Anyone who has shot, or been close to a pcp being shot without a silencer/moderator will know that the shot is accompanied by a very audible blast. But the loudness of the blast can differ different pcps. I have never fully got to understand why this is but was reminded of it when reading Russ Douglas'review of the Lobo Mustang in AGW in which he warns you that it is LOUD.
But why?
In my simplistic way of thinking, clearly too simplistic, for any calibre and desired mv, the volume of air being expelled depends on the bore and the air released by the valving. Now I can appreciate that the air pressure largely determines the ultimate velocity of the pellet and the length of the barrel influences the velocity as well. So, question: what are the critical factors that determine the loudness of the shot? In a regulated pcp for which we assume the air pressure is, or should be constant for each shot, is the determining factor the barrel length? Is this the same for the 'sweet spot' of an unregulated pcp? If the mv is the same for a regged and an unregged rifle using the same pellets and the barrel length is the same, will the loudness be the same? If so, could the loudness be modified by careful manipulation of both barrel length and air release pressure from the valve?
Any pneumatic airflow experts around who can clarify the clearly complex interactions?

Just wondering over coffee this morning.
Cheers, Phil