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Sound is a longitudinal wave created by vibrating objects. It needs a medium (solid, liquid, or gas) to travel — it cannot travel through a vacuum. The speed of sound depends on the medium and its temperature: about 343 m/s in air at room temperature, faster in liquids (~1,480 m/s in water), and fastest in solids (~5,960 m/s in steel). Warmer air also carries sound faster than cooler air.
Clap two blocks together and listen from various distances to sense that sound takes time to travel. Compare sounds transmitted through a table (solid) vs. through air by pressing an ear to the table while someone taps the far end. Calculate the time delay of thunder after lightning using the speed of sound.
Every sound you hear — a friend's voice, a guitar chord, a clap of thunder — starts with something vibrating. A guitar string vibrates, pushing air molecules back and forth. Those molecules push their neighbors, which push theirs, and a longitudinal wave of compressions and rarefactions spreads outward from the source. When this wave reaches your ear, it vibrates your eardrum, and your brain interprets the vibration pattern as sound.
The speed of sound depends on what the wave is traveling through. In air at room temperature (about 20°C), sound moves at approximately 343 m/s — fast enough to cross a football field in about one second, but slow enough that you can notice the delay between seeing distant fireworks and hearing the boom. In water, sound speeds up to about 1,480 m/s because water molecules are packed much more tightly than air molecules and can transmit vibrations faster. In steel, the speed reaches about 5,960 m/s — nearly 17 times faster than in air.
The pattern is simple: sound generally travels fastest in solids, slower in liquids, and slowest in gases. This is because the particles in solids are closest together and most strongly bonded, making it easy for a vibration to jump from one particle to the next. In gases, particles are far apart and move randomly, so vibrations spread more slowly.
Temperature also matters. In warmer air, gas molecules move faster on average and collide more frequently, allowing sound vibrations to be transmitted more quickly. The speed of sound in air increases by about 0.6 m/s for every 1°C rise in temperature. On a hot summer day (35°C), sound travels at about 352 m/s, while on a freezing winter day (0°C), it drops to about 331 m/s.
One crucial fact: sound cannot travel through a vacuum. In the famous tagline "In space, no one can hear you scream," the physics is correct. With no particles to compress and expand, there is nothing to carry the wave. This is why astronauts communicate by radio (electromagnetic waves, which do not need a medium) rather than by shouting. The difference between sound waves (needing matter) and light or radio waves (traveling through empty space) is one of the most fundamental distinctions in wave physics.