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I play a berimbau for Capoeira. One of the most fragile (and most expensive) bits is the cabaça, a hollow gourd used as a resonator.

cabaças

I'm not very familiar with the qualities of the resin used for 3d printing. If I were to take this to our local Maker Lab and have them scan and print a copy, how likely is it that it would work? My fear is that the plastic would be too sound deadening.

If you want a less exotic parallel, imagine the body of a guitar. That's a resonating chamber.

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    $\begingroup$ "Scan and print a copy" - It doesn't work like that. It will be "scan, spend hours remodelling the part to obtain a printable file using the scan as a reference, print". $\endgroup$ – Tom van der Zanden Aug 4 '17 at 16:12
  • $\begingroup$ Shows my ignorance. What if I just started out by modeling it in a program? My question still stands on whether plastic works for resonating. $\endgroup$ – Sean Duggan Aug 4 '17 at 16:44
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    $\begingroup$ Of course it's possible to print one; but until someone does, we won't know how well or poorly it resonates. $\endgroup$ – Davo Aug 5 '17 at 10:13
  • $\begingroup$ Best case scenario is someone had done it before. :) $\endgroup$ – Sean Duggan Aug 5 '17 at 11:54
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    $\begingroup$ There are a ton of ocarina models available online, along with various other hollowbody instruments. The relationship between wavelength and resonating power is absurdly complicated, depending on material properties, material cross-section, cavity shape,and so on. $\endgroup$ – Carl Witthoft Aug 7 '17 at 17:17
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I can't answer this from a technical 3D printing angle. But, from a musical angle:

Where the body of an instrument has the primary function of enclosing a vibrating air column, the material has often been demonstrated to perhaps make a difference, but only a subtle one. As an example, a recent range of plastic trombones, although not first-class instruments, have proved extremely playable (and have the great advantage of being virtually indestructable).

I suggest you try this. Play your instrument, dampening vibration of the cabaça with your hand or with a piece of cloth. Just damp the shell externally. Don't obstruct the hole or put anything inside. Now, fill the cabaça with cotton wool or similar. If the first makes little difference, you're probably good to go with a plastic cabaça. Of course, if the second makes little difference either, we might have to suspect that the cabaça is mainly decorative!

You could also experiment with alternative resonators of a similar size and capacity, available 'off the shelf', not worrying too much about a cosmetic match. They might sound even better!

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    $\begingroup$ Using plastic mostly affects the tonality and has little to no effect on the playability of the instrument. Does this sound correct to you? $\endgroup$ – tbm0115 Aug 7 '17 at 18:09
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Following up on this, the answer is, yes, this works quite well. I printed this cabaça model from Thingiverse using PLA on a Lulzbot Mini and put it on my berimbau today to test it out. I can't make a direct comparison because the printed resonator is smaller than the gourd one that I own, but the sound is good. I am not certain whether it is actually cheaper (it used a fair amount of PLA because it's probably about a half-inch thick, and it took a few tries to get a good print due to it taking about 9 hours and needing to be monitored for the filament breaking), but it may be more accessible for people who can't ship a gourd in from Brazil. It is definitely more durable.

Exterior view of *cabaça* on the printer (click to enlarge)

Interior view of *cabaça* - Note some stringing on the inside (click to enlarge)

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  • $\begingroup$ the breaking filament might be more a thing of how you feed the filament and your filament quality - drying it for a short time in the oven and proper storage can prevent this. $\endgroup$ – Trish Aug 2 '18 at 6:43
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Here is a great answer to the underlying physics of resonance:

enter image description here

"A object rings because it has acquired energy in a way that it resonates - it vibrates at a frequency and with enough energy to generate sound waves. As long as the object has sufficient energy, it will continue to ring. It cannot ring forever as the sound waves gradually reduce the amount of energy that the object has. But generating sound waves not the only way that an object can lose energy, One of the ways that plastics differ from metals is that plastics are better able than metals to dissipate energy internally. Any plastic object will show some amount of viscosity dampening since all polymeric materials have a non-zero loss modulus curve. Metals can also have internal dissipation mechanisms, but they are far less prevalent than in plastics."

So, we all know it, you can make plastic musical toys. They never resonate as well as metal, glass, or wood; but, they often can work. Here is an example of a Trumpet made out of ABS! enter image description here

This plastic's magazine also makes it's pitch for using plastics in musical instruments (most of which is also ABS).

That said, if Young's Modulus is the key (as the first comment said), PLA should be even better as its' Young's Modulus is a bit larger than ABS.

All that said, all the plastic bells I have seen are generally pretty quiet. Plastic has a habit of absorbing energy and thus not being a very good resonator. If you want to try, I think you are going to have to find a way to harden the plastic. Maybe Acetone treated PLA as that seems to harden it. Maybe try something simple like printing a plastic bell. If you can get that to ring, you are well on your well.

enter image description here

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I'll take a stab here, but my gut instinct is to say that a printed part will not sound the same as your original gourd resonator.

