In discussing Doppler effect, we use the word “apparent frequency”. Does it mean that the frequency of the sound is still that of the source and it is some physiological phenomenon in the listener’s ear that gives rise to Doppler effect? Think for the observer approaching the source and for the source approaching the observer.

The frequency of the sound remains the same that of the source. The apparent frequency refers to the frequency perceived by the observer due to the relative motion of the observer and the source.
When the observer approaches the source, the time between two consecutive points of the same phase decreases, so he perceives the sound as having a higher frequency than when standing.
When the source approaches the observer again due to the relative motion the second consecutive point of the same phase is produced nearer to the observer, so the two consecutive points of the same phase reach observer in a shorter time than both were not moving. So again the observer perceives the sound as having a higher frequency. This phenomenon is named as Doppler Effect.


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