If a sound source moves toward an observer, the observed frequency is

If a sound source moves toward an observer, the observed frequency is

• A. Higher
• B. Lower
• C. The same
• D. Higher or lower depending upon the exact frequency

Answer: (A)

The observed frequency increases due to the Doppler effect when the source moves toward the observer. As the source approaches, the emitted wavefronts are released closer together, so the observer encounters more wave crests per second. The speed of sound in the medium stays constant, which means the wavelength shortens while the frequency heard rises. A handy way to see this is f' = f · v/(v − vs) for a source moving toward the observer, where v is the speed of sound and vs is the source speed toward the observer; since vs > 0, f' > f. So the observer hears a higher pitch.

The observed frequency increases due to the Doppler effect when the source moves toward the observer. As the source approaches, the emitted wavefronts are released closer together, so the observer encounters more wave crests per second. The speed of sound in the medium stays constant, which means the wavelength shortens while the frequency heard rises. A handy way to see this is f' = f · v/(v − vs) for a source moving toward the observer, where v is the speed of sound and vs is the source speed toward the observer; since vs > 0, f' > f. So the observer hears a higher pitch.