Doppler Effect for Approaching Source

Steps:

• Identify the scenario: source (horn) moves toward stationary observer (man) at constant velocity.
• Recall Doppler formula: observed frequency $f' = f \frac{v}{v - v_s}$, where $f$ is emitted frequency, $v$ is sound speed, $v_s$ is source speed (positive toward observer).
• Since $v_s > 0$, $f' > f$; higher frequency means shorter wavelength (${\lambda }^{\mathrm{\prime }}$).
• Conclude wavelength heard is shorter.

Why D is correct:

• For approaching source, waves compress, reducing wavelength while increasing frequency, per Doppler principle.

Why the others are wrong:

• A: Incorrect; heard frequency exceeds emitted frequency due to bunching of waves.
• B: Incorrect; frequency is constant for steady velocity, independent of distance.
• C: Incorrect; sound waves travel at constant speed $v$; no acceleration occurs.

Final answer: D