The least exposure that has a measurable effect on unexposed film. 1
The threshold of ‘hearing’ is the sound pressure level at which people hear only 50 per cent of the time. Approximately 0 dB SPL at 1 KHz.
The threshold of ‘feeling’ is the sound pressure level at which people feel discomfort 50 per cent of the time. Approximately 118 dB SPL at 1 KHz.
The threshold of ‘pain’ is the sound pressure level at which people feel actual pain 50 per cent of the time. Approximately 140 dB SPL at 1 KHz.
References
1 Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston