The folks at Ammo To Go have run some tests on suppressors with both pistols and rifles and published the results here. Spoiler: Rifles don't get noticeably louder or quieter with changes in barrel length, and everything gets about 25 dB quieter with a can. Nice, but only the .22s get below the hearing damage level.
While barrel length doesn't seem to make much difference on the rifles, I have noticed that shooting PCC class in IDPA that the pistol caliber carbines are enough quieter than the pistols that the shot timers sometimes have trouble hearing them. Add a can, and they could become quite user friendly. Alas, no PCCs were tested.
One might note that in the case of the Virginia office shooting, the use of a suppressor on a .45 cal pistol likely had no effect at all on any ones ability to hear the gun and locate the shooter.