Airgun report depends on: FPE/Bore expansion volume
For a given barrel length, a larger caliber has more expansion volume. The larger the expansion volume, the lower the muzzle pressure at a given FPE.
Greater expansion volumes are more air efficient. If you have to think about this, a pistol of a given caliber producing a given FPE is much louder than a rifle in the same caliber, producing the same FPE. The greater report of the pistol represents waste energy that is not transferred to the projectile.
What a PCP moderator has to deal with is muzzle pressure and air volume. Increasing either of these makes the job more difficult. But, higher muzzle pressure produces more shock when the projectile "uncorks" from the muzzle.
Yes; all else being equal; a PVP with a valve that bounces or stays open too long will be harder to quiet down, despite not adding FPE to the projectile.
Talking about valve dwell; a reg setting of 2500 PSI tuned to produce the same FPE as with the same reg set at 1500 PSI, would need to have the valve dwell be shorter at the higher pressure, and longer at the lower pressure. Thus, a given FPE achieved at a lower reg setting will be louder than when using a higher reg pressure. This is because what produces FPE from a given platform is average pressure down the barrel length. The lower the reg pressure, the higher the muzzle pressure when achieving a given FPE, because the valve would have to be open longer.
If your reg is set very low you may have to keep the valve open all the way to the muzzle to reach a given FPE. Then reg pressure, average pressure and muzzle pressure would be almost identical; and the PCP very loud. So, for lower muzzle report at a given FPE, choose a higher reg pressure setting, and short valve dwell.
Larger calibers are harder to quiet down than smaller ones - if you use the same moderator diameter. Think of the muzzle diameter as the diameter of a loudspeaker, if you like.
Consider the sound of a shotgun shooting at 1200 FPE, and compare that to a .223 centerfire shooting at the same FPE. The shotgun has a "big" booming sound, while the rifle has a sharper crack that seems very loud for its FPE. Both are "loud", but the much higher muzzle pressure of the smaller caliber produces higher sound pressure near the muzzle. Thus is more unpleasant and more damaging to your hearing.
Any projectile that is supersonic is going to produce at least 130 dB, regardless of what suppression may be added to end of the barrel. Generally, larger supersonic projectiles are louder. The more they exceed the speed of sound, the louder they tend to be also. As one selling point for airguns is their comparative lack of noise, it is better to keep your projectiles sub-sonic. If you want more FPE, use a heavier projectile. And/or a larger caliber.
