I don't really know but the sonic bang heard might be caused by the displacement of air as the projectile travels so that the bang is heard by any observer as the bullet passes overhead. The noise ,which is only compressed air is heard at all points along the trajectory, so at a distance from the muzzle a subsonic pellet has little noise.

If the pellet is subsonic then there is not a lot of noise from a pcp and the pellet itself makes little noise. The hiss as it passes a rabbit would be enough to spook it. Is it just the airblast as the pcp empties its charge that is noisy and needs to reduced? That blast is not like a big powder burner bang. Or even a subsonic .22 LR bang.

There is a bit more noise from a springer but that noise is really confined to the close proximity of the shooters ear and is produced by all the hardware thrashing about. {The quiet of the Gamo IGT is noticeable.} {Had to get that in} Subsonic pellets don't break the sound barrier hence no boom. Supersonic pellets do but the shockwave peals off,  I would think, into the atmosphere after the pellet passes the constriction of the silencer. If the pellet goes supersonic then maybe it does not stay supersonic for long before it drops back below the speed of sound where the shockwave reforms on the pellet and I guess would produce another bang or at least some noise. 

I wonder what the noise would be like from any  airgun at the 30 - 40 -50 yard mark. 

I remember from working in the butts at training that all one really heard was the crack of a supersonic round going over head. No report from the gun being discharged two or three hundred yards away.

I look forward to reports of silencer effectiveness but I can't help comparing with fishing lures that are made more to catch fishermens' wallets than catch fish.