Physics
When two similar sources vibrate with slightly different frequencies, a regular rise and fall occurs in the loudness of tone. This phenomenon is known as

Beats
Doppler effect
Echo
Resonance

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Physics
Why are we able to hear short wave broadcasts better than long wave broadcasts ?

Short waves are unaffected by atmospheric disturbances.
Short waves are more energetic than long waves.
By convention, short waves are meant for long distances, whereas long waves are reserved for short distances.
Short wave broadcasts are made by nearby radio stations.

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