she may be "finished", but she sure ain't finished!

There's trial by fire, and then there's trial by water...

Not part of the original plan, but she got left out in the rain!  This was just after a second and hopefully comprehensive polyu coat on the soundboard, wrestplank, and case sides, and the first coat on the inside of the soundbox and the top edges and inner faces of the case lumber.  It was cloudy and misty but not raining, I got called off to an important event on short notice, no time to move the instrument back under a roof, and a couple hours later, the "mist" had turned into regular Northwest rain.  Yikes.

Seems like no immediate harm was done; the fresh finish seemed to readily repel the water (perhaps even more vehemently, since it was outgassing all those solvents), and two days later, there is no visible sign of the ordeal.  But it's certainly a good test, a harsher test than I ever planned to subject the thing to.  We shall see if enough water managed to sneak past the finish barriers (coverage is definitely not 100%, there are regions which have been kept bare wood for later gluing steps), to cause any problems later; this could include obvious things like warping and cracking, or more insidious things like mold stains or rot forming underneath the finish in future years...

Anyway, at this point, I don't know of anything special to do about it other than just let it dry for a good long time, and not very fast.  I had been planning to keep it in a semi-indoors location, a garage, for the (solvent) drying process, but turns out there is a convenient place to park it outdoors, next to the garage, under an awning; I think this location may be better, for the same reason I originally worried that it might be worse: higher humidity.  Now that the instrument has probably attained the highest internal humidity since its wood parts were green, and the highest it'll ever see again (I hope!), I want to *very gradually* lower it down, to the "Pacific Northwest indoor winter" level, which is still reasonably high.  I have some reason to hope that there will not be dramatic damage from this water incident, at least, thanks to my ill-advised experiment with the "wet-pack" on the spineside 2x4, back when I was building the frame.  That extended water exposure to a small region surely wicked more water into the unfinished lumber than this relatively brief shower could have, through the small areas without finish.  Didn't kill it, so I guess maybe it made it stronger?  Well, one can hope.  I now definitely don't plan to put any string tension on this instrument, until after the New Year at minimum.

That was the "exciting" part.  The rest has been more routine, so far:

I trussed up the instrument for travel, putting padding on the convex corners, and duck-taping the lid piece to the top to keep the soundboard protected inside.

Fits quite readily into a late-model Toyota Rav4 (electric version: note the 50 Amp charging cable plugged into the left side; full charge in 20 min), with room for about 1.5 passengers and a driver (and a dog!).

This is the first time I've tried to move the instrument around very much, since it has gained quite a lot of weight with the case sides and wrestplank being attached.  It's still a breeze to handle for two people, and by grabbing it in the right places to balance comfortably, I am able to readily carry it around all by myself.  It's a long, awkward shape to navigate through doors and in spaces with low ceilings, but it's really delightfully light in weight.  Even if it only sounded slightly worse than a piano, rather than way way better, it'd still be worth building, just for the portability.  To move much of a piano with that same Rav4, I think you'd need the hitch, and a trailer.

Here you can see the soundboard, back side already finished before coming here; it's leaning on the lid piece, which has been painted flat black on the top side, and flat white on the bottom side, where the art will go.

I did some masking with duck tape and chicken feed bags, then painted the bottom panel flat black.  Then took the masking off, and sprayed the whole bottom and the sides with polyu (coat #1).

The next day I positioned it rightside-up, and sprayed the soundbox and upper surfaces (coat #1), and the soundboard and sides (coat #2).  That was the Day of the Rain...

Now, I'll just let it sit for a while where it is, under a roof protected from rain, but at outdoor humidity level.

Next up, a little Holiday Travel, and then I'll be back to finally glue the soundboard into place.  Lots of little brass-plated nails.  And steel nails, for the hitch-pins (glad I tested and found out in advance that the brass nails won't cut it for the hitch-pins).  And I'll drill out the wrestplank holes to final diameter (glad I hadn't already done this before the Rain Thing).  And then, time to string it up -- and see if it breaks!  And then, well, I'll pretty much be done with construction on the pieces that I now have.  It'll be time to start acquiring the wood and other materials, with which to build the keyboard, action, and the stand.  And a few more pieces needed for the lid.  I have a plan for a nice way to do the latch.  Not that I ever finalize a design before beginning construction, but I'll certainly have some time during this upcoming travel, to intensively go over the design of these elements.  I find I can compensate in part for my lack of either experience or common sense, by delaying implementation of designs for as long as possible, even when it seems that the design is done and what am I waiting for; this seems to give my brain time to bubble things up from the subconscious, things that bother me but I don't know why, better ways to do things, etc..  Haven't yet found a way to market this technique to Corporate America, but I think they could really benefit...

