In sound propagation, a series of which two features are formed by oscillating particles?

• A. Crests and Troughs
• B. Compressions and Rarefactions
• C. Peaks and Valleys
• D. Refractions and Diffractions

Answer: (B)

Sound in air travels as a longitudinal wave, so the particles in the medium move back and forth in the same direction as the wave. This motion creates alternating regions of higher pressure and density (compressions) and lower pressure and density (rarefactions). The series of compressions and rarefactions is what propagates the sound through the medium. The other terms describe different ideas: crests and troughs (or peaks and valleys) are features of transverse waves, not the density variations in a sound wave, and refractions and diffractions refer to bending or spreading of waves, not the pattern created by particle oscillation.