A stationary sound wave is formed in the air between a loudspeaker and a wall. A microphone, linked to a cathode-ray oscilloscope (CRO), is moved along a straight line that lies directly between the loudspeaker and the wall. The CRO trace amplitude increases to a maximum at a point X, decreases to a minimum and then increases again to a maximum at a 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 could show the CRO trace of the sound detected at X?
Physics 9702 · AS & A Level · Stationary waves