Noise 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
• C. Any unwanted sound
• D. Velocity

Answer: (C)

The essential idea is that noise is what we consider unwanted sound. In everyday use, noise refers to sounds that are disruptive, annoying, or not desired in a given situation, rather than all sounds in general. So the best description is “any unwanted sound.” The other statements describe general properties of sound waves—such as a pressure variation in a medium or a longitudinal wave produced by vibrating objects—without capturing the idea of unpleasantness or undesirability. Velocity isn’t a definition of sound or noise at all. Keep in mind that what counts as noise can depend on context and the listener, but the core concept is unwanted sound.