Listening to the trains one morning at the age of five, he noticed that when the sound reached a certain pitch, it made the window vibrate.
5岁的一天早晨，他听到了火车的声音，注意到当声音到达一定高度时，窗户开始震动。
2
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 pitchof the sound suddenly drops).
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。
3
Even a simple note on my guitar has an array of harmonic frequencies. But the frequencies have a special harmonic relationship, which is why you hear it as a single sound with a single pitch.