Answer: 100 Hz.
- A 100 Hz
- B 200 Hz
- C 400 Hz
- D 800 Hz
Correct answer: A. 100 Hz
Explanation: Direct sound (source moving away from person): f<sub>direct</sub> = 800 x 320/(320+40) = 800 x 320/360 = 711 Hz. Echo (source moving toward wall then back): f<sub>echo</sub> = 800 x 320/(320-40) = 800 x 320/280 = 914 Hz. Beats = 914-711 = 203 Hz. Closest: 200 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.