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Doppler Effect

If a source of sound is moving towards (away from) the listener, the frequency heard is higher (lower) than for the same source at rest, this is the Doppler effect.

Here the source emits equally spaced pings. Detect them by moving the microphone (click stop/start if necessary). Notice that as the source passes, the frequency drops, like an airplane passing overhead.

The speed of sound is about 330 m/sec, at and beyond that speed a shock wave is generated. Try it. Full explanation in my lecture.