In an experiment to produce a stationary sound wave in air, a fine powder is initially evenly distributed along the length of a horizontal glass tube which is closed at one end. At the open end of the tube, a loudspeaker emits a sound wave of a constant wavelength. A stationary wave is formed and the powder accumulates in regularly spaced piles, as shown.
Which statement explains the positions of the piles of powder within the tube?
The piles are where the air molecules vibrate with maximum amplitude.
The piles are where the air molecules vibrate with minimum amplitude.
The piles are where the air molecules vibrate with the highest frequency.
The piles are where the air molecules vibrate vertically.
