OSHA says, "Exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure level." That's where the 140 dB comes from. In my experience 140 dB sounds pretty loud and is in the range that temporary ringing may occur (it does for me). For comparison an unsuppressed non-braked .308 is usually about 168 dB. Every +10 dB is a 10x increase in the sound pressure level.