Standing wave node-to-antinode distance determines wavelength

Steps:

• In a sound standing wave, position X is a displacement node (minimum amplitude), Y is an antinode (maximum amplitude).
• Distance XY = 30 cm = 0.3 m = λ/4, so wavelength λ = 1.2 m.
• With speed v = 300 m/s, frequency f = v/λ = 300 / 1.2 = 250 Hz.
• At reduced speed 30 m/s, new frequency f' = 30 / 1.2 = 25 Hz, producing a lower-frequency sinusoidal trace on CRO.

Why B is correct:

• B shows a low-frequency sine wave, matching the reduced f' = 25 Hz from halved speed (same λ in fixed setup).

Why the others are wrong:

• A: High-frequency wave, implies unchanged or higher speed.
• C: Distorted or clipped waveform, not sinusoidal sound.
• D: Flat or no signal, ignores ongoing wave propagation.

Final answer: B