Asound-producing object is moving toward an observer. the sound the observer hears will have a frequency that actually being produced by the object.
higher than
lower than
same as

The answer is higher than.
A sound-producing object is moving toward an observer. The sound the observer hears will have a frequency higher than that actually being produced by the object.

Why?
As the source of the waves is moving toward the observer, each of the successive wave crest is emitted from a position closer to the observer than the previous wave.
Thus each wave takes slightly less time to reach the observer than the previous wave. So, the time between the arrival of successive wave crests at the observer is reduced, increasing the frequency. 

higher than 

This phenomenon is known as the Doppler Effect. It occurs when a sound producing object is approaching towards an observer, it can be noticed that the sound is higher than what it really is. The sound usually fades as the object passes by.

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