When a sound source is moving away from an observer, the observer will hear a lower pitch. this is due to

Lower Pitch Due to the change to a lower frequency

Answer;

Decrease in frequency

Explanation;When a sound source is moving away from an observer, the observer will hear a lower pitch. This is due to decrease in frequency. Pitch of sound depends on the frequency of sound waves and also on the loudness and spectrum of sound. A high pitch sound corresponds to a higher frequency sound wave and a low pitch sound corresponds to a low frequency sound wave.Based on Doppler's effect, which is the change in the observed frequency of a wave such as sound wave when the source or the observer moves relative to the medium of transmission, as the sound source is moving away from the observer, the sound reaching the observer will have a larger wavelength but lower frequency. Thus the sound heard by the observer will have lower pitch.

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