Airgun Moderator Design, Performance, and Development - Shroud Dampers (part 7)

FX Crown VP w/ shroud damper

In case you missed it, here is a link to PART 6. Fear not, we haven’t forgotten the Leshiy, or any of the other projects, this is just a quick hit for a little thing we discovered while doing testing. Then we’ll be back to, what is now apparently the regularly scheduled programming on this blog….. how the heck did that happen anyway?

So this is an oddball little project, however while testing all of these moderators I noticed there is a bit of a ringing sound with each shot which really sounds like it is coming from the shroud. On the .22 caliber Crown you can barely hear it. On the .30 caliber VP Crown it is very noticeable.

Let me rewind for a second. Normally you would assume this would be coming from your air tube, however there are two notable things about that, the first is that the FX Crown doesn't have an air tube it runs a carbon or aluminum bottle, and the second is that it is regulated, so there is no major air-shock to the tank anyway. The plenum should also be fairly well damped given its location and elements of its installation, so color me skeptical that it is coming from there because there are all kinds of factors going on which would impede resonance in this case. Obviously ruling it out though would be something you'd have to do experimentally, hence this mini-project.

I'm also not a big fan of the look of my crown when it has an erection. Color me weird, but I'd rather put a small light weight moderator of equivalent length on my crown's muzzle than extend the shroud which I think looks goofy and does a lousy job at sound mitigation anyway. On the .30 Crown it is actually earsplittingly loud if you shoot it indoors. If you pull your shroud off, you'll also notice that the shroud stop is part way up the tube, which essentially reduces the shroud's volume when it is retracted. So step one was loosening that shroud stop and gently moving it rearward so that it pins the shroud in its fully retracted position. This is necessary for silencer testing on the .30 as it blows itself open with each shot otherwise. Also it will stop the POI shift caused by rotating the shroud to different positions, another annoying “feature” from the floating shroud.

So back to the main story, sound loves to run up and down tubes. It really really does, particularly if the tubes have flat faces at the end. And guess what the Crown has? A nice long tube which is basically flat at both ends. So after finding this extra shroud volume, why not eat up a little bit of it with some damping material? Thankfully I have plenty of options kicking around, so I took some quick measurements and zipped out a little disc of rubber and some foam to, hopefully, damp the sound which is inevitably running up and down my shroud.

FX Crown .22 w/ shroud damper

Having done this, to both Crowns, audibly the ringing is gone. But I’d like some sort of data to back it up. I should note here that this damper is NOT meant to reduce peak sound signature, just clean up the noise which follows.

This is the Pilum design on the .22 before any modification to the shroud retainer or damper. Note this was done on a different day from the rest of these tests. This moderator on this day metered an average of 57 (peak).

So this is the exact same moderator, same gun, same everything except on a different day and now with the shroud stopper moved rearward and with the damper installed. The average for this moderator on this day day metered 51.8 (peak). 

So these two different peaks are outside their respective standard deviations, however they don't exceed what is POSSIBLE day to day variation. My experience, and looking at other tests between the two days, suggests that it is unlikely the peak difference was solely random and atmospheric differences. I can't explain fully why the peak is lower, but that doesn't mean it is the shroud damper. That said, after the peak, to my eyes the traces look much quieter. The damper appears to be working. Further, that faint ring is now audibly gone. 

Now on the .30 Crown things are a little different. First the ringing is much much more obvious and obtrusive. Second though is that, for a true apples to apples comparison, I tested with just the shroud extended, no moderator, and changing nothing between the two tests other than the damper. These were tested back to back.

This is the .30 with just the shroud extended. The scales are different between this and the above traces by the way. This thing is earsplittingly  loud in factory form. If you think you're going to shoot it inside, think again, any sound-reflective surface or room causes my ears physical pain anyway and leaves my ears ringing. It metered an average peak of 637.6, and the ringing is very very noticeable. (to the subjective ear anyway) 

This is the .30 with the shroud extended still, but now with the damping material added. Note that adding a damper without moving the shroud stopper rearward eats up reflex volume. It metered an average of 617, however that is not statistically significant as the two are within less than a standard deviation of each other. When airguns are this loud and messy, things are just a lot less consistent. Subjectively, this also seems to have done away with the ring. 

Both looking at the traces, and to my subjective ear, this really appears to work. I’m happy with the project’s outcome, and for once something worked as intended on the first try.

Stay tuned for more on the Leshiy. There have been some very exciting developments, and the Leshiy is now considerably quieter than before.