As we were moving the instrument, a curious passerby asked what it was, and then when will it be finished.  I guesstimated another six months.  So there you go, I'm on record with a projected completion date; let's make it, specifically, the upcoming Summer Solstice.  Seems like longer than it should take -- unless I consider the past history of all my other projects.

assorted fixes

A few tasks that needed to be done before the finishing steps.

I carved a wedge (using the original long strip that I sawed from the frame), to fill the gap between the case side and the frame 2x4, on the spine side.  This area will take large amounts of strain and torque, being the attachment point for the wrestplank, so I wanted to make sure the repair was strong.  Ugly, but strong!

Clamping the wedge repair also gave me the opportunity to draw-in the bottom edge of the case lumber, which was warped outwards on this end.  Still not perfect, but overall it's improved.

And here, I was mixing up the Titebond-sawdust filler paste, for gluing the wrestplank down.  Looks like coke!  After it was mixed, it looked even worse, turning into a goopy pudding with a rather disgusting consistency, which nevertheless seems to have done the job required (which was to fill narrow gaps around the ends of the wrestplank, up to 1mm or so).

And there's the wrestplank, finally glued into place.  I used long sheetrock screws (now sealed with glue in their countersunk holes) to self-clamp it down.  I think all conditions were as right as I could get them for good gluing.  We'll see, when I put it under tension!

Here's the "two-pronged fork" I made, to help me locate the positions of the bridge blocks for drilling.

Here's the even-uglier-than-the-other-one repair I made, to fill in another gap between the frame and the case on the spineside, this one at the tail end.  I jammed a bunch of plywood pieces in there, drizzled in copious glue, let it dry, and then sawed off the jagged ends.  I hope that this forms a reasonably rigid composite structure; unlike the other repair, this one does not have to take a lot of force, and the instrument probably would have worked with no repair at all: but this makes me feel better, and now you can't see the ground through a crack next to the soundboard...

Here, and also in the previous photo a bit, you can see the crossbars I added to the bottom.  These will eventually interface with the instrument stand in some way, when I get around to building that.  For now, the stand is two columns of stacked Home Depot buckets!

One of the last chances to see inside; after applying finish, I will glue the soundboard into place.

The imminent closing of the soundbox is another reason I had to rapidly make a decision about reinforcing the bridge blocks with screws.  Once the soundboard is glued on, there'll be no going back, no reasonable way to change my mind if I hadn't already put the screws in.

So, I decided to go with the screws.  Having already marked the locations of the blocks with my two-pronged fork, I then had to make a guide for the hand-drill, just a hole drilled in a block on the drill press so it'd be straight.  By adjusting the position of the drill bit in the chuck, I could set the depth of the holes just a little longer than my screws, so that the holes wouldn't pierce through the tops of the bridge blocks.  (At one time I did consider letting this happen, probably not detrimental, but, nah.)

My little Ryobi hand drill surprised me: it was able to do all the holes (well, 45 out of 49, 4 having been done the day before) on a single charge.  And this was slow going, hard work, drilling into oak.  Too fast, and the wood would start to burn.  I was sure I'd have to recharge multiple times to get through all of the holes.  I was just hoping to get as many as ten holes per charge.  But it just kept going and going...

I dripped a dab of Titebond II onto each hole, let it sit for a while to hopefully start working its way into the hole and not just sitting on top (but I'm not sure to what extent this happened); and then I drove in the screw, through the glob of glue.  Hopefully this caused at least some glue to end up down in the hole, helping to seal the screw into place and helping to prevent splitting, just like the glue "sizing" on the outside faces of the blocks.

Next up, in terms of things which must be done before I transport the thing to the finishing location.  I need to tape-mask the regions inside the soundbox which need to stay free of finish, for gluing down the soundboard.  And I need to trim the lid piece.  You can see that I'm using it here, to give a surface from which to work on the soundboard.  The lid is as yet just a rough-cut piece of plywood.  It will eventually have a rim of 1x2s to make it more rigid, since it is the same 3/16" plywood as the soundboard, i.e., very flexible.  I need to trim it to just about final size, and also to cut the second, smaller rectangular lid piece, hinged to the larger one, which covers the front end of the instrument including the wrestplank area.

During the finishing operations, I will spraypaint the top surface of the lid pieces flat black, and the inner surface flat white.  This will serve as a primer to prepare the surface for artistic painting.  Perhaps even before building the rim of the lid and the hinges and all, I will then be able to pass the lid plywood pieces to an artist friend (I hope!) to decorate.  As an artist myself, I understand the pressure of these sorts of commitments to help someone else with their project; fortunately, I won't be too impatient to get the lid pieces back, because I'll be busy for "some unknown length of time" building the keyboard and action.

bridge moldings, sideways force, and swearing off oak

So the more I consider it, the more I don't trust my bridge moldings just glued to the plywood.  Chances of de-lamination (ripping loose) seem too high.  My monochord has survived for weeks with this arrangement, one of the things I built it to test.  However, I don't think I want to operate so close to the edge: even if the blocks do hold the tension, they are extremely fragile.  The tall nails sticking out of the top provide plenty of leverage, so that bumping into the nails sideways is likely to tear the block off of its footing.  I know this from experience...

