Definition
The sound intensity level at which a normal human ear begins to feel physical pain rather than simply hearing the sound, generally accepted as approximately 130 decibels (dB).
Plain English
The point where a sound is so loud it stops just being loud and actually starts to hurt your ears.
Context Anchor
Seen in human factors, aircraft noise, cockpit noise, and hearing-protection discussions.
Derivation
Threshold' comes from Old English 'therscold,' meaning the sill at the entrance of a doorway -- the point you cross to enter. Used here in the sense of a crossing point: the level at which sound crosses from merely loud into physically painful.
Why Pilots Care
Exposure near or above this level, even briefly, risks immediate pain and long-term hearing damage in high-noise environments like turbine aircraft.
Grounding Statement
Standing close to a running piston engine without hearing protection puts the ear near this level; the discomfort you feel is the threshold being reached.
Intuition Check
Do not treat the threshold of pain as the point where noise first becomes harmful. Hearing can be damaged by noise that is uncomfortable or prolonged even if it never hurts.
Example Sentence 1
Unprotected exposure to a running turboprop on the ramp can approach the threshold of pain, which is why ground crews always wear hearing protection.
Example Sentence 2
Maintenance crews monitor ground run noise to keep levels safely below the threshold of pain for nearby personnel.