Answer: 800 Hz.
- A 710 Hz
- B 750 Hz
- C 800 Hz
- D 850 Hz
Correct answer: C. 800 Hz
Explanation: Wall receives f' = 600 x 340/(340-60) = 600 x 340/280 = 728.6 Hz. Driver now moving toward wall: f<sub>echo</sub> = 728.6 x (340+60)/340 = 728.6 x 400/340 = 857 Hz. Approx 800-850. Closest: 800 Hz.
A standing wave forms from two identical waves travelling in opposite directions and interfering; nodes (always zero displacement) and antinodes (maximum displacement) stay fixed in place, unlike a travelling wave where the whole pattern moves.
Concept context
Wave types, superposition, standing waves, Doppler effect, and sound intensity.