts of the bottom of the horseshoe vent around the tongue, to provide some space for air to get to and past the tongue and allow it to start. Eventually I got it to sound, albeit poorly, in the suck direction, and it even made a tiny bit of sound in the push direction.

I had a theory that the triangular profile resulting from filing the underside of the horseshoe vent was causing the airflow to be cut off too gradually in the blow direction, so I next made a new horseshoe piece, this time with a square-sided recess milled into the underside so there was air space all around the tongue. This was supposed to cut the flow off more cleanly when the tongue swung up into the horseshoe vent.

bidi_reeds_3

This did work a little better, however it was very inefficient, very quiet, and worked much better in the suck direction than the blow direction. I figured that the reason it worked unequally was because the reed tongue was profiled only on the top surface, so when it passed into the bottom vent it cut the airflow cleanly and suddenly whereas when it passed into the top vent it cut it progressively from the tip towards the root.

In order to try to solve this asymmetry, I built a second, more complicated, reed. On this one the reed tongue is set into the bottom frame by half the thickness of the reed stock, it is profiled equally on top and bottom of the tongue, and I also restricted the air pocket to the last third of the tongue, which I tried to profile fairly flat so that it cuts the airflow fairly cleanly in both directions.bidi_reeds_4

The second reed was the most successful prototype I built, however it revealed the biggest flaw with the idea. When set up carefully it works pretty equally in both directions, however the amplitude is very limited compared to a standard reed:

I believe I now understand the reason for this, however it is a little tricky to explain. Before starting my experiments I had observed that with a standard reed playing at normal volume, the tongue swings well above and below the restriction point at the entrance to the vent. I imagined that with the bidirectional reed, it would swing past both restriction points and generate a similar amplitude level, perhaps with a different tone. This was based on a couple of misunderstandings about how reeds work.

My current understanding of what happens with a standard reed when it first starts up is that the tongue gets drawn down towards the frame (it needs to be set such that at rest there is a slight gap between the tongue and frame so air can start flowing). I don’t fully understand the physics behind why this happens, but it seems to me that the faster the airflow into the vent, the harder the tongue gets pulled down. The tongue descending towards the vent opening restricts the airflow into the vent, the force pulling the tongue down reduces, and the tongue springs back up, eventually peaking slightly higher than its rest position. Because it is higher, the gap between the tongue and the frame is larger and more air is able to flow through it than on the first cycle, so it gets drawn down a bit further, and springs back a bit higher than before. Over the course of a number of cycles, the amplitude builds up and up until the tongue is swinging a long way below the top of the vent. In order for this build-up to work, it’s important that every time the tongue swings a bit higher, it results in more air flowing through the vent, which causes the tongue to be pulled down harder and the amplitude of the oscillation to increase. Eventually the oscillation reaches an equilibrium level that depends on the pressure differential between the top and bottom of the reed frame. If you squeeze the bellows harder, the tongue oscillates to a greater height and the ‘packets’ of air being chopped up by the tongue passing through the frame are larger and more energetic, which results in a greater volume of sound from the instrument.

What goes wrong with my bidirectional reed that prevents it developing a decent amplitude at normal bellows pressure is that the second vent restriction, the one ‘above’ the tongue (whichever direction that happens to be), cuts off the air supply whenever the tongue tries to swing higher than the opening into the second frame. It’s impossible for the amplitude of the oscillation to ever build up any higher than the second frame, because it restricts the air supply as the tongue swings higher instead of allowing more air through. It is a lot like the governor on an engine, which throttles the fuel supply whenever it tries to exceed a certain speed.

It is possible to increase the amplitude at which the limiting occurs by increasing the distance between the two vent openings inside the reed, however there is a limit to how far you can take this. If the distance is too wide, the reed oscillations take several seconds to build up to an audible level, or never start up at all. It also becomes impossible to deliberately play the reed very quietly: you end up with a reed that has essentially no dynamic range.

