Why can I hear this beep at 6 kHz and others cannot?

Question

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?

Answer 1

I listened to the file several times and couldn't hear it. In a spectrum analyzer, I can see what you're referring to. It's at -90 dB, so you'd have to crank up the volume pretty far to be able to hear it.

Because the speech volume in the recording is at -20 dB, most people will set the volume to a level where that's comfortable to listen to, and the beep will disappear below the noise floor.

The spectrum indicates the sound is in a narrow frequency band (or even a single frequency), so you might be able to get rid of it with a very narrow notch filter.