Reference the sonic crack of a suppressed supersonic rifle round, it has been my experience that velocity, time of flight and terrain will all have an impact on what the shooter perceives.
Velocity applies as the faster the bullet screams away from the shooter the less time it has to act on the ear.
Time of flight applies in the same way, especially in hunting. The shorter the distance to your game the less the sonic crack will have time to act on the ear.
Terrain will apply with respect to the perception of the sound of the sonic crack in that, On a square range with berms backstop and roof the sound of the crack will be reflected back at the shooter and it will sound louder than it will in the field while hunting.
In a canyon it will sound louder than in a wooded area.
Although video is an extremely poor media to try and quantify how loud a sound is (microphones in most cameras try and compensate for lows and highs reference noise) here are a couple of videos I've made of suppressed shots in the field.
These videos are of shots taken on game so if you're not interested in seeing harm come to animals, don't watch:
This is my suppressed 17 Hornet using factory loaded Hornady 20 grain ammo and the suppressor being used is a Gemtech Titanium Trek 223 suppressor. Distance to the targets is 200 and 225 yards respectively:
http://youtu.be/RME1NmPmEMo
The next one is my 220 Swift, 50 grain Zmax loaded by me, velocity is 3750 fps distance was about 150 yards. the rifle is suppressed using an AAC M42K 5.56 can.
https://www.youtube.com/watch?v=lvOKqBnOLZU
In both instance's, to me (the shooter) the sound of the bullet hitting the animal seemed louder than the crack of the bullet.
