These state-of-the-art earphones, for example from Apple, could be dangerous if you adjust them to create canceling sound against external noise, because then you are guaranteed not to hear a single siren, even if no music is played.
Thirty years ago, I also had a silicone impression made of my ear canal and used these earplugs very successfully in the rehearsal room. But I didn't like wearing them in the car.
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