Standing waves cause powder to collect at nodes

Steps:

• Standing sound waves in a closed tube have nodes (zero displacement) and antinodes (maximum displacement).
• Powder particles are displaced by air vibrations and gather where vibrations are absent.
• Piles form at nodes, spaced by half-wavelengths along the tube.
• This identifies piles as minimum amplitude locations.

Why B is correct:

• In standing waves, nodes are points of minimum amplitude (zero displacement) per wave interference principles.

Why the others are wrong:

• A: Maximum amplitude occurs at antinodes, where powder is repelled.
• C: Frequency is uniform throughout the standing wave.
• D: Sound waves in air vibrate longitudinally (along the tube), not vertically.

Final answer: B