What phenomenon explains the change in perceived pitch when a moving sound source approaches or moves away from the listener?

What phenomenon explains the change in perceived pitch when a moving sound source approaches or moves away from the listener?

• A. Echo
• B. Doppler effect
• C. Reflection
• D. Interference

Answer: (B)

The change in perceived pitch comes from the Doppler effect—the way motion between a sound source and the listener changes the frequency of the waves that reach the ears. When the source moves toward you, waves are emitted closer together, so more wave crests arrive each second and the pitch sounds higher. When the source moves away, waves spread out and fewer crests hit per second, so the pitch sounds lower. This is why a siren climbs in pitch as it approaches and drops after it passes. Echo involves sound bouncing off surfaces, not a change in pitch from motion; reflection is the bouncing itself; interference is about how waves combine and create patterns, not about frequency changes due to motion.

The change in perceived pitch comes from the Doppler effect—the way motion between a sound source and the listener changes the frequency of the waves that reach the ears. When the source moves toward you, waves are emitted closer together, so more wave crests arrive each second and the pitch sounds higher. When the source moves away, waves spread out and fewer crests hit per second, so the pitch sounds lower. This is why a siren climbs in pitch as it approaches and drops after it passes. Echo involves sound bouncing off surfaces, not a change in pitch from motion; reflection is the bouncing itself; interference is about how waves combine and create patterns, not about frequency changes due to motion.