The clincher was examining the trigonometry of the forces, particularly the sideways force.  Give any kind of side-angle to the string hitching position, and the forces mount rapidly (as double the sine).  And I will need a lot of side-angle, to avoid excessive length on some of the string tails, due to my straight-line bentside.  Angles of 20 degrees or more quickly get above 25% of the string tension, applied as sideways force.

So I'm thinking I should probably have designed these blocks to have screws (or dowels) going all the way through the plywood.  I don't really like putting so many holes in the soundboard, either; especially in the treble, it amounts to a pretty severe perforation, a "dotted line" along which I hope it won't tear or buckle.  If I had planned ahead, the holes would have helped in positioning the blocks for mounting, and the screws would have obviated the need for my complex and awkward clamping rigs.  Instead, I now have the delicate task of locating the right place to drill through the plywood from the back side, and into the bottom of each bridge-block, just to the right depth for the screws I will use, not penetrating the upper surface.  Tricky, but doable I think.

I made a little helper item to mark the positions of the blocks for drilling: a pair of matching 1x2s, glued together at one end in a sandwich with a small piece of the soundboard plywood, so that they form kind of a two-pronged fork, with a space in between equal to the thickness of the soundboard.  Thus, when I slide this "fork" over the edge of the soundboard and bring the end of the upper 1x2 up against the corner of one of the bridge-blocks, the end of the lower 1x2 shadows its position on the back side of the soundboard, and I can mark the spot with Sharpie.  Taking the time to glue-up this little tool was much preferable to struggling to mark the positions with straightedges and my T-square, the same struggle repeated 49 times.

Next I need to make a little block with a straight hole drilled through it, on the drill press; this will let me drill straight-ish holes to a specific depth using the handheld electric drill.

My diced-up moldings for a bridge idea seems to have acoustic merit, and with a conventional solid-wood soundboard, they'd probably work fine glued on the surface.  It's when I combined that cheapskate-ordinary-materials idea with another cheapskate-ordinary-materials idea, the plywood soundboard, that unforeseen (but arguably foreseeable) problems arose.  The cheaper and easier-to-obtain materials end up costing quite a bit of labour and design complexity, to get acceptable performance out of them.  Oh well, that's pretty much my whole strategy here, to devise a way to build a good instrument using almost no money, but almost unlimited amounts of time and tedium.  For some of us, it's a good bargain.

In other news, I have noticed that my oak nut molding has split, all along the top along the line of the nails.  So far, the bridge moldings still remain un-split.  So basically every example where I've tried this nails-into-oak thing, has split (also the monochord shows splitting), except those cases (the bridge moldings) where I used copious glue-sizing to pre-empt the splitting.  I shoulda used glue on the nut, as well!  Oh well, now I can see if it makes a difference.  None of the nails feel noticeably loose, which would clearly be a problem.

I'll make this beast work somehow; but in the future, no more oak for instrument parts!  There's gotta be something more appropriate, and I'm thinking it might be maple.

"It's oak, so it broke"?

scaling and inharmonicity

Work progresses.  I am gluing the wrestplank into the frame as I write this.  I used a mixture of sawdust and Titebond as a filler compound; first time I've tried this idea, seems like it's going to work.  And there are a couple of places on the frame where I need to carve specially-formed wedges and glue them in, to fill voids due to my imprecise "planing" operation with the hand-saw.  I wasn't sure initially, but now I am fairly confident that the overall structure will be adequately strong, even with these flaws and repairs.  A couple other details remain, like attaching crossbars to the bottom, for attachment to a stand; then it'll finally be ready for the major finishing and painting of the case, which I'll handle "off-site".

As all this continues, I have some time to contemplate the design.  Playing around with the two-string "monochord" has been extremely informative.  By placing the monochord on the soundboard of the real instrument, I've been able to probe and analyze the resonance of the different regions of soundboard.  But perhaps even more importantly, I've been able to experiment with the tuning and tone quality of this iron string.  This has caused me to reconsider my scaling factor on the instrument: my original plan was to go for close to the maximum possible scaling length, but now I am not so sure.  At least with this one particular length of note, I believe I like the sound better when it is tuned significantly lower than the highest "safe" pitch.

