possible to use Fusion for guitar top modal frequency simulations ?

[rumpelstielz]

hey everyone, just been playing around in my spare time with one of the parts of Fusion i really don't know anything about: simulation ... and physics in general for that matter (my highschool physics teacher would keel over laughing if she knew i was even considering sth like this) ... the simulation feature it's one of those things that 'looks like it's related' but might be something else entirely.

as a musician and luthier i'm familiar with a topic of much mystery, lore and speculation: how exactly wood transmits sound in such a complex structural object as a stringed instrument and how exactly different wood species, design features, components and construction strategies relate to 'sound'. Whereas the basic theory of propagation of forces and sound waves in an acoustic guitar for example are generally accepted:

String vibrations between two anchor points [nut and bridge] are transmitted via the bridge to the sound board which moves air amplified by the corpus and thus genrates the 'sound' .. the guitars top acts like the speaker membrane and contrary to common belief the hole is not where the sound comes out it's more like the bass port of a subwoofer as a vent for air pressure to modulate frequency and volume response

there are so many variables that come into play and so many frequencies interact that no one really knows for sure what's going on and how different instrument designs end up propagating the sound waves.

one technique luthiers have used is 'modal tuning' using chladni patterns to visualise how the top (the membrane generating the sound) vibrates at certain frequencies ( http://www.goreguitars.com.au/main/page_about_design_modal_tuning.html 

some do it before the top is glued to the body, some do it on the finished guitar clamped down, and others bury the body in the sand for it (to remove the back and sides from the equation) .. and it's still unclear how exactly that relates to 'sound quality', sustain, projection, volume, attack/dynamics and a myriad of yet to be discovered  psychoacoustic phenomena and how the base parameters change in a typical playing situation (which can be very different than the 'lab' situation).

while i personally don't believ that there is a scientific recepie for building great guitars, the nerd in me is wondering if the Fusion modal frequency simulation thingie can be used to analyse different designs in a similar way and see how different, body shapes, bridge positions/size, string tension, and sound port (the hole) size/shape/position affects the vibration of the top. I haven't had time yet to dive deeply into simulation but managed to quite quickly create an object, pin it down at some points and then see how it deforms at different frequencies .. but the problem is 'it looks like' it's working but i have actually no idea what exactly i'm looking at 😛

two things i have noticed so far in the simulation engine:

• the frequency modes only go from 226.1 Hz to 1626 Hz, i'm wondering where this range comes from and why lower and higher frequencies are not available. Typically the interesting frequencies in a guitar are between 70 an 2500Hz with probably some interesting partials up to 5kHz.
• wood can't be used as a simulation material .. is this a general limitation or did i just not set it up properly ?

i've attached a simple template file of what i would like to simulate but haven't hooked it up yet since i'm not sure yet to what precision it makes sense to model this, what parts it makes sense to include in this simulation (could Fusion handle a more complex model including acurately modeled strings and bracing and how exactly i would pin the anchor points and set up materials.

Maybe someone here is fit in the simulation features of Fusion and/or has an interest in guitars or physics and can tell me if this is worth investigating or if i got this all horribly wrong.

If there is a point getting into this then probably combining it with the generative engine would be the logical next step.

i know that in the luthier world this is a highly controversial topic and even as an amateur i can see how the methodology is often flawed, unrelated or downright wishfull thinking or marketing, i also simplified the whole issue a lot here and am fully aware that there is much more to it and that this is only one tiny part of the equation.

personally i just find it interesting to dive a bit deeper into the inner workings of these amazing instruments and if i have a tool for this sitting on my puter i might as well use it to play around 😛

[TrippyLighting]

I don't know too much about this either, but I am wondering if @John_Holtz , @Andrew.Sartorelli or @Anonymous can chime in ?

Very interesting topic!

[Anonymous]

This is a challenging sim for most FEA programs for a few reasons...but you could get a simulation result here.

1 - wood is orthotropic and fusion 360 can only handle isotropic materials. This means your results will be off by some degree.

2 - fusion 360 can handle all frequencies. A modal response only shows the resonant frequencies of a body (this is why your results looked limited)

3 - guitar bodies and strings are very high aspect ratio so it's tough to get a good model to mesh. This is partly because f360 only uses tet elements.

I have actually tried to simulate a guitar string with preload with no success.

