Answer #31

The correct answer is B: Particles vibrate perpendicular to the direction of the wave propagation. Sound is a mechanical wave, meaning by definition it has to have a physical interaction with a medium in order to propagate (choice D). This is as opposed to an electromagnetic wave (choice C) where a medium is not necessary an propagation can even occur in a vacuum.

Mechanical waves can be either longitudinal or transverse. Sound is a LONGITUDINAL wave, meaning that particles vibrate back and forth PARALLEL to the direction of propagation. This is shown in the figure below [echo.physics.longitudinal wave.jpg]. The areas of higher densities of particles are called compressions and the areas of lower densities are called rarefactions (choice A).

In contrast a TRANSVERSE, wave the particles move perpendicular to the direction of the propagation (choice B), as shown in figure.