And now for the latest installation in the thrilling adventures of airgun moderator design. You can find part three here. In this episode I test the culmination of quite a lot of designing and testing and tweaking to create an effective 1/2-20 UNF “universal” moderator which I’ve named Levitas, latin for lightness or fleetness of foot. A fitting name for a moderator which is a mere 120mm in length, 35mm in diameter, and 65 grams in weight, but has the sound attenuation of a much heavier and greater volume design. This series started off as a pet project, but I’ve gotten so much interest in moderator design and so many requests for a 1/2 UNF version I decided to build one. So after much time, effort, and refinement, here it is.
Adapting the original concepts to a 1/2UNF universal application is more challenging than it sounds. The FX Crown's unique design, and unique associated moderator, was more than just a gas diode. The large thread allowed for significant flow to be tapped off from around the bore. Were size no constraint a blast chamber integral to the moderator could be used to fulfill the same function, however this is volumetrically inefficient when it comes to this level of sound attenuation. Further, the lack of a flow-through system along the perimeter will put additional load on the gas diodes as there is no alternate pressure relief, they simply have to eat it. This leads to one final concern, which is air stripping. Without the expectation of some sort of muzzle brake on the rifle to pull turbulent gas off the pellet near the muzzle, the moderator must now possess this as well.
So, how to go about it? Well in this case I went with a three stage system. The first stage is, of course, the aforementioned blast baffle/air stripper. A symmetrical conical baffle designed to bear the brunt of the blast out of the airgun's muzzle, and cleanly strip that turbulent air off the pellet to maximize accuracy. Following that, the second stage, are two of a new and more efficient gas diode which stops or at least delays forward air flow. Unlike the rev.1 and rev.2 gas diodes, which were comparatively volumetrically wasteful, these waste much less of the precious internal space. They also offer cavernous flow-chambers, reflexing vastly more air. All in all, they are a massive design improvement, and I really couldn't be more pleased with them. The third stage is something incorporated all the way back in the rev.2 gas diodes, sound damping. Why should flow and sound in a moderator be the same from end to end? It wouldn't, not if the moderator is working properly anyway, so it stands to reason the design of the moderator itself should change from end to end to match this. Most moderators out there seem to either focus on flow disruption or sound damping, but my approach of first flow stopping then multi-layer sound damping appears to be more effective than either flow stopping or sound damping alone. The CAD images don't capture this sound damping structure well, showing just the skeleton around which is it installed, however it is built of high surface area foam, a new type of foam I’ve never before featured, and rubber. Again this is a combination of materials arrived at after much testing of different combinations and designs.
Before we get to the test results, I also want to bring up something I’ve touched on before, which is peak sound vs. total sound. It is difficult to relate and difficult to quantify, this concept of total sound, because in essence you’re trying to put a number of a squishy human perception. I’ve done a series of experiments with RMS (root mean square), however so far these have not yielded good consistent data. You see there is more to a moderator than just the brief “uncorking” sound produced when the pellet exits and the maximum pressure wave gets out, there is also the sound of your moderator draining that pressure slowly over time. A bare muzzle may have a very high peak, but the subsequent drop back down to zero will be very rapid. Meanwhile a moderator design which effectively slows the flow of air out may have a very quiet peak however because the moderator spends a long time, relatively speaking, draining out all that air until it equalizes with ambient pressure, it may produce more total sound. How this relates to human perception of loudness is more complicated still, because both intensity and duration of sound will affect perception. At longer ranges though it seems the peak is what is most audible, and most likely to alert a prey animal or nosy neighbor. Thus quantifying this is next to impossible, which is why we associate a representative sound readout with each data sample. Below you can see an example in high tech MS-Paint, with the peak (what I measure) circled in purple, while the subsequent sound profile bracketed in teal.
