What is the standard definition of sound?

• A. Any pressure variation in air, water, or other medium that the human ear can detect
• B. A musical tone audible to humans
• C. A vibration of a surface
• D. A random noise

Answer: (A)

Sound is a pressure variation that travels through a medium and can be detected by the human ear. This emphasizes that sound is about what the ear perceives—the audible pressure fluctuations produced when something vibrates and set forces ripple through air, water, or solids. In practice, this definition focuses on audible events within the ear’s sensitivity range, which is why it’s framed around detectability by humans.

Think of it this way: a vibrating object creates waves of compressions and rarefactions that move through the surrounding medium. If those fluctuations are strong enough and within the ear’s hearing range, we perceive them as sound. That’s why a simple surface vibration isn’t by itself sound—the important aspect is the propagating pressure variation in a medium that the ear can sense. And while random noises can be sounds, the defining feature here is audibility, not whether the sound is musical or structured.