Answer: Above 20,000 Hz.
- A Less than 20 Hz
- B Between 20 Hz and 20 kHz
- C Above 20,000 Hz
- D Between 1 Hz and 20 Hz
Correct answer: C. Above 20,000 Hz
Explanation: Ultrasound is above human hearing range: frequency > 20,000 Hz (20 kHz).
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.