Sounds are the result of air vibrating, and if they’re reproduced at, say, twice the speed that they were originally recorded at, the vibrations hit our ear twice as many times per second – i.e. twice the original frequency, which makes them sound higher in pitch.
In the case of musical notes, doubling the speed raises the pitch of each note by an octave. Some records exploit this effect, with trumpet players being recorded at half-speed so that when replayed at the right speed, they sound like they’re hitting really high notes perfectly.
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