Ask any doctor: any exposure, however brief, to noise above 100 db is a permanent hearing loss. Fireworks, machinery, guns, cars, music. If the exposure is continuous (work), even 80 db causes cumulative damage - see OSHA regs.
Them why does it "come back" after a few days?
Only some comes back, and your subconscious has adjusted to the partial deafness.
In addition, really low frequency noise (below about 15 Hz, caused by chassis vibration, etc.) is inaudible but causes headache and stress. That's why riding in a loud car is "tiring" after a few hours, in addition to hearing problems.
