消防车鸣着警笛开了过去。

The fire trucks sirened by.

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车鸣噪音， 被耽误的行程，车尾气污染…

The noise, delayed journeys, car pollution…

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这是由多普勒效应(类似一辆警车或消防车鸣着警笛从你身边经过，之后音高突然下降)引起的。

It is caused by the Doppler effect (something similar happens when a police car or fire engine with its siren blaring drives past you, and the pitch of the sound suddenly drops).

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