The monochord speaking length is about 54 cm, which is the max scale length for the note E4, with this type of iron string.  This iron is very fragile, and I got some good lessons in this as I attempted to string up the monochord.  I broke the first string I tried, probably due to abrasion near the tuning pegs.  I very carefully brought a second string up to pitch (E4), and kept it there for a few days as I played around with it, monitoring among other things how quickly it went flat and how often I had to re-tune it until it seemed to stabilize: in this regard, performance was satisfactory.  However, on one of the re-tunings, I broke the string again.  Did I go too sharp over the note?  Did I change the pitch too abruptly?  Had the string been damaged somehow, from all of the handling and banging on it with different implements?  Hmmmm.  I don't know, and I don't like it.  I want these strings to hold up to rather inhospitable conditions, and after all, I am planning a percussive action.  The strings need to not break at the drop of a hat.

So I replaced the string again (not too happy to use up my supply of this specialty instrument wire, not even on the real instrument, but clearly more testing was needed).  This time, I decided to leave it below pitch for a while, just so I could bang on it with less fear of the string breaking.  I started out with the pitch at B3: way below the intended pitch.  I was surprised at how good this tone sounded: nice harmonics, loud volume, and none of the pitch instability or other symptoms of way-too-low string tension.  Well, that's nice, but I had to know if the string could really take its intended design tension, or not: I might have a systemic problem in my calculations.  So I brought it up to E4, carefully.  The string didn't break, it held the pitch fine, everything seemed perfect -- except, something was lacking in the tone quality.  I had hoped to hear the quality get better, and certainly the volume to increase, as I brought the pitch way up from B3 to E4.  But actually, the increase in volume was not dramatic, and frankly the tone quality seemed to markedly decrease, at least in my subjective opinion.  Hmmmmmmmmmmmmmmmm.  So where's the sweet spot, then?  B3 is perhaps a little too low, though it does sound better than E4.  I tried C4, and D4.  Both sound better than E4.  I think that overall, I like C4 more than D4, but it's a tough call: too much of "whatever it is" that tuning flatter brings in, starts to sound bad, a muddy overload of harmonics.  But not enough of it sounds bad in the other way, too thin and sparse, not enough of that delicate spidery tone which I greatly prize.  And there is no particular reason to assume that the same degree of flat-tuning which sounds good here, in the middle of the range, will have the same positive effect in the far bass, or the far treble...  Darn, the world got more complicated again, I hate when that happens!  Actually I love it, this is the real design process in action.

Basically, my string lengths are pretty much irreversibly defined at this point, but not the pitches.  Instead of running from E2..E6, I could run from C2..C6, or D2..D6.  I don't really have to decide absolutely, until quite late in the construction of the keyboard and action.  Indeed, I could probably design the keyboard to come apart and be rearranged, so that (with significant effort) the pitches could be changed after the fact.  But really, I just want to get it right in the first place.  I'm hoping I can be happy with C2..C6, because that makes for a nice keyboard layout.  Starting and ending on D, I don't like.  I don't like arrangements that "break up" the groups of two and three sharps in the layout.  So, starting and ending on B, C, E, or F are the best choices.  (Yet another thing that I don't like about pianos.)  One will note that my choices of pitch-compass are deriving from considerations like how the pattern looks on the keyboard.  Others would be concerned with the notes required to play specific pieces from The Literature, but I am mostly interested in improvisation and new compositions: whatever pitches and tone quality this experimental instrument turns out to have, I will adjust my playing to fit, and I will find ways to use whatever sounds the instrument can make.

My realization that a shorter scale length may sound "better", by my definition, is made purely from subjective observations.  My simplistic understanding of the math and physics, had me convinced that "longer is better" when it comes to string-scaling.  Trying to reconcile the math with the reality that my ears are telling me, I have re-visited the "Inharmonicity Equation"; and I think I can see some explanation for what I'm hearing.  Basically, if the aim is to reduce the inharmonicity in the string vibrations (which is not a "given", this in itself is one of my working assumptions which should be carefully examined), then there are three parameters over which the instrument designer has control, assuming the string material is already decided: string length, string diameter, and string tension.  Increasing the tension does reduce the inharmonicity, but it does so by the least dramatic amount of the three parameters.  Increasing the length helps more, and most dramatically helpful is decreasing the string diameter.  I have been aware of all these factors, but I have probably tended to treat them as all about equal in importance.  In fact, the string diameter seems to be the real key.

The Inharmonicity Equation defines a value, B, which is the coefficient of inharmonicity.  The higher B is, the more inharmonic are the partials (they always pull sharp from where they should be).  Distilling out the constants into a value "K", the equation for B reduces to:

B = K * d^4 / (L^2 * T) 

So B depends on the 4th power of the diameter, a dramatic sensitivity.  Increasing T divides down B, but only in a directly inverse relationship.  Increasing L obtains results much more effectively, by a power of 2.

This does not explain why strings seem to sound better at less than maximum tension; however, it shows that, if one subjectively feels that a lower tension leads to a better tone, one doesn't need to feel "guilty": the inharmonicity is probably still quite low, and the change in tone quality is probably coming from other factors, affecting the balance of amplitudes of the partials, not necessarily their frequencies.