Sound is defined as:

• A. Any pressure variation in air, water, or some other medium that the human ear can detect
• B. A longitudinal wave, created by vibrating objects, that spreads from one location to another
• C. Velocity
• D. Frequency

Answer: (A)

Sound is the pressure variation in a medium that the human ear can detect. When objects vibrate, they create alternating compressions and rarefactions that travel through air (or another medium) as a wave. Humans perceive only those pressure fluctuations within a certain range of frequencies and amplitudes, so sound defined this way emphasizes what we can hear. While a vibrating object does generate the wave and the wave travels through the medium (a longitudinal wave in air), velocity and frequency are properties of the wave, not the definition of sound itself. This definition neatly ties the physical phenomenon to human perception, which is why it’s the best choice.