Porous Moderator Design Tests

[OldSpook]

I re-plotted the two signals on the same graph in both the frequency and time domain. For frequency domain I used a Hamming window of the full length of the signal. Since I am writing the code, I can have any length FFT I want. The FFT length is equal to the signal length. (It happens to be 18843 points). I cut off one of the time domain signals (to make it the same length as the other) so both frequency domain outputs would share a common x axis. I'm not saying that I took the data perfectly, but here it is. In this configuration, the solid one is quieter, it's obvious in both domains.
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I did this in python (numpy & scipy) to process and plot the waveforms. I exported the Audacity signals as matlab files, (rather than a wav file) and ran my program on these data files sequentially. The frequency domain signal is slightly filtered with a Savitsky-Golay filter to remove some of the fuzz without changing the shape.

I put substantial bracing on the baffles, as I was concerned about the structural integrity of the weak mesh. The six radial braces hold more than 1/2 the height of the cones. I could see that high power would cause some issues. Haven't had any good days to test accuracy.

I was not suggesting that your data was incorrect. I was suggesting that there must be a reason for our different results which we may not have figured out yet. But as for the difference between solid infill and gyroid infill the top post in this thread speaks to that. The same device 100% gyroid at something like 65% wrapped in tape and not wrapped in tape. That's what's in that video.

What we need to do is figure out why. Also there is a video here somewhere where I test that gyroid unit against a 100% solid unit sliced & printed from the same stl file.

 

[WobblyHand]

I was not suggesting that your data was incorrect. I was suggesting that there must be a reason for our different results which we may not have figured out yet. But as for the difference between solid infill and gyroid infill the top post in this thread speaks to that. The same device 100% gyroid at something like 65% wrapped in tape and not wrapped in tape. That's what's in that video.

What we need to do is figure out why. Also there is a video here somewhere where I test that gyroid unit against a 100% solid unit sliced & printed from the same stl file.

Wasn't taking it that way, so no worries. I do have a full 30% gyroid infill with 0 perimeters LDC I can look at. Finally got a 4 flute tap which is so much easier to start than a 3 flute. Just tapped the bendy rubbery LDC. I can wrap the LDC with some tape and see if it is quieter. I expect it would be. Unfortunately it isn't the same size as what I have data for (37x148). It is 40mm in diameter and 166mm long.

The graphs I showed were LDC's sliced from the same stl. The difference was simply the cylindrical modifier inside the LDC.

 

[OldSpook]

Wasn't taking it that way, so no worries. I do have a full 30% gyroid infill with 0 perimeters LDC I can look at. Finally got a 4 flute tap which is so much easier to start than a 3 flute. Just tapped the bendy rubbery LDC. I can wrap the LDC with some tape and see if it is quieter. I expect it would be. Unfortunately it isn't the same size as what I have data for (37x148). It is 40mm in diameter and 166mm long.

The graphs I showed were LDC's sliced from the same stl. The difference was simply the cylindrical modifier inside the LDC.

Okay that's good. So I looked for the post that I was referring to and that post was on my personal profile and management made me delete all of the technical posts that had to do with air guns on my personal profile. They did give me a chance to copy them to the appropriate forum but I had over a month of data in there and there's no way I was going to do that so that data is gone except for the raw data on my system.

I could duplicate it for you if you'd like I don't know that it would provide any information you don't already have.

TPU comes in various hardnesses I wonder if that has anything to do with it? I'm using matterhackers TPU right now.

Well either way there's plenty of room for more experimentation in this thread

 

[OldSpook]

@WobblyHand & @denovich Here is something we should be thinking about that I often forget. Every change we make to a design reshapes the "radiation" pattern of the moderator. I think of them like antennas because that is where my background comes from WRT wave propagation. That is going to influence our readings because none of us has a full blown sound lab. I test with one sensor located at 45 degrees counter clockwise from the line of the bore 5 yards from the muzzle. The mic is 40" off the ground. There is clutter in my data from local reflectors which begins to show up about 14 ms into a trace. My backstop is 75 feet from the muzzle and ~68 feet from the mic. I use it as a marker in a lot of my analysis. Anyway you get the idea. It would be much better to have three or four sensors but ...

 

[WobblyHand]

Okay that's good. So I looked for the post that I was referring to and that post was on my personal profile and management made me delete all of the technical posts that had to do with air guns on my personal profile. They did give me a chance to copy them to the appropriate forum but I had over a month of data in there and there's no way I was going to do that so that data is gone except for the raw data on my system.

I could duplicate it for you if you'd like I don't know that it would provide any information you don't already have.

TPU comes in various hardnesses I wonder if that has anything to do with it? I'm using matterhackers TPU right now.

Well either way there's plenty of room for more experimentation in this thread

If you can find the two wav files on your experiment and you can post it, I'd appreciate it. I'd like to plot it with my software might be interesting to see.

The cheap skate that I am, I have been using Overture TPU which is supposedly 95 shore. It seems soft to the touch. I was trying to measure the diameter and the micrometer ratchet was squishing it somewhat. Was not easy to get a consistent measurement. Just another way of saying it seemed soft. I don't have a profile specifically for the Overture TPU, I have been using the profile for Amazon Basics TPU instead. I do have some Sainsoft TPU which does have a profile in PrusaSlicer. It appears the Sainsoft will print faster, according to the slicer.

