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There’s an official name for the horrible sounds that emanate from a blackboard: chalk squeal. Scientists say that the explanation involves both friction and resonance.
When you hold a large seashell to your ear, you can hear a distant roaring sound. It’s as if the great rumble and crash of the ocean waves is somehow trapped within the shell. So when you bring a seashell home from the beach, it keeps the memory of the sea alive. But even if you don’t have a seashell handy, and it’s too cold to go to the beach, you can always hear the ocean. Just pick up an empty coffee mug, and hold it to your ear, tilted slightly away. Ah, there it is—the gentle, echoing roar of the waves.
Have you ever stood on a city sidewalk and heard the distant wail of an approaching fire engine? As the fire truck comes into view, the sound of its siren becomes louder and more frantic, the wail higher and higher pitched. Then, as it passes, the opposite effect occurs: The sound of the siren drops in pitch, getting lower and lower as the vehicle vanishes into distant traffic. But the fire truck’s driver hears no such change; to him, the siren that he flipped on 20 blocks ago has sounded at a steady pitch.