Using a sound absorbing material in the moderator makes NO difference in sound level. I have used many baffle shapes and they all work with very little difference between the different designs in sound level.
If you think about what a moderator does, it is a translator. It translates a high pressure, high velocity pulse of air to a low pressure low speed pulse of air. It does this via generated turbulence. This induced turbulence impedes the column of air in front of the pellet as well as the driving air behind it. The moderator has no way to discriminate between the two. This turbulence does attenuate the pellet velocity achieved a bit. My last moderator (25 cal.) changed the end velocity from 940 to 900 ft/sc.
A couple things I want to highlight here. First is I've done a fair bit of testing, and not with some cellphone app or cheap-o sound meter, to scientifically compare airgun moderators of identical external size. This includes back-to-back tests on several moderator designs where the ONLY thing changed was the sound absorbing material, it was otherwise cut to identical size, shape, and thickness. The difference in performance was about 10%, some designs more some designs less. Either way, it was a far cry from "NO difference."
https://www.airgunnation.com/topic/fx-crown-bespoke-moderator-tesla-gas-diode/page/3/#post-398384 As far as differing baffle shapes go, again I've done a series of tests on this, all with an identical size design, just changing baffle shapes. The conical baffle design wasn't a bad performer by any stretch, but was significantly outperformed by both the gas diode design and the foam fill design.
https://www.airgunnation.com/topic/fx-crown-bespoke-moderator-tesla-gas-diode/page/2/#post-381182 As far as a moderator necessarily reducing velocity, this is far from the case. In firearms, suppressors (which work largely on the same principles) are seen as largely increasing velocity, which they do. In the case of airguns, what sort of velocity change, up or down, you'll see really depends a lot on the design of the moderator as well as your gun. The pellet will be traveling nearly as fast as the pressure wave which limits the ability of air to "pile up" ahead of it. Obviously some certainly can and will, so how effectively the design strips that air off the pellet will be a factor. Another factor will be how good the gas seal in your barrel is, because the less air escaping ahead of the pellet, the less pressure it'll have to push its way through. Sadly I don't have a big pile of data on this subject to quickly reference to compare different designs, but saying that freebore boost is a firearm-silencer-only phenomenon I'm not confident is true.