To make matters worse, as well as the limited volume issue, there are several other disadvantages to this type of reed:

  1. They seem to be less efficient (i.e. they use more air than a standard reed operating at a similarly low amplitude), possibly because the way it is constructed to allow it to perform equally in both directions has the side effect of not cutting the airflow very cleanly in either direction.
  2. They are considerably more difficult to make than a unidirectional reed, probably something like 75% of the work of making a pair of standard reeds. A lot of the extra work has to do with making both frames a tight fit around the tongue without catching on the sides. Because nearly all of the cost of a hand-made reed like this is in labour time, it wouldn’t be a large cost saving to make an instrument with half the number of bidirectional reeds.
  3. They are significantly bigger and heavier than a standard reed because of the need to be able to screw the two parts together; you would save a little compared to a pair of standard reeds but not as much as you might think.
  4. There are a bunch of issues around the fact that what you would call the ‘set’ on a standard reed is fixed at manufacture-time by the relationship between the height of the recess and the thickness of the tip of the tongue. You can’t easily alter it deliberately, and it is possible to alter it accidentally as a side-effect of tuning the reed. It’s also important for the tongue to be set precisely central between the two frames, otherwise it starts poorly or not at all in one direction or the other.
  5. I don’t know for sure, but I suspect this design would be more susceptible than a standard reed to getting dust and fluff caught inside it and impeding its operation, because the air gets forced through a narrow recess inside the reed.
  6. It goes without saying that this type of reed is useless for an Anglo instrument because it produces the same note in both directions.

Following up on a slightly different line of inquiry, I made two final experimental reeds, one which only had a rectangular recess right at the tip, and another which was very similar but with a triangular recess instead. Neither of these worked as well as the second reed, I suspect because they only have a tiny amount of space for air to squeeze past the tongue. They sound in both directions (just about), but are very inefficient and quiet.

bidi_reeds_5

bidi_reeds_6

Here is an audio recording I made of the four experimental reeds plus a standard reed for comparison. The first reed had the second horseshoe fitted.

Although this work didn’t lead to a usable product, it was still a useful exercise for me in that I believe I now have a significantly better understanding of how concertina reeds actually work.

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Reed Prototypes Part 2: Tongue

The way a free reed works is that a tongue made from a springy material (usually spring steel or brass) is made to oscillate through a close-fitting window in a metal frame by the flow of air through the reed. Each time the tongue passes through the tight part of the frame it briefly interrupts the air flow. This regular chopping-up of the air flow produces a tone with a significant harmonic content (it’s a long way from being a pure sine wave).

I’m using hardened spring steel for my reeds (IIRC it was 0.8mm thick on this size of reed), which I found practically impossible to cut with hand shears, so I bought an old bench shear on eBay. I got it cheap because it was seized up with rust and the blades were blunt and dented, but it is a nice heavy-duty machine:

handy_shear_before

After restoration, it works really well (though it would be nice to have an extension tube on the handle):

handy_shear_after

I need to come up with a better way of cutting the strips consistently to the right width. To complicate matters, they are slightly tapered. Initially I scribed them and lined them up under the blade by eye, which worked better than I expected but was rather fiddly and time-consuming.

marking_reed_tongue

sheared_reed_tongue

The shearing action bent the tongue slightly so I straightened it before doing any more work on it:

straightening_reed_tongue

Next I cleaned up the edges by draw-filing while it was held in a toolmakers vice:

filing_tongue_edges

It’s important to make the tongue a very close fit in the frame, but not so tight that it’s prone to catching if the reed pan expands and presses on the sides of the frame:

fitting_reed_tongue_1

A low-power back-lit stereo microscope is a big help with fitting the tongue to the frame. Although the gap looks off-centre in this picture, that’s because you’re only seeing the view from the right eyepiece.

fitting_reed_tongue_2

In a concertina, the tongues are usually not a consistent thickness along their length: they are profiled so as to bring the pitch up or down and to balance the volume of the reeds across the range of the instrument. Because I don’t fully understand all the parameters yet, I decided to start out by copying the profiles of the reeds in an antique instrument. I took the tongue I was copying out of its frame and measured it in several places with a point micrometer (I found it wasn’t difficult to put it back in the same place, and it still produced the same pitch afterwards).

measuring_reed_profile

I did the profiling by hand using a triangular Bahco saw file and an Eclipse hand vice, on top of an oak block with a shallow step cut into it. You can tell roughly what pitch you are at by ‘pinging’ it. I found, at least with this size of reed, that it is very easy to take a hair too much thickness off the belly area and the pitch suddenly drops by a couple of semitones. You can bring it back up by taking a lot of thickness off the tip, but then you have a weak reed that sounds slightly odd.

