I agree with previous posters - a processing thing rather than anything physical. Until I got into my forties, I could easily hear things like ultra-sonic pest-repellers etc., and even now, often pick up on things that no-one else can hear even if they're trying to listen. So, definitely nothing wrong with the physical equipment.
My brain's sensitivity seems to depend on a lot of things...
- What am I paying attention to at the moment? In a flow-state (hyper-focused)? I won't notice my name called from right next to me. Can't sleep at night? That car alarm on the other side of the city is deafening.
- Am I in control of the sound, or is it unpredictable to me? Me doing hoovering/drilling? Not nice, but I can manage it. Moth fluttering around in the next room? Intolerable!
- What kind of sound? Dawn chorus? Beeeaaauuutiful! Babbling voices in a bar/restaurant/bus-station! Run, run, escape, sanctuary!! !
- Stressed out? Please, someone give me a pencil to burst my eardrums with!! !!
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