Porous Moderator Design Tests

[denovich]

OldSpook,

I'm using BambuStudio (which is related to a few other slicers). I experimented with 15%-30%. How the infill is generated is not really consistent from one percentage to the next. At lower infill percentages it combines strands together to make fewer but thicker inner walls. You keep getting more walls until about 20% when the infill strands start getting smaller but more numerous. It gets more mesh like. I didn't notice a significant difference in sound between the percentages, so I went with what I needed structurally. I tried it "side vented" into lambswool about 2mm thick wrapped around the insert and stuffed in a metal shroud. I also tried it taped and without the wool. I think it was better taped and it was certainly less of a pain in the ass to repack.

I was printing on a Bambu X1C and I was printing fast.
I needed a brim to keep it from detaching from the build plate. (Especially if you turn off top/bottom shells)

I printed with PLA and PETG... I think PETG is structurally the better choice.
But better than PETG in the actual business of suppressing sound was TPU.
TPU had by far the largest impact on sound of any "refinement" on basic design I've tried.

I thought it would be a pain to print, but I slowed it down just a bit and it printed like a dream. Prints nicer than PLA, I just can't use the automatic filament changer.

 

[denovich]

And I think you can make a case that a rubber silencer is more obviously airgun only.

Printing in white revealed that these actually do collect a fine coating of lead dust.

This is the insert I'm currently using. TPU with fancy baffle (@subscriber jazzed up my basic design) had a little better performance than my best mesh design. It was way better structurally.

 

[subscriber]

denovich,

It can be tricky to quote someone in absentia, with or without naming them, so thanks for popping up.

In case anyone has interest in the part you referred to above, where I added the inner aerofoils to your design, to make the Tesla Valve fully functional; here it is; including the zipped STL.

View attachment Tesla Valve added to MD insert V2 STL.zip

 

[denovich]

As deployed here in a 40mm OD CF shroud.

https://www.airgunnation.com/threads/clocking-problems-with-a-moderator.1302363/post-1624669

 

[OldSpook]

@subscriber you took that poke well .

I agree with the sentiments that you have expressed in the last couple of posts entirely.

It looks to me like there are not many vendors or hobbyists building moderators who are not selling "snake oil". I am not saying the moderators on the market don't work. I am saying we mostly don't understand how they work.

When you add to that the subjectivity of "opinion" in testing, which is absolutely rampant in the industry, you get a marketplace dominated by the "cool" factor. "It looks cool! It's about a foot long and as big around as your wrist. Obviously it works!" Well that's what she said anyway.

Then you test it. On a rifle that normally shoots MOA it shifts the POI a couple of MOA in one direction or another and can't hold groups smaller than twice what the rifle does without a moderator. While it has a lower frequency it is literally louder than the bare muzzle because it compresses a bunch of the high frequency noise into the lower part of the spectrum. Nevertheless shooters say, "It sounds better!" And that sells moderators. That's what I mean by "snake oil".

The market is flooded by vendors offering mediocre products that look good and have a pleasing pitch. Sprinkled in there are a few designs that really stand out. Everyone knows which ones they are.

What we really need is a standard test methodology. Everyone thinks they can trust their ears until they actually start recording and analyzing the audio.

Last year there were a couple of fellows that I argued with about testing these devices with cell phones. Their argument was that it's too difficult and cell phones are not accurate enough. I think they were half right and I mean that with respect because I was half wrong. Testing is a very difficult task but a cell phone samples at 44 kilohertz and that's fast enough to learn a lot if you actually record the sound and then analyze it with mixer software.

In the beginning I advocated for the use of software that measured the peak impulse. I have since learned how wrong that idea is and I am beginning to understand why it is wrong as well. The devil is in the details and you have to really look at the details to "see" what a moderator is doing. I've heard half a dozen "experts" (like myself ) say things like "The sound of the shot is very short only a few tenths of a second." That proves how poor our understanding of what is going on is actually. The sound of the muzzle blast has come and gone and echos are returning from nearby objects in a few HUNDREDTHS of a second. In 25 thousandths of a second echos are returning to the microphone from objects that are 5 yards away. Our ears just are not designed to "hear" the differences between sounds of that short duration.

Of all of the vendors STO seems to be the only one who attempted to make a scientific study. And even in that case the study was used to optimize individual designs. It is commendable that they published their work. Presumably other vendors do similar studies but the industry as a whole seems to jealously guard it's "secret sauce".

