If something other than the three acoustic variables rhythmically oscillates in a wave, the wave is ____.

• A. Not a sound wave
• B. A sound wave
• C. An electromagnetic wave
• D. A seismic wave

Answer: (A)

Sound waves are mechanical disturbances that propagate through a medium by rhythmic fluctuations in three acoustic variables: pressure, density, and particle velocity. If something other than these acoustic variables is the quantity that oscillates, the wave does not fit the definition of a sound wave. For instance, if the oscillating quantity is an electromagnetic field, the wave is an electromagnetic wave, not a sound wave, because it carries energy through oscillating electric and magnetic fields and doesn’t rely on pressure or density changes in a medium. Seismic waves are mechanical too, but the crucial point here is that the oscillation is not one of the acoustic variables, so this would not be classified as a sound wave.