So I've been doing a lot of research into moderator design and performance. I spend a lot of time listening to airgun shots over and over again, and one thing with the Crown that kept coming up over and over again is this faint "ring" with every shot, irrespective of the moderator. When I got the VP Crown in .30 caliber, what was a faint ring on the .22 was now clearly a very loud and significant ring. It didn't seem likely it'd be the tank, as this is a regulated gun, and the plenum almost certainly isn't conducive to resonance.What is left that could be causing this? The shroud.
The shroud is also the perfect thing to resonate, because it is basically a tube with flat faces at both ends, and sound just loves to bounce up and down something like that. FX didn't even put a taper or ribbing or crenelation or anything on the shroud stopper. The obvious solution, if this indeed is the problem, is a damper. Maybe even a multi-stage damper. Quantifying the results though I thought would be quite a pickle, as you wouldn't expect it to have much impact on the peak if any, just the noise in the trace afterward.
So I did two things, and you'll notice they're different. First is the obvious regarding the damper itself. I put a rubber washer at the back and then three foam discs in front. It'll effectively break up the wave front, I hope, keeping it from ringing up and down the tube. The .30 though brought another problem to light. You don't want your shroud extended when you're using a moderator because it will exacerbate any concentricity issues you may have, it is volumetrically inefficient, and will mess with how your moderator functions. The .30 has enough go-juice that it will shoot the shroud forward with every shot if you don't extend it. So the solution is to do what is seen on the left, on my .22, and that is move the shroud stopper rearward until the shroud is permanently locked to the rear. This frees up additional reflex volume, which should make some moderators perform better. On the VP in .30 on the right I left the shroud stopper forward (as pictured) only for two tests (shroud extended w/ and w/out the damper) so I could have an apples to apples comparison of the damper's performance. After that I locked it to the rear as well.
So what happened? .22 Crown first.
This is my Pilum design on my .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 for the .30 cal VP Crown.
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.
While there is certainly room for debate, looking at all the traces, it seems to me that in both cases adding damping material to the shroud has a positive effect on sound attenuation. Not necessarily peak, but the sound which follows the peak. Subjectively, it works like a hot damn. It is also helping me produce cleaner data for my moderator experiments, as now I don't have that same resonance in every trace which just so happens to be roughly equal to twice the length of the shroud at the speed of sound.
I should also mention that this is probably applicable to other guns. Almost everything these days comes with a shroud. Adding a little damping to it may just help your sound attenuation too.
The shroud is also the perfect thing to resonate, because it is basically a tube with flat faces at both ends, and sound just loves to bounce up and down something like that. FX didn't even put a taper or ribbing or crenelation or anything on the shroud stopper. The obvious solution, if this indeed is the problem, is a damper. Maybe even a multi-stage damper. Quantifying the results though I thought would be quite a pickle, as you wouldn't expect it to have much impact on the peak if any, just the noise in the trace afterward.
So I did two things, and you'll notice they're different. First is the obvious regarding the damper itself. I put a rubber washer at the back and then three foam discs in front. It'll effectively break up the wave front, I hope, keeping it from ringing up and down the tube. The .30 though brought another problem to light. You don't want your shroud extended when you're using a moderator because it will exacerbate any concentricity issues you may have, it is volumetrically inefficient, and will mess with how your moderator functions. The .30 has enough go-juice that it will shoot the shroud forward with every shot if you don't extend it. So the solution is to do what is seen on the left, on my .22, and that is move the shroud stopper rearward until the shroud is permanently locked to the rear. This frees up additional reflex volume, which should make some moderators perform better. On the VP in .30 on the right I left the shroud stopper forward (as pictured) only for two tests (shroud extended w/ and w/out the damper) so I could have an apples to apples comparison of the damper's performance. After that I locked it to the rear as well.
So what happened? .22 Crown first.
This is my Pilum design on my .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 for the .30 cal VP Crown.
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.
While there is certainly room for debate, looking at all the traces, it seems to me that in both cases adding damping material to the shroud has a positive effect on sound attenuation. Not necessarily peak, but the sound which follows the peak. Subjectively, it works like a hot damn. It is also helping me produce cleaner data for my moderator experiments, as now I don't have that same resonance in every trace which just so happens to be roughly equal to twice the length of the shroud at the speed of sound.
I should also mention that this is probably applicable to other guns. Almost everything these days comes with a shroud. Adding a little damping to it may just help your sound attenuation too.
