Couple of things that were not explained in the video:
They correctly said that the first round would be louder than the following rounds, the dB rating would decrease on the sound meter with the next few rounds (5 total). This is commonly referred to as first round pop (FRP) and is caused by the oxygen within the can at the get-go. The O2 facilitates combustion, within the can, of unburnded propellant before it exits the can, thus raising the pressure levels more than subsequent rounds which have less remaining oxygen to deal with.
A way to decrease FRP (and subsequent rounds at least for awhile) is to shoot "wet". A bit of water (I prefer wire line pulling gel or lube) placed in the can prior to shooting, provides more cooling of the hot gasses - no FRP! But, there's a downside, you can get "speckle face" with some guns. I was shooting my P229 40 off a sand bag with an Osprey can. My face was closer to the gun that would be if held at arm's length. Shot a couple of mags full before I realized I was peppered with blackened gel.
The muzzle blast (gas jet) is further reduced by the baffles and can extracting heat from the gas, thus reducing its volume. This is more of a factor when firing a few rounds slowly, as the can has a chance to cool between rounds. Reduction of the gas jet velocity and volume (by cooling), also significantly reduces secondary recoil, which by some accounts makes up over half the felt recoil.