That serves two purposes. It protects proprietary information and it keeps secret the fact that most of these vendors don't have a clue what their moderators are really doing. If it's cool it sells, so who cares? Everything is thereby reduced to the "designed by feel" approach.

There has to be a better way.

 

[OldSpook]

View attachment 421218

And I think you can make a case that a rubber silencer is more obviously airgun only.

Printing in white revealed that these actually do collect a fine coating of lead dust.

This is the insert I'm currently using. TPU with fancy baffle (@subscriber jazzed up my basic design) had a little better performance than my best mesh design. It was way better structurally.

Very nice work. Good job on the printing!

My only concern with TPU is wear and tear and cleaning. I don't know enough about it yet. Any material which can flex under load is probably an improvement over a material which does not, especially for absorbing sound energy and converting it to heat. That's the holy grail here, isn't it? Catch the sound energy and convert it to a different form of energy. Absorb it don't reflect it.

I have not tried to print TPU yet. I want my designs to be self-contained. I prefer to print them as one piece but I am willing to epoxy them together with "West System". I want my users to be able to clean their moderator by soaking in a glass of warm soapy water or by rinsing with alcohol or acetone. That's one of the reasons I tend to use PETG. PETG is also more UV resistant than other plastics. I am not saying it is a better choice for this application. If I find TPU can survive ten or twenty thousand shots and could be cleaned in that manner I'd be more inclined to use it. Don't mistake this for criticism. It isn't.

 

[OldSpook]

I've studied the Gyroid Infill pattern as drawn by Prusa slicer.

I sliced a cube 1" x 1" x 1" and prepared it for export. The percentages shown correlate with the weights and volumes such that a 90% infill indicates 90% of the volume is composed of extruded plastic.

That's what we needed to know to calculate the other information @subscriber wanted.

That is to say, a 50% infill setting will have equally spaced pores which are approximately "layer height" tall and "nozzle diameter minus overlap" wide. For example an extruder running a .6mm nozzle and a layer height of 0.3mm will have equally spaced holes and plastic which are 0.3mm tall and 0.6mm wide by 0.6 mm deep. This is for Prusa slicer because it sets the extrusion width to be 10% wider than the nozzle diameter. The calculation might be different in other slicers.

In the test at the start of this thread the gyroid infill was set higher than 80%. I do not recall the exact percentage. So I have calculated from the weight a better approximation of the infill percentage that I used in that test. Best guess using available data is that the infill setting was 65%. The moderator was still very porous as evidenced by the ease with which air could be blown through it when one end was plugged. Even at that density I am concerned that too much volume is being occupied by the plastic which would reduce expansion area.

This warrants further testing. Perhaps we make a few moderators like that first one printed at different densities and then tested before wrapping to see which provide the most moderation?

 

[subscriber]

Gyroid infill reminds me of the porous inside of bone; or something organic.

How the infill percentage translates to pore size seems a little indirect, but here is a little info:

 

[OldSpook]

Gyroid infill reminds me of the porous inside of bone; or something organic.

How the infill percentage translates to pore size seems a little indirect, but here is a little info:

Nice image here.

Gyroid - Wikipedia

en.wikipedia.org

Good discussion here but a bit tedious...

Gyroid - Wikipedia

en.wikipedia.org

 

[beerthief]

I like this ! interesting to read about , thankyou

 

[denovich]

If I find TPU can survive ten or twenty thousand shots and could be cleaned in that manner I'd be more inclined to use it. Don't mistake this for criticism. It isn't.

TPU is incredibly tough. Layer adhesion is basically perfect. The GoPro mounts I've printed in TPU for my racing drones have proven to be almost indestructible. I have some TPU prints that are 6 years old now but are otherwise as strong as new.

Gluing can be a little tricky. I used CA glue to bond TPU to PETG. I used accelerator and gave it plenty of time to setup. Also gave the joints plenty of surface area. I'd ruin the parts trying to separate them. You can print TPU on PETG and vice versa. A bit of a pain having to do a manual filament change, but you could do that to get a monolithic part.

Another option: DCM and MEK are known to work on PETG, so a solvent based glue that had either/both of those would be likely work too. Weld-on makes some good options.

 

[OldSpook]

TPU is incredibly tough. Layer adhesion is basically perfect. The GoPro mounts I've printed in TPU for my racing drones have proven to be almost indestructible. I have some TPU prints that are 6 years old now but are otherwise as strong as new.

Gluing can be a little tricky. I used CA glue to bond TPU to PETG. I used accelerator and gave it plenty of time to setup. Also gave the joints plenty of surface area. I'd ruin the parts trying to separate them. You can print TPU on PETG and vice versa. A bit of a pain having to do a manual filament change, but you could do that to get a monolithic part.

