A standing sound wave is formed between a loudspeaker and a wall. A microphone is linked to a cathode-ray oscilloscope (c.r.o.) and is shifted along a straight line joining the loudspeaker and the wall. The amplitude of the c.r.o. trace increases to a maximum at point X, decreases to a minimum, and then increases again to a maximum at point Y. The separation between X and Y is $33\,\text{cm}$. The speed of sound in air is $330\,\text{m s}^{-1}$. Which diagram shows the c.r.o. trace of the sound detected at X?
Physics 9702 · AS & A Level · Stationary waves