Airgun Moderator Design, Performance, and Development - Post-Peak Sound (part 20)

Catch the previous episode HERE.

Catch the “BIG TEST” series, which preceded this post, here:
Standard flow - FX Crown .22 shrouded producing 32 foot pounds. Flow Factor 460

Moderate flow - FX Crown .30 shrouded producing 80 foot pounds. Flow Factor 1,111

High flow - FX Dreamline .30 unshrouded producing 87 foot pounds. Flow Factor 6,500

I’ve mentioned post-peak-sound repeatedly. It is that noise after the spike, which is difficult to quantify (yes, RMS has been tried and the results are…… messy) but which significantly affects perception. This is a story of attempts to make a better high flow Sarissa, as well as a test of the Brevitas at vastly more airflow than it was meant for and a quick glance at some materials experimentation.

Let me take you on a little journey here quickly. The idea of the current moderate and high flow diode architecture is to use the struts as flow paths to feed the diodes, and to enhance the feedback loop of sorts whereby the more backpressure is generated, the more it feeds the diodes to pinch off flow, resulting in more backpressure, so on and so forth. Obviously airflow is necessary for all this to function, it is something of an oversimplification, but you get the point. A challenge this creates though is one of length, this diode architecture occupies rather a lot of it per-diode which is to say per-choke-point. So I had an idea to create a quadrant diode, that is to say a diode which occupies a pie-slice of the circular area of the moderator tube. Each diode would also feed the next diode, just like in the “rev.2” architecture, but they wouldn’t be stacked on top of each other, they’d be stacked next to each other but slightly displaced longitudinally. Thus it would form a spiral, which would coil down the length of the moderator. These diodes would also be paired up, creating a fang-like obstruction to flow through the bore path. (paired to prevent POI shift and pellet destabilization by being symmetrical) Thus the “vipercoil” diode architecture. It is incredibly promising as an avenue of research, because it allows more choke-points, more impediment to flow within the design.

So what is the catch? Well there are two actually. The first issue is that, as you might imagine, at the start and end of the coil you have a whole bunch of partial diodes and space-filling to do, where that space is largely useless. And so back when I developed this initially and tested it in the context of a moderate flow Falx, the results were no better or slightly worse than the “rev. 2” diode architecture in terms of peak. So it languished for a while, but then the Sarissa came along, and with it a high flow variant unshacked from the constraints of length the way the Falx is. So I decided to try it again, in a high flow context, and superficially IT WORKED!!!! Peak reduction was just over 10 points (average) compared to a high flow Sarissa. So why then is this not the latest and greatest Sarissa-H you can buy now? Because of that post peak sound, the sound of the moderator draining.

At the top you can see the vipercoil diode, the bottom the rev.2 diode. While the vipercoil has a small advantage in terms of peak, that post-peak draining sound makes the result sound noticeably louder to the shooter. Were I just measuring peak, I’d have never noticed. It is a great illustration of the value of a dataset comprised of more than just a number.

And another example of both high peak and high post-peak-sound. Don’t let the scale fool you, this was properly properly loud. This is mounted directly to the muzzle of the 87FPE .30 cal dreamline, 6,500 flow factor. The Brevitas really wasn’t rated for this much flow, or having no shroud to store some of that air/pressure. But I did abusive testing on it to ensure at lower flow factors it’d be good to go, and I was curious as to just how loud it really was. The answer is about 1213.0, with quite a sustained sound. In fact it appears, at this level of violence, some oscillation is definitely a problem for this design. Learn something new every day eh?

The final post-peak-sound subject at issue for this blog post has to do with the sound damping. Now to be clear, this affects both peak and post peak sound, really a win all around if it can be improved. The work with the Sarissa standard flow taught me a hard lesson that I needed to look at this area more closely. It is an extension of my original research, years ago now, to put the standard of felt up against everything I could lay my hands on to find the best materials available.

Well now I’m starting to take this a step further, fabricating my own damper materials from scratch, and experimenting with acoustic metamaterials. It is hard to say where exactly this will go, out of four tests three were failures, but one showed promise bringing a standard flow Falx down in peak by an average of about 7 points, compared to an otherwise identical regular Falx back to back. When you’re already down around 50 points, that is a significant performance enhancement. Shown below is an overlay of two traces, one from a successful damper experiment, one from an unsuccessful one. Clearly visible is the reduction in both peak and post-peak amplitude.

While it isn’t exactly ready for prime-time, it does provide a direction for other promising enhancements. Of course scalability of these parts is always a question and challenge, exotic materials made like this by hand one-at-a-time will not have the speed or consistency necessary for anything but prototypes. The goal here though is very much experimentation, baby steps at first to learn what I can, because it is challenging and fascinating. After that, if the experiments are successful, I’ll work on the manufacturing engineering aspects to try and get these into other people’s hands so they can enjoy them as well. :)