Porous fill moderators, any success yet?

[WobblyHand]

Been watching other places, but would rather talk about it here.  I was playing around with a basic design and wondered if I could have different fills in different areas.  Previously I had made a design (the crazy large chamber with skinny aerodynamic steering tubes) that I had printed.  I decided I could print the same design in TPU, but use a 50% gyroid infill with 0 perimeters.  It printed ok, but my basic design had pretty weak walls.  You can squeeze it pretty easily in the middle, which is not ideal.  Since I used 50% infill on TPU, the threads didn't come out all that great either.  You can see them, but it is difficult to start the threads easily. 

Anyways, thought it might be interesting to use different infill in different areas.  I don't have a multi material printer, so I'm kind of limited to either layer changes, or some kind of differing infill, which at least according to PrusaSlicer can be different in different geometric regions.  So I made a more traditional type of design and will use it for my tests.  I propose to make only the baffles and the supports inside the moderator to be porous, but the exterior walls and ends will be solid.  This can be done by creating a cylindrical region with different geometric infill, than the 100% infill exterior shell and body.  I was able to do this slicing after a little playing around.

[Mr.P]

Very interesting!

[WhatUPSbox?]

Bruce, have you picked your sound measuring approach? You are coming up on ideal measuring (anechoic) conditions. Fresh snow absorbs 50-90% of sound. 

[WobblyHand]

Bruce, have you picked your sound measuring approach? You are coming up on ideal measuring (anechoic) conditions. Fresh snow absorbs 50-90% of sound. 

New computer, new microphone (Dayton Audio iMM-6 calibrated Measurement Microphone).  As for snow, well we are getting some more rain, so the snow will be nearly gone.  Still need to figure out some of the audio stuff.  I'll probably use Audacity.  Just need to work out the input gain levels to maximize SNR.  (Signal to noise ratio)

[WobblyHand]

I upgraded my printer from MK3S+ to MK4.  I just resliced the stl using the fastest print speed.  Seems to have halved the time of a print, which is welcome.  We'll see if it prints, or I get spaghetti.  Using TPU with solid walls with porous baffles.  Think I will also get a bigger nozzle for this printer, just for grins.

My last PETG print had some lifting of the corners, I think due to it being long and thin.  Also had more stringing than previously.  So I added a brim to the moderator to help with adhesion.  Also will use some AquaNet on the satin sheet.  Haven't printed TPU on this upgraded printer, so hoping for the best.

Anyways, the guts look like the attached.  The internal cylindrical volume that is 30.8mm in diameter and 124mm long is meshed with gyroidal fill without perimeters.  The rest of the model is meshed at 100% fill.  Now kicking off the print...

[TorqueMaster]

I'm sort of interested in the concept of porous fill, but haven't experimented with it.

I did print similar style moderators to yours, identical to each other except one in TPU and one in PETG, and on average the TPU was 2.5db quieter.  Something to consider before assuming the porous fill is the /sole/ reason for any improvement. 

[WobblyHand]

I'm sort of interested in the concept of porous fill, but haven't experimented with it.

I did print similar style moderators to yours, identical to each other except one in TPU and one in PETG, and on average the TPU was 2.5db quieter.  Something to consider before assuming the porous fill is the /sole/ reason for any improvement. 

Since my printer is back online now, I can play around with stuff.  I figure I will make one solid in TPU, and maybe a porous one in PETG.  This printer is faster than my old MK3S+, which helps a little.

[WhatUPSbox?]

I think Audacity has a slider for microphone recording level. When I was doing some measurements, I would periodically use a P-17 as a check on the audio and microphone geometry.

[subscriber]

We have a member on GTA that has tried porous infill, with some success.  He has posted results on AGN, and sent some to me by email.  Rather than name him, I have invited him to participate in this thread.

[WobblyHand]

We have a member on GTA that has tried porous infill, with some success.  He has posted results on AGN, and sent some to me by email.  Rather than name him, I have invited him to participate in this thread.

Believe I know of whom you are speaking.  My print finished with no drama.  It's a little bit smaller than some of my others.  The combination of solid walls with porous interior results in a stiffer LDC, certainly stiffer than a pure porous LDC in TPU.  Need to test it this weekend.  I may also print it using 100% infill everywhere, just to see if it makes a difference.

[WobblyHand]

I think Audacity has a slider for microphone recording level. When I was doing some measurements, I would periodically use a P-17 as a check on the audio and microphone geometry.

Kind of a reference "pop"!  I'll have to try that.  Pretty sure I have an unmodified P-17 around.

My testing will be rather primitive for a while.  I'm indoors, so it will have to pass the indoor test first.  If it's reasonable indoors, I could test it outdoors, which will have better acoustics.  I'm not going to build an anechoic chamber, that's for sure.

[WobblyHand]

Well, tested on a 2240 with a SSG first, (which is not much of a test,) and it's fine.  Sure it could be quieter.  Then on my PP700SA 0.22.  It's definitely muffled, and not bad inside this small bedroom.  I'd shoot it outdoors without compunction in this city neighborhood.  The 3mm solid walls in TPU are nice, no sharp edge to the sound.

Think I need to make some other test pieces with different fill, ie solid TPU, and perhaps in PETG, with and without gyroidal infill.  For interior testing, I need to get some acoustic material, I'm getting lots of bounce off the walls.  Think I need to mount a couple of foam panels to make the room deader.

