An observer moves toward a stationary sound source emitting 500 Hz at 30 m/s in air (v = 330 m/s). What is the observed frequency?

An observer moves toward a stationary sound source emitting 500 Hz at 30 m/s in air (v = 330 m/s). What is the observed frequency?

• A. About 640 Hz
• B. About 550 Hz
• C. About 545 Hz
• D. About 530 Hz

Answer: (C)

When you move toward a sound source, you encounter wavefronts more often, so the frequency you perceive increases. This is the Doppler effect for a moving observer: f' = f_source × (v + v_o) / v, where v is the speed of sound and v_o is the observer’s speed toward the source. Plugging in the numbers: f' = 500 Hz × (330 + 30) / 330 = 500 × 360 / 330 = 500 × 12/11 ≈ 545 Hz. So the observed frequency is about 545 Hz. The wavefronts are compressed as you approach, raising the pitch.

When you move toward a sound source, you encounter wavefronts more often, so the frequency you perceive increases. This is the Doppler effect for a moving observer: f' = f_source × (v + v_o) / v, where v is the speed of sound and v_o is the observer’s speed toward the source.

Plugging in the numbers: f' = 500 Hz × (330 + 30) / 330 = 500 × 360 / 330 = 500 × 12/11 ≈ 545 Hz.

So the observed frequency is about 545 Hz. The wavefronts are compressed as you approach, raising the pitch.