 

[OldSpook]

If you can find the two wav files on your experiment and you can post it, I'd appreciate it. I'd like to plot it with my software might be interesting to see.

The cheap skate that I am, I have been using Overture TPU which is supposedly 95 shore. It seems soft to the touch. I was trying to measure the diameter and the micrometer ratchet was squishing it somewhat. Was not easy to get a consistent measurement. Just another way of saying it seemed soft. I don't have a profile specifically for the Overture TPU, I have been using the profile for Amazon Basics TPU instead. I do have some Sainsoft TPU which does have a profile in PrusaSlicer. It appears the Sainsoft will print faster, according to the slicer.

This may be all I have left of that session. The .WAV contains the trace shown below. Each shot was trimmed to 100ms. The porous was the red one shown in the video at the top of this page (PETG) and the solid referenced here was sliced and printed from the same stl file. It had a CF sleeve and was PETG (and that may be the problem I am seeing the same with TPU). The porous one was wrapped in aluminum duct tape. May not be a reliable comparison because of that. Mic was 5 yards at 3 o'clock and 40" AGL. There is plenty of echo which could be cleaned up by trimming out everything but the muzzle blast (about the first 12 ms of each shot). That will impact the numbers I reported I expect.

 

[WobblyHand]

@WobblyHand & @denovich Here is something we should be thinking about that I often forget. Every change we make to a design reshapes the "radiation" pattern of the moderator. I think of them like antennas because that is where my background comes from WRT wave propagation. That is going to influence our readings because none of us has a full blown sound lab. I test with one sensor located at 45 degrees counter clockwise from the line of the bore 5 yards from the muzzle. The mic is 40" off the ground. There is clutter in my data from local reflectors which begins to show up about 14 ms into a trace. My backstop is 75 feet from the muzzle and ~68 feet from the mic. I use it as a marker in a lot of my analysis. Anyway you get the idea. It would be much better to have three or four sensors but ...

My test environment is horrible. That's because it is indoors at the moment. I can see the echo response of the room, based on the time delays I see and accounting for the speed of sound. This is easy to see if one plots the time domain response in dB's rather than linear. Pretty easy to pick out the echo stuff. Can't test higher powers, simply because I can't move the mic far enough away to prevent clipping. So serious testing will require going outdoors or doing some acoustic quieting of the main sources of echo. Need to come up with a way to do this reversibly so my wife will allow me to live

Yes, these LDC's are antennas or transducers of sorts. And we don't know the response over angle. I'd wager that measurement could be done similarly to that of an antenna, but it would get quite tiresome, or require some automation. (Just like measuring real antenna patterns is usually automated.). Having multiple sensors then begets the problem of calibrating the sensors to one another, a single sensor is easier in that respect. Like an antenna, we probably would find that there are some unexpected asymmetries in the patterns of LDC's. It happens. We would not know if the asymmetry is pointing up, down, or to the sides ahead of time - which could easily account for some of our differing results. Just saying, as if it isn't obvious by now, that these kinds of measurements are technical - and there's quite a few things to account for.

 

[OldSpook]

Yes I have built and tested many antennas for all parts of the spectrum up to 5 gHz and down to a few hundred kHz. The longest unlicensed shot I ever built was just over 25 km between White Stone, Va and Reedville, VA. That was back around 2006 maybe?

I found the raw video files. They are attached in the zip file.

I tested with two different rifles using the same pair of moderators. I tested the PETG "Ember" which was the solid design and the same stl sliced with Gyroid at something like 65 or 70 percent the only solid parts were the last couple mm of the end caps on that device. Both moderators are PETG as I said. Be advised there is some strong language in the second video when I discover that I can't remove the adapter without going back to the shop to get a tool.

It is a large file, even zipped. I will delete it when you tell me you have it. No point in forcing Mike to store that much data.

 

[WobblyHand]

Downloading now, but it is going slowly. 1.2MB/minute. Time for a snooze. Only a 1/2 way there...

 

[WobblyHand]

@OldSpook Downloading now, but it is going slowly. 1.2MB/minute. Time for a snooze.
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Ok, got it! Thanks! It unzipped ok, and both started to play, so I think you can remove the file.

 

[WobblyHand]

My experience is with L-band, S-band and way up at 24 GHz and 77 GHz (millimeter wave). Used FDTD (finite difference time domain) methods for electromagnetic simulation. Astounding to see the wave propagating along the the antenna and into space - quite insightful as well. We were eventually doing whole vehicle simulation at mm wave frequencies. They required clusters of computers with exabyte storage.

 

[OldSpook]

My experience is with L-band, S-band and way up at 24 GHz and 77 GHz (millimeter wave). Used FDTD (finite difference time domain) methods for electromagnetic simulation. Astounding to see the wave propagating along the the antenna and into space - quite insightful as well. We were eventually doing whole vehicle simulation at mm wave frequencies. They required clusters of computers with exabyte storage.

