During a studio recording, I heard a beep and paused the recording. I played it back and neither the actress, sound technician, or assistant director could hear it. I imagined that it could be the remote control unit of the air conditioning, which was turned off but still listening.
Listening to the tapes, I heard it again and found it in the frequency spectrum:
It is very, very faint, so here is the same picture with gamma correction and a note on the beep:
Here is the recording.
I asked a musician and he didn't hear it either. I did frequency repair and it almost worked; doing it on the voice as well made it sound tinny. If I delete all frequencies above 6k, the voice sounds muffled; if I delete frequencies below, the voice becomes unintelligible. At 6k, it should be well within the frequency range of a human ear:
Frequencies capable of being heard by humans are called audio or sonic. The range is typically considered to be between 20 Hz and 20,000 Hz.
Why can I hear this beep sound and several people cannot?