After clamping the profiled reed into the frame, you have an extra bit of tongue sticking out of the back of the reed that needs to be removed (you deliberately shear the tongue too long so you have something to grip while profiling and fitting it). Because it is hardened steel, the extra bit is very easy to break off, and the fact that the clamp doesn’t quite reach the end of the frame means that the sharp stub doesn’t stick out significantly past the end of the reed:

Here I am doing the initial rough-tuning of the reed before trying it in the instrument. Note that removing some metal from the belly (using a 600 grit diamond needle file) caused the set of the tongue to alter, i.e. the tip bent down slightly. This caused the reed to choke the second time I tried to sound it. There needs to be a slight gap between the tongue and frame when it’s at rest or no air will flow through it and it won’t start oscillating by itself.

From left to right, we have the original antique Lachenal reed I was copying, my first working reed (using the best of the aluminium frames), and my first brass-framed reed:

three_reeds

My first brass reed in the instrument:

reed_in_pan

The profiling of this first reed isn’t a perfect copy of the original: rather than being a smooth curve from the belly up to the tip, the profile curves up too sharply and then plateaus before the tip. The effect of this is that although the pitch is right and the dynamic range seems OK, the tone has less upper harmonics. When I compare it in the instrument to the original reed next to it, it sounds ‘softer’ with less of the piercing ringing overtones of the original. I suspect this is because most of the bending action is happening near the clamp rather than spread out along the full length of the tongue. Something to work on improving in my next prototype!

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Reed Prototypes Part 1: Frame and Clamp

Recently I made my first prototype concertina reeds. There’s a lot to write about so I’m going to divide it into two articles, this one will cover the frames and clamps, and the next one will cover the tongues.

My plan was to make a drop-in replacement for one of the low ‘B’ reeds in my vintage Lachenal English. I think this instrument was probably one of Lachenal’s higher-end ‘broad scale’ models, because I also have another steel-reeded Lachenal that in many cases has narrower reed tongues in the same pitch. I’m not totally sure why the narrow scale models existed or were originally cheaper than the broad scale equivalent (the extra metal and labour is fairly trivial). They were probably quieter at the top end of the dynamic range, which might make them better for a student instrument.

I understand Wheatstone’s earliest reeds were made totally by hand, piercing them with a jewellers’ saw and cleaning them up with files. This must have been very labour-intensive, highly-skilled work, and prone to inconsistency. At some point fairly early on, perhaps when Louis Lachenal was hired to mechanise production(?), they changed to using fly-presses and dies to punch out the reed frames. This was much faster, worked well, and the presses could be operated by relatively unskilled workers, but the disadvantage is that precision dies are very expensive to make. To save on tooling costs, instead of making a different set of dies for every pitch of reed, they made do with a handful of sizes and made up for the gaps between them through careful tongue profiling. Until relatively recently, the need to invest in a set of press tooling was a significant barrier to entry for new reedmakers.

Enter CNC machining. I understand other makers have successfully used laser cutting or possibly wire EDM, but I have my own small CNC milling machine so that is the process I am going to use. It is fairly slow (certainly compared to a a press tool), but it works pretty well and I hope to get to the stage where I can load in enough brass for half an instrument worth of reed frames, and set it going with minimal supervision while I work on another task. As well as cutting out the shapes of the frames and clamps, it can also cut the vent slots (albeit with filleted corners), drill the clamp holes, engrave the note labels and a logo, counterbore the clamp screw holes, and even chamfer the top edges of the frame so they fit nicely into the dovetailed slots of the reed pan. Initially I’m planning to copy all the dimensions of my prototype reeds from the Lachenal instrument, but in future when I understand the design parameters better I will be able to make frames that are the optimal size for each pitch.

My first attempts at milling frames were using scrap aluminium. It took me quite a few failed attempts before I got one that seemed pretty good (the antique Lachenal frame I was copying is on the right):

aluminium_reed_attempts

Next I moved onto brass prototypes, immediately running into problems with it cutting very badly and breaking 1/16″ end mills:

failed_brass_reed_shoe

A microscope view of an end mill with clogged flutes, from a run that I aborted before it snapped:

clogged_end_mill

I think the reason for my problem was that the chips weren’t clearing from the slots properly so on subsequent passes they were getting re-cut and generating a lot of heat. I experimented with a lot of parameters, but basically what worked was making the depth of cut shallower, increasing the spindle speed to 10K RPM (the maximum my machine’s spindle can handle), significantly increasing the feed rate (to make bigger chips), and adding a compressed air blast to blow the chips away. I also changed from two flute HSS bits to three flute cobalt bits, though I’m not certain that helped with the chip clearance (it did allow me to increase the feed rate a bit more). It also proved necessary to make some proper mechanical clamps to hold the plate to the spoil board, because double-sided tape wasn’t holding it securely enough:

sheet_metal_mill_clamps

Here is a (21 minute long) video of the process of milling a single reed shoe prototype. Don’t bother watching the whole thing unless you’re really fascinated! This isn’t quite the final program: I subsequently altered the bevelling operation slightly so that the frame wedges more securely into the reed pan.

The CNC program includes small tabs that prevent the parts coming loose during machining. Afterwards these need to be manually cut. I found that it was possible to break them out with a small chisel but it left rough stubs that I then had to clean up with a file, so I changed to cutting them with a jeweller’s saw:

cutting_reed_frame_tabs

Because I cut the vents using a 1/16″ end mill, this leaves 1/32″ radius fillets in the corners, which should ideally be dead sharp. I’ve been manually cleaning these up using a fine square needle file with one edge ground smooth. I put the reed frame over the small square hole in my bench peg (see previous photo), hold the safe face of the file flat against the end of the vent, and carefully file sideways into the corner until it’s as sharp as possible without leaving a nick:

reed_vent_squaring

The clamp screws I’m using are a bit smaller than the originals; they are M1.6, stainless steel, with 2.5mm diameter allen heads:

clamp_screw_comparison

In my testing they are strong enough for the purpose and take up less space than the originals, and the finer pitch and allen heads make them easier to tighten and loosen without damaging them.

When tapping the clamp screw holes in the frame, it’s very important to keep the tap perpendicular to the hole. After researching tapping machines and complicated guides, I came up with this simple method that works surprisingly well (though I still managed to break a tap the first time I tried to tap one of the brass shoes!):

I added counterbores to the holes in the clamps because it was easy to do and significantly reduces the height of the reed without weakening the clamping ability. It also improves the accuracy of the location slightly. Because I was already using an engraving operation for the note labels, I added a simple brand to the clamp (HC=Holden Concertinas):

reed_clamp

Because the screws start out a couple of mm too long, I put them in the frame and grind them almost flush with the bottom of the frame, then finish them off with emery paper on a sheet of glass:

grinding_clamp_screws

One of the defining characteristics of traditional concertina reed shoes is that the underside of the vent is relieved (i.e. the bottom of the slot is slightly wider than the top). My current understanding is that this allows the reed to work properly even at very low bellows pressures, i.e. it enables you to play quietly if you want to. It also has an effect on the tone. I’m not doing this on the milling machine because there are good reasons to cut them out from the top and it would be a bit tricky to turn them over and accurately register them for an extra operation. Instead, I made a special jig that allows me to file the vent to a consistent angle using a flat needle file with two safe narrow edges.

The clamp part of the filing jig started out as an old war-surplus hand vice with damaged jaws:

vent_filing_jig_1

vent_filing_jig_2

I trued up the jaws and modified the profile of the front jaw so that there is room for the file to tilt down below the level of the back jaw:

vent_filing_jig_3

Next I added an adjustable brass frame and a PTFE roller to guide the file, as shown in the following video. I align the top edge of the vent with the top of the back jaw, paint the inside of the vent with a black marker pen, and file until I’ve almost but not quite removed all the ink.

Here we have my first brass reed frame in my Lachenal reed pan. You can see how much lower-profile the new screw heads are: I think this might help with air flow inside the reed chambers. It took several prototypes before I was totally happy with the tightness of the fit in the dovetailed slot. There is a small area around the clamp that isn’t fully bevelled, giving a nice friction fit without compressing the sides of the frame adjacent to the vent.

reed_frame_finished

finished_brass_reed_frame

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