So enough foreplay, how about test results? I used the same protocol as always, with the moderator separated from the pickup by 1 meter, perpendicular to the muzzle. Excluding the Crown Shroud test, which was done with the shroud extended, all tests were done with the Crown shroud collapsed. I also pulled the old foam filled moderator out to once again provide fair comparison against a “typical” foam/felt filled moderator (your hair curlers and washer baffles) design of equivalent material, design, volume, etc.
Shroud Extended - 214.4
Been there, done that. It is loud. It is the baseline, it is what it is. I always insist on repeating the baselines because some days, some atmospheric conditions, god knows some moon phase just results in different numbers. So I always strive to meter the baseline/s so that the later data has good context.
Rev.1 Gas Diode – 122.6
Again, another baseline. The rev.2 is a major design improvement, but it is always good to see progress. Here we can also see one of the things which I strove to improve upon, which is the quantity of noise produced AFTER the peak.
Foam Filled Moderator – 172.4
Again, this is an open cell foam filled moderator, intended to be a test analogue for the common high surface area moderators out there which use felt or foam. Here we see a phenomenon I alluded to before, all the way back in the first tests. The sound just tapers faster on these sound damping, rather than flow-stopping, designs. So you get higher peak sounds, the moderator is definitively louder, however it empties that air more quickly and more quietly. We can see that in the image, the initial spike is quite high, but the subsequent taper is more rapid. One has to wonder if part of the reason why felt/foam baffle-less moderator designs are so popular because of this phenomenon: while they may technically be louder, particularly at close range some people may perceive them as quieter. Borrowing or purchasing a popular and highly regarded commercial moderator design, such as a Donny FL, might be a good idea both to get a better understanding of what sound profiles they produce but also to provide a baseline for comparison that other people can relate to.
Rev.2 Gas Diode – 115.3
Still doing comparisons and baselines, what can I say? The second lowest peak, and one of quietest total sound profiles of all the moderator designs tested today. Why? Well this moderator you'll recall is a combination of gas diodes and sound damping in the center, and with off-axis flow-through around the perimeter, again with sound damping toward the muzzle end. The result is a design which slows the initial blast, and effectively and quickly drains the pressure and quiets the resultant sound. Thus there is both a fairly mute peak, and good quick tapering of the remainder of the sound profile.
Levitas (Rev.4 Gas Diode) (1/2-20UNF attachment) – 81.2
So this is the culmination of all this testing, the moderator which this is all about. More efficient gas diodes and a sound damping system work together to bring the peak down to the lowest moderator yet tested. And I don't mean just in this round of tests, I mean of all time; this is the quietest airgun shot I’ve ever sampled. I was surprised and pleased, to say the least. Remember this moderator maintains the same overall length, 120mm, as the other designs which ALL were direct thread into the Crown shroud. Integrating a 1/2UNF thread mount means you now have to eat up precious length and volume inside the moderator for the threads, so I would have been exceptionally happy if it were able to simply match the rev.2 gas diode, beating it by a significant margin was an unexpected but pleasant surprise. You'll notice though that the peak is not much higher than the rest of the sound profile. Both of the previous gas diode designs (revs.1&2) had pickups around the perimeter, out of the bore axis, which tapped off gases from inside the Crown Shroud and likewise vented them out of line with the bore. This is more useful than you might think, as it allows you to place dampers directly between the sound source and the exit for at least some of the air. This isn't possible with a 1/2UNF mount, for obvious reasons. While such a design could hypothetically be integrated into the blast chamber, with the loss of volume and effective length it just wasn't feasible thus was omitted. Even still though, it is great performance by any standard, so I'm very happy with it.
What will the next advancement be? Not sure yet, but as long as people find this interesting we’ll keep experimenting. I do want to test other moderators. A dirty secret of the firearm suppressor industry is that values can vary significantly from day to day due to ambient conditions, and my testing shows this is no different with airgun moderators. That said the values are reasonably close enough as to provide a very general basis of comparison, which would be very nice to have. So if anyone has a moderator they would be willing to loan for testing, if it has a 1/2UNF thread I’d be happy to give it whirl. Watch this space!
Comments? Questions? Fire away.