Another option: DCM and MEK are known to work on PETG, so a solvent based glue that had either/both of those would be likely work too. Weld-on makes some good options.

Just ordered 2 Kg of the stuff. Thanks for the heads up and information. I'll give it a shot next week. I have a tiny moderator design I'll use to test the principle we are discussing in this thread. Those should be off the printer tomorrow. Next week I will apply what we find out tomorrow to incorporate the TPU into the design.

 

[subscriber]

The gyroid shape seems like a colony of mobius strips during mating season

 

[OldSpook]

Gyroid Infill Test

These images show the construction and dimensions of the moderators under test. I nicknamed this moderator the "Micro25" as it is .25 caliber and far to small for any practical use other than isolating one design feature for test purposes.

These moderators are 25 caliber, 66 mm long and 30mm in diameter. Their total volume is 27.66 ml. They weigh 28, 33, and 39 grams respectively. From that you can compute the weights of the infill for each moderator. If we solve for the weight of all other components we get about 23 grams. The weight of the infill in the 25% moderator is ~5 grams, in the 50% moderator it is ~10 grams and in the 75% moderator it is ~15 grams.

They have the gyroid infill as we have been discussing in this thread. The percentage of infill is shown on the moderator.

I shot two rifles in this test. The first rifle was a 17.25 foot pound .17 cal Stormrider (Crossman 10.5 @ 860). The second rifle was a 61 foot pound .25 cal Condor (H&N Barracuda 31.09 at 940).

What we want to discover is the "optimal density" of the gyroid infill in our application. We only have three data points so some graphing will be required later.

Stormrider Test Results

First lets get to the audio comparisons. I shot the video, stripped out the audio and removed clipping by applying attenuation to the entire sample. Lets have a look at the samples for the Stormrider.

NOTE: I should have made a moderator with no infill so that we could subtract out the effects of the moderator body itself from the ones with gyroid infill. As an exercise later we will try to estimate what such a moderator would do.

First up 250 ms of the bare rifle looks like this.


I have selected 138 ms of the first shot of each sample set in this spectrum trace so that you can compare them to each other. I have not yet analyzed for RMS over the sample set so any conclusions will have to wait.



I have analyzed the RMS levels for these 4 samples. I selected the first 80 ms of each shot and averaged the three shots for each sample. Then calculated the RMS level for each sample. This is the result.

Gyroid Percentage	RMS Power dB	Delta dB	Delta Percent Power
0	-14.46	0	0
25	-15.21	-0.75	-15.86
50	-15.41	-0.95	-19.65
75	-15.20	-0.74	-15.67

This data indicates the 50% infill is 24% better at damping the muzzle blast than the 25% infill and 25% better than the 75% infill at damping the muzzle blast. Given only three data points, actually projecting an optimal fill percentage is an exercise in futility BUT we can reasonably assume it will be found very near to or below the 50% ratio. Another test might be useful if we build two moderators one with 37% percent infill and one with 67% infill. There is something satisfying about the idea that the optimal APPEARS to be very near 50%.

Keep in mind this 25 caliber moderator has an 8.35mm bore and is only 66mm long and is being tested on a 17 caliber rifle yet it is eating 20% of the muzzle blast. That's not bad.

Next we will have a look at the Condor data.

Condor Test Results

I have again selected the first 138 ms of the spectrum for each sample set to display one above the other.



The RMS levels for the Condor were handled in exactly the same way they were handled for the Stormrider.

Gyroid Percentage	RMS Power dB	Delta Power dB	Delta Power Percent
0	-13.22	0	0
25	-14.80	-1.58	-30.5
50	-14.97	-1.75	-33.17
75	-14.47	-1.25	-25.01

That's interesting. It looks like efficiency increases with power applied. We also are now shooting a .25 caliber pellet through a hole which is a considerably tighter fit. A law of diminishing returns seems to apply to that hole in the muzzle. You can make it bigger but it will cost you, especially in smaller tubes. That's the good news though. Smaller (shorter) tubes are where you can get away with that tight bore.

I have double checked the math by rerunning the entire analysis process.

In both tests the 50% infill came out on top. It might be a good idea to make a couple more moderators and refine this number further but at this point if I were to have to pick a value I'd go with 45% infill.