If I were a better designer, this would go faster. . Not that great in putting in the finesse touches that would make for a more pleasant looking design.  Having trouble putting in the chamfers and whatnot in the design.  Seems FreeCAD has some issues there.  But for the moment, I'm still going for function.  I'd like to somehow add some Tesla valves to circulate the air using these porous walls.  Seems like that would be quite lossy with rough porous walls.  Got to study the TV some.

[Back_Roads]

 Hang some heavy blankets in the test area

[WobblyHand]

Hang some heavy blankets in the test area

Not a bad idea.  It would knock down the echo a bit.  Was almost thinking of shooting down a tunnel of a blanket shaped like an upside down U.  Or even an old rolled up carpet.  Need to come up with something sort of like that which would allow adding a microphone.  I want to measure things, so I can see if one unit is better than the other - and by how much.

[WobblyHand]

Starting to get the hang of Audacity.  Not the easiest thing to work with, in my opinion.  Got a non clipped sample of a firing.  Totally jury rigged, nothing is fixtured, so it's not reproducible yet.  I will need to set up a mic stand, and a bunch of things, before it's stable.  But here's a picture of the Audacity screen.  Need to figure out how to get the numbers out of the system - so I can do my own processing.  And yes, how to save the files. 

Microphone is 18" away from muzzle of LDC to the side.  Microphone gain set to 12.5% so the signal didn't clip.  High resolution sampling (16 bits) 44.1 KHz.

Was able to save the spectrum frequencies and values, in a text file as well as the .wav file.  But so far I can't save the audio as a series of 32 bit floats, or 64bit floats.  Actually it saves the data in binary as a raw file, which a Mac interprets as an image file.  That's useless.  Guess I need some file converter.  Or I'll write a program.

[subscriber]

Bruce,

I realize that you are currently focused on sound capture and measurement, but have a question about gyroid foam: 

Would it be useful to stand off a layer of gyroid foam from the ID of the outside wall, so that open space could act as an air capacitor?  Yes, we think of the foam as both flow restrictor and capacitor, but I wonder if the full layer depth of the foam acts that way, for a sudden impulse of air.  The thin layer of foam should be supported at intervals around the circumference for structural integrity.

If the foam is thin, then its capacitor function has to be limited.  If the foam is thick, then is air making it all the way through, before air on the high pressure side stagnates and tries to flow to a low pressure area nearby?

So, I am suggesting the primary functions of gyroid foam, to act as flow restrictor and pressure wave de-bouncer, rather than air capacitor or expansion space.  Perhaps a thin layer of dense gyroid over a thicker layer of more open gyroid would fulfill these functions, rather than the same density all the way through. 

[WobblyHand]

Bruce,

I realize that you are currently focused on sound capture and measurement, but have a question about gyroid foam: 

Would it be useful to stand off a layer of gyroid foam from the ID of the outside wall, so that open space could act as an air capacitor?  Yes, we think of the foam as both flow restrictor and capacitor, but I wonder if the full layer depth of the foam acts that way, for a sudden impulse of air.  The thin layer of foam should be supported at intervals around the circumference for structural integrity.

If the foam is thin, then its capacitor function has to be limited.  If the foam is thick, then is air making it all the way through, before air on the high pressure side stagnates and tries to flow to a low pressure area nearby?

So, I am suggesting the primary functions of gyroid foam, to act as flow restrictor and pressure wave de-bouncer, rather than air capacitor or expansion space.  Perhaps a thin layer of dense gyroid over a thicker layer of more open gyroid would fulfill these functions, rather than the same density all the way through.

There's a lot packed in your questions. 

I am thinking about how to measure things.  My first shot showed me I have acoustic bounce in my small room - which is not surprising at all.  Actually I see evidence of that in a secondary peak which corresponds to round trip distance of a bounce back to the microphone.  Kind of happy to see that, simply because I was expecting it.

As an actual EE, I'm trying to understand what you mean by a capacitor in this context.  A capacitor stores electrical energy, specifically charge.  What are you meaning by the term?  Are you intending to somehow trap the air for a while?

To really understand the complex dynamics of a blast of air into a complex material like gyroid foam, well at the moment it is not within my grasp.  I'd like to think that CFD might help there, but I have yet to teach myself enough to run a simulation.

I sort of grasp what your questions are, but I don't know enough about this subject area to answer them.  Really what we are trying to do is to dissipate the higher frequency energy first, and also the lower frequencies, by delaying and extracting the kinetic energy of the air as much as we can before letting it out the exit port(s).  The question really is what are the more effective and practical ways of doing this in a compact size.

[WobblyHand]

Practically speaking - I have only taken baby steps in using this foam.  I suppose it might be possible to generate shells with differing meshes, but I haven't looked into it at all.  The existing slicer may not allow it.  That doesn't make it impossible, but I don't know anything about how the slicer works.  For instance, it might be quite useful to vary the density of the foam exponentially. 

[subscriber]

Bruce,

There are a full range of analogies between electrical and mechanical engineering; including hydraulics and pneumatics: https://en.wikipedia.org/wiki/Mechanical%E2%80%93electrical_analogies

[WobblyHand]

Bruce,

There are a full range of analogies between electrical and mechanical engineering; including hydraulics and pneumatics: https://en.wikipedia.org/wiki/Mechanical%E2%80%93electrical_analogies

That article was helpful in that I got the basic idea.  Unfortunately, there were not many links to pursue to get more details.