system
Given the following post, respond as if you're commenting on Reddit:
human
The 'book detector' when I walk in/out of my university's library causes a high pitched sibilance in my earphones. Why?

On entry or exit at my university's library, stock standard iphone 5 earphones let out a high pitch noise while listening to music. Why?
gpt
The book detector is emitting a radio signal which would energize and RFID chips in the book detector tags. Your earphones are acting as antennas and picking up the signal. You can sometimes get the same effect if your phone is sending/receiving a lot of data near some speakers.