Here's a free class on modal analyses and resonance in general if you want.

https://www.udemy.com/fusion-360-simulations-modal-frequencies-analysis/?couponCode=RELEASEPARTY1Q18

There are 13 remaining free spots so if anyone else wants to learn more...feel free to join.

[melvinbrian3d]

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@Anonymous wrote:

Here's a free class on modal analyses and resonance in general if you want.

https://www.udemy.com/fusion-360-simulations-modal-frequencies-analysis/?couponCode=RELEASEPARTY1Q18

There are 13 remaining free spots so if anyone else wants to learn more...feel free to join.

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Thanks for sharing!!!

[Anonymous]

Sure! If you have any questions please feel free to ask me through the course or on these forums. Either work for me.

[rumpelstielz]

@Anonymousmany thx for your insights, i kinda suspected that the complexity of the issue would be a problem with this, but the good news for me is that i'm not totally thick and am looking in the right place, just signed up for your course, seems like a good idea to at least learn the basics of FEA properly

1/ i was suspecting sth like this but now i know what it's called, thanks 🙂 Is it possible to work around this and 'model a material' .. the main material of interest would be spruce, which has a pretty simple and predictable grain pattern and density (darker lines are harder), would it be possible to model a blank with materials of differing strength and then bool out the top shape for use in analysis ?

so given that the meshing will be primitive, the material properties of the model innacurate, and the complexity of the model much reduced compared to the real situation do you think there are still some meaningful insights to be extrapolated from a Fusion simulation result ? When you tried to simulate just one guitar string did it fail because it was too complex or because the Fusion sim engine lacks some tools you needed for it ?

Since i still know nothing about sims yet i'm wondering to what extent the sim model can be simplified before it becomes meaningless,  for the strings alone these are the parameters we have and the materials although manmade are probably also complicated since for half of the strings it's one material wound around another materials core and then coated:

if i put aside the whole acoustic tuning part of the analysis for a moment it would also be interesting for me as a builder to see how different bridge models/positions/thicknesses affect the deformation of the top under tension but then the model would have to take into account all the string parameters and the/a mix of frequencies and the transfer of the vibrations through the bridge and not just a single driver on the top. What would then be really interesting here is to map the string lengths and frequencies as user parameters and then see how the bridge and top behave under more 'real world' conditions .. meaning: chords and harmonies with  .. because this is where IMHO many chladni efforts falls apart: analyzing how a single sine wave generator makes a free standing top resonate in a specific frequency doesn't seem to have much relation to how 6 continually changing frequencies generated by drivers of continually changing scale length interact in a highly complex structural system, cause when you fret a string the length of the string changes and the vibration characteristics change dramatically and none of the modal tuning techniques take that into account. So my hope was that with Fusions modeling tools it would be easy to build a pretty accurate model of all the parts and thus get a bit closer to sth that relates to when the instrument is actually played in a harmonic context.

re: 3/ i'm not sure i understand what you mean by 'high aspect ratio' and 'a good model to mesh' (does good mean complex in this instance ?).

[Anonymous]

1 - "meaningful" is subjective. If you want to learn the tool...sure. But if you want to be able to use these results to predict the acoustic behavior of a complex design...then no it's not worth it. That's a very expensive simulation package in a completely different market segment. But you may start to understand the correlation between component design properties and their resulting resonant frequency. If you don't need all out resonant response accuracy...and just want to see how the different parameters interact....then it may be meaningful.

I should also say that the mesher in F360 isn't primitive as much as it is limited. Tet elements are the standard used by most if not all solvers in this market segment...it's just that packages such as solidworks include shell elements as well which help with long thin sheets. 

I didn't play around enough to see why the guitar string model was failing. You have to include a fairly high preload in those simulations to get the true resonance of a string...so it may have been having trouble with the combination of high aspect ratio elements (guitar strings are long and very skinny), preloads and overall size. 

Fusion 360 is not the right tool for that type of complex interaction, and I'm not sure what would be. That's an incredibly difficult simulation with so many variables that can vary from batch to batch and person to person. Typically, modal analyses are only linear and there will be some non-linearities in what you describe from a contact perspective. 

3 - High aspect ration means the length is much greater than the diameter or height dimension. Guitar strings, sheet metal, wood panel...all of those are high aspect ratio. That is bad for a mesh, and you need a good mesh to get good results. 

Also, displacement units in a modal analysis are not real or correct. Only the mode shape and frequency matters.