I believe the acoustics rely on the hardness, shape, and size of the material. In which case, a gourd is a hard and often thin material (after gutting it). Typical 3D printing materials will have a minimum thickness which may get in the way of achieving the same shape of the gourd resonator and plastics are typically going to be softer in hardness than your gourd.

So, in short, I think if your try to replicate the resonator with 3D printing it will not sound the same. That may not be a bad thing, depending on what you're looking for.

Also, who's to say the resonator has to be shaped like that? 3D printers allow us to manufacture parts that have historically been impossible to make and many instruments that we use today were designed hundreds of years ago with far less advanced tools available. I say its worth trying a replica of the gourd and then exploring other shapes to print that may affect the tonality of your instrument.

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Of course. I've printed a 7 octave organ rank, so it's certainly possible. Organ pipes are quite special in that the material properties of the pipe play a secondary role in the tone, the primary role being played by the length and cross-section of the pipe, the elements surrounding the mouth of the pipe, and its other openings (end opening, and extra openings that sometimes are used to modify the overtones). It's also well understood how the pipe material properties affect the sound, so it's possible to design the printed pipe to mimic properties of either a wooden or a metal pipe, and anything in between.

Depending on how big of a role the resonator's stiffness plays in the sound of the instrument, you might need to replicate some of mechanical properties of the Gourd shell - e.g. its stiffness - in the printed shape. Thus the inside of the print will have the shape of the inside of a Gourd, but the exterior may need to be much farther away, and then the infill percentage will play an important role, as well as the infill shape.

The shape of the interior of the gourd can be approximated by measuring some dimensions and sculpting an interior surface in a 3D modeler or CAD. Most likely a 3D scan is not necessary, unless you can get it done cheaply and with no fuss.

You will also need to print full-size instruments. So this will not be cheap in terms of material used and printer time. I would benchmark the printed instrument at 3 shell thicknesses: 0.5, 1.0 and 1.5 inches, and 3 infill percentages: 20%, 35%, 50%. All with 3 perimeter layers, and cubic infill. The thick 1.5in shell with high infill will be the stiffest of them all.

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If an instrument sounds "right" has as much to do with how it is shaped as with how the material it is made of is layered - or not - as this changes it resonance. Let's take a few examples:

Metal, cast resins & hard woods all are very dense and contain (little to) no hollow spaces to dampen their resonances. This makes them the classic choice for making instruments, as just the shape changes the tune now.

Balsa wood is very very light and brittle, and it is very grainy, creating some hollowness inside the wood that should resonate, changing its resonance a lot.

I don't exactly know how the gourd is in its composition, but for percussion perposes, 3D prints are more on hollow side than on the hard wood side for low infill. Very dense (35%+) prints can have similar properties to solid resin casts, but this depends on the type of filament and printer used.

Let's look for a compromise!

From my own experience with musical instruments, there is a vougue of carbon fiber reinforced resin instruments in classics. In broad strokes, these were manucaftuered by casting resin over carbonfiber mats pressed in to a mold and then sanded and polished. In other terms, 3D printing of carbon fiber filament is around in the FDM scene, even if it highly abrasive and demands a nozzle like the ruby to print.

With the right interior design (that is, an almost hollow print that just contains struts that don't seperate cavities inside the print), a carbonfiber print could possibly be filled with resin and become a 3D-printed composite, granting a much closer sound to the gourd while providing extreme durability.

However, such a design will take many hours of work before the first print can start. Atop that printing carbon fiber is a specialty thing not many print shops can do or do regularily. And finally, the postprocessing of filling the "gourd shell" with resin is a very delicate process, followed by coating inner and outer layer and then tuning... I estimate such a thing to be at least as pricy as several real gourds.

Addendum

After some months and this question popping back up, I realised some more things that might be used in the design of this:

  • Thin shell, armored up. Think about this the following: Print a 2 or 3 shell thichk wall, that will be the most inner. Then reinforce it with resin and carbonfiber mats until the sound is right. This would allow to tune the gourd to some degree.

  • Solid print, slidified. You might take your model and print it solidly, yet in the design you could include tree like hollow spaces that are accessible from the outside. with a syringe. Pushing in resin via these holes would create veins of harder resin within the gourd, which might alter the tune and strengthen it.

  • Postprocessing. Ignoring the resin for a bit, you can also make a PLA gourd more stable by allowing it to cure in the oven for some time. I had some success baking it at about 100°C for about an hour. For mor information on how this changes the modulus of different materials, I suggest CNC Kitchen: Thin Walls & PLA Annealing and Changes in annealed PLA

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