There may actually be no point in giving you the audio but before you listen to this audio sample, I assure you the second three shot samples in the clip are 1.75 dB LESS than the first three. It really is 30% less power than the first three shots sample. Clearly human hearing is non-linear in it's response to sound vibrations. The audio is slowed down to 1/10 speed. This means 200 Hz goes sub-audible and 10 kHz sounds like a 1 kHz tone. Anyway here is the audio for the bare rifle and the 50 percent infill Micro 25.

EDIT: 11-01-2024 Testing has now essentially confirmed that gyroid at 36% infill is just about an optimal starting point for testing new designs. I've run these comparisons on very different moderator designs and in both cases an infill around 36% looked best.

 

[OldSpook]

@denovich THANK YOU for pointing me towards this material!!!

EDIT: Prediction of future events removed. This material is VERY good, but there are other similar materials which are as good.

This is a study of the difference between PETG and TPU when used as a sound dampener in a baffle structure.
The EMBER design is the design I've settled upon for comparing apples to apples in my tests. More about it here.


This design (the Ember) is already an excellent moderator. It performs exceptionally well for it's size. it is inherently accurate, having a stripper design as the first baffle ahead of the muzzle. I was asked to make this design specifically by one of the members of this forum and a gentleman who has been testing these things tirelessly for more than a year now. We named it after his grand-daughter.

What you see here are my efforts to improve the design through materials science. The moderator when printed as a solid design in PETG plastic performs at about the same level as a DonnyFL Tanto perhaps a bit better. The same design when extended to 150mm in length by the addition of a large hollow "expansion chamber" in the center of the unit performs to about the same level, perhaps a bit less, as the Weirauch moderator on my HW110.

Two changes are documented here. First I printed the unit in a manner which makes the internals porous. I used 50% infil and printed the baffles so that they serve a dual function. First they continue to work as a baffle by redirecting muzzle blast (although less effectively). They also create turbulence across the entire cross section of the moderator like a wool mesh would do (kind of like a lawn mower with a spark arrester muffler acts). Second I substituted TPU for PETG to introduces a "damping" or "absortion" that does not allow sound waves to create resonances in other parts of the moderator (and the gun for that matter). This same plastic is used as rubber feet when silencing appliances etc because it does not transmit vibrations well.

The end result is documented here.

Here is the audio from that clip trimmed but not massaged beyond recognition. There are three shots each from the bare rifle, the PETG Ember with gyroid construction and the TPU Ember with gyroid construction.

Here is an image of the first shot of each of those three samples, juxtaposed one above the other so that you can see the differences of the spectrum traces. There is 250ms of data showing here.



I reduced each shot sample to the first 80ms of each shot, aggregated these and averaged them to obtain the RMS values.
RMS for the bare rifle was -14.7 dB, for the PETG Ember it was -18.0 and for the TPU Ember it was -21.3. That tells us the improvement for Gyroid mesh is 3.3 dB (53%) and the improvement for TPU with Gyroid mesh is 6.6 dB (78%).

These are the best numbers I've ever seen using this test methodology.

Today while I was testing I printed off replacement "innards" for my Talon-P suppressor. I did one in PETG and one in TPU.
Perhaps tomorrow... if the squirrels are not laughing at an old man walking his dog in the morning .

 

[Super-p]

@OldSpook I'm really interested in purchasing one of your carbon fiber moderators for my FX Dynamic. How do I get one. Thanks

 

[cavedweller]

@old

@OldSpook I'm really interested in purchasing one of your carbon fiber moderators for my FX Dynamic. How do I get one. Thanks

@OldSpook - i would love to try one of your new angled moderators as well for a .25 cal - should i PM you or??

 

[rc4fun]

He dosen't have the post count.

 

[JungleShooter]

I enjoy reading about cooperation, serving the community, discovery and research, and improving our gear.

Our silencer guys are doing it again!
Thank you!

Matthias

 

[Mr.H]

TPU is incredibly tough. Layer adhesion is basically perfect. The GoPro mounts I've printed in TPU for my racing drones have proven to be almost indestructible. I have some TPU prints that are 6 years old now but are otherwise as strong as new.

Gluing can be a little tricky. I used CA glue to bond TPU to PETG. I used accelerator and gave it plenty of time to setup. Also gave the joints plenty of surface area. I'd ruin the parts trying to separate them. You can print TPU on PETG and vice versa. A bit of a pain having to do a manual filament change, but you could do that to get a monolithic part.

Another option: DCM and MEK are known to work on PETG, so a solvent based glue that had either/both of those would be likely work too. Weld-on makes some good options.

I'm on a GluBoost kick lately. Long working time before it sets up. Wondering if you've ever heard of it?