Fun stuff yeah!? Well then you will remember freeantennas.com ... It's pretty much dead now bluehost is a nightmare. Need to call them up and give them a little bit manure for messing with my Apache installation... They just can't seem to leave things alone. The site is still there but they've mucked up the permissions when they updated Apache and there wasn't enough traffic there anymore to make it worth my time try to figure it out. Maybe this summer.

 

[subscriber]

I THINK the baffle cones were not stiff enough to take the pressure at 25 fpe and were closing up around the pellet as it passed through the moderator. There was no observable clipping.

Is it possible if the baffles are too soft for the power dissipated, that they not just slump inwards or towards the exit, but vibrate? A sort of farting noise generation.

 

[OldSpook]

Is it possible if the baffles are too soft for the power dissipated, that they not just slump inwards or towards the exit, but vibrate? A sort of farting noise generation.

Imagine a flexible baffle that closes after the pellet passes forcing air to bypass around the sides and then reopens when the pressure equalizes.

 

[subscriber]

"Non-contact wipes" would be great, OldSpook. Yes, the concept is contradictory; like jumbo shrimp.

I actually have some valved moderator ideas modelled up in CAD and posted on GTA, but they don't look like what you are proposing. A closer match might be this:

I suggested baffle bores that look like tricuspid heart valves, when talking to someone else about such things via GTA forum PMs. Getting the valves to close fast, but not ahead of the pellet, and without the valve slamming shut producing more sound than it blocks, could be tricky. Perhaps some PCP "balanced valve" type principles could be used.

After starting some designs without making parts, I came back to "simple". If an active system can be simple, I am all for it. It would not just need to work. It will need to last. Unless it is just a proof of concept.

 

[subscriber]

I found the GTA thread that includes a discussion about active muffler valve systems: https://www.gatewaytoairguns.org/GTA/index.php?topic=209681.msg156473566#msg156473566

GTA does not allow non-members to see images, so I am pasting some below.

The first image is an animated gif, created by GTA Forum member jackssmirkingrevenge

Jack also posted this soft duck bill design, actuated by the shot. It is a copy in principle, of one made from metal:

Jack also posted these:

I posted this virtual wipe idea that I have been cogitating for some time. Flap valve retention parts are missing:

Variants:

That thread has more potentially interesting designs, but I think the above may be more than enough.

I don't like long narrow tubes with a small clearance for the pellet to travel down. While the idea is to capture the bulk of the air inside the valved chamber, air trying to overtake the pellet in a narrow tube is likely to upset or steer it. So, I consider much of the above as conceptual.

 

[denovich]

I keep my TPU in a bag with about 250 grams of desiccant (when not printing) and print from a dry box at 55C. The TPU has been dried maybe 24 hours at least. I think my printer is printing a little too hot, so I could reduce the temperature a bit. Which TPU are you using?

I found a similar thing with all porous walls, I could barely even start a tap in the porous threads, and the unit wanted to twist up like a pretzel when screwing it on the adapter. I was able to try it, but it was kind of loud without walls. I didn't bother with taping it up, simply because I didn't think it would be a long term viable option. I also didn't have a tube to slide it into to try.

This roll was Overture High Speed TPU. I also do the desiccant in the bag trick + dry box. Printing at 230C... using stock Bambu TPU settings, but capping the max print speeds at 170mm/s (vs 250 standard).

 

[WobblyHand]

This roll was Overture High Speed TPU. I also do the desiccant in the bag trick + dry box. Printing at 230C... using stock Bambu TPU settings, but capping the max print speeds at 170mm/s (vs 250 standard).

Ah, ok. I was using plain Overture TPU. Didn't even know there was such a thing. My dryer only goes up to 55C, maybe I need to get a better one to bake out the filament. I have been using the vacuum bags and gobs of desiccant, which has helped, at least for PETG. Thinking it isn't quite good enough for TPU though.

Still kind of a 3d printing neophyte. Been trying to avoid becoming an expert in printing, since I just want to make stuff, (and not eff with printing) but it seems more tweaking is necessary. There he goes, falling into yet another rabbit hole...

 

[WobblyHand]

I hesitate to mention this, simply because it may be a dumb idea, but I'm considering making an acoustic tunnel out of duct board for my experiments. Have located a local place to get the duct board. I'll fab a rectangular tunnel and fire through it. Should knock down the room echo some. If more cancellation is needed, structures can be placed to knock down the acoustic energy in key places. Kind of like building an special built anechoic chamber. Could be a total failure, but hey it's worth a try to get a somewhat more stable acoustic setup here.

But for the moment, I'll see if I can process@OldSpook's audio and see how it looks. Tomorrow is going to snow here, so it would be a good time to play with those files.

 

[subscriber]

This active valve moderator might also be of interest:

Hache moderator by Mechainnova

I am sharing with yours my last TequilaGUN accessory, it is a unique LDC named ”HACHE MODERATOR”, the particularity that do it unique new product is that the LDC has a ”H-SOFT” baffle that it is closed once the pellet pass through it. This condition decrease a lot of the sound intensity. To...

www.airgunnation.com