You ask a good question! I have been working on brake design for over 15 years and have been using the advice ov an engineer who though now retired used to do super sonic wind tunnel testing in the aircraft industry. I must tell the truth and add the engineer is my father so I am cheating.
Any way most of the stuff out there on brakes is hype!!! Angled holes spirled designes to counter act rotational forces and all that stuff is bunk and sales pitch. Gas above the speed of sound ( and yes the speed of sound goes up with temp.) does not like to turn corners so the holes in a brake are only effective as far as their frontal area. In reality a rifle with a brake is no louder then a rifle with the barrel cut off before the brake. The muzzel brake does not increase noise or blast. However if this brake is ported on all sides as many are then the shock wave is directed or reflected back to the shooter and those around him.
If you look at the brake design I use you will notice that the under side of the brake is not ported and this is done for several reasons. First is the muzzel blast as all of you are talking about. The other reason is that I want to get rid of muzzel jump and leaving the bottom of the brake clean holds the muzzel down to the point where even off the benck you do not have to hold on to the forearm. As to noise this brake is as bad as any if you stand next to a building or your friends step forward and get even with the muzzel but other then that ,like I said earlier it is no worse then a barrel cut off behind the brake. What makes brakes bad is the holes on the bottom!!!! Off hand the blast is reflected from the ground to the shooter and those around and off the bench it is horendous and a real problem.