Sound travels as a wave of pressure moving through a physical medium, whether that’s air, water, steel, or any other substance with particles that can vibrate. It cannot travel through a vacuum. At room temperature (20°C), sound moves through air at about 343 meters per second (roughly 769 miles per hour), but it travels more than four times faster in water and nearly seventeen times faster in steel.
What a Sound Wave Actually Does
When something vibrates, like a guitar string or a clapping hand, it pushes the air molecules next to it. Those molecules bump into their neighbors, which bump into theirs, creating a chain reaction of tiny collisions. This produces alternating zones of high pressure (where molecules are bunched together) and low pressure (where they’re spread apart). These zones are called compressions and rarefactions, and together they form the wave.
The key thing to understand is that the particles themselves don’t travel with the sound. Each molecule just oscillates back and forth around its resting position, like a row of people doing “the wave” in a stadium. Nobody leaves their seat. What moves forward is the pattern of pressure, not the air itself. This type of wave, where particles vibrate in the same direction the wave travels, is called a longitudinal wave.
Why Sound Is Faster in Solids Than in Air
Two properties of a material determine how fast sound moves through it: stiffness and density. The stiffer a material is (meaning the harder it is to compress), the faster sound travels. The denser the material, the slower sound travels. Speed comes from the balance between these two factors.
Air is highly compressible, so sound moves through it relatively slowly at 343 m/s. Water is much harder to compress, which pushes the speed up to about 1,522 m/s in seawater. Stainless steel is harder to compress still, and sound races through it at roughly 5,790 m/s. Even though steel is far denser than air, its extreme rigidity more than compensates, giving it a much higher sound speed overall.
This is why you can sometimes hear a train coming by pressing your ear to the rail long before the sound reaches you through the air. The steel carries the vibration far more efficiently.
How Temperature Changes the Speed
In air, temperature has a direct effect on sound speed. Warmer air molecules move faster and collide more readily, transmitting pressure waves more quickly. The relationship is straightforward: for every degree Celsius the temperature rises, sound in dry air speeds up by about 0.6 m/s. At 0°C, sound travels at roughly 331 m/s. At 20°C, it reaches 343 m/s. At 35°C on a hot summer day, it climbs to about 352 m/s.
Humidity also plays a small role. Water vapor is lighter than the nitrogen and oxygen molecules it displaces, so humid air is slightly less dense than dry air. This means sound moves a bit faster on a muggy day than on a dry one, though the difference is modest compared to the effect of temperature.
What Happens When Sound Hits a Boundary
When a sound wave reaches the boundary between two different materials (say, air meeting a concrete wall), some of the wave’s energy reflects back and some passes through. How much reflects versus transmits depends on a property called acoustic impedance, which combines a material’s density and its sound speed into a single value.
If two materials have similar acoustic impedance, sound passes between them easily with little reflection. If the impedance values are very different, most of the sound bounces back. This is why you can still hear voices through a wooden door but a thick concrete wall blocks most sound. It’s also why sound has such a hard time moving from air into water, or from air into solid materials. The impedance gap between air and nearly every solid or liquid is enormous, so most airborne sound energy reflects off those surfaces rather than entering them.
How Sound Fades Over Distance
Sound from a point source spreads outward in all directions, like an expanding sphere. As it spreads, the same amount of energy covers a larger and larger area. This follows the inverse square law: double your distance from the source, and the sound intensity drops to one quarter. Triple the distance, and it falls to one ninth. This is why a shout that’s painfully loud at one meter becomes barely audible a few hundred meters away.
On top of this geometric spreading, the medium itself absorbs some energy. Air molecules convert a small fraction of sound energy into heat with each vibration. Higher-frequency sounds (like a whistle) lose energy to absorption faster than lower-frequency sounds (like a bass drum), which is why distant thunder sounds like a deep rumble. The sharp, high-frequency crack has already been filtered out by the atmosphere.
How Your Ear Receives Sound
Your ear is essentially an impedance-matching device. The problem it solves is significant: sound travels well through air, and your inner ear is filled with fluid, but the impedance mismatch between air and fluid means that roughly 99.9% of airborne sound energy would normally bounce off a fluid surface. Your middle ear exists to overcome this barrier.
The process starts at the eardrum, a thin membrane whose impedance closely matches that of air, so it vibrates readily in response to incoming pressure waves. These vibrations pass through three tiny bones (the smallest in your body), which act as a lever system. The last bone, the stapes, presses against a small membrane called the oval window that opens into the fluid-filled inner ear. Because the eardrum is much larger than the oval window, all that vibrational energy gets concentrated onto a smaller area, dramatically increasing the pressure. This amplification bridges the impedance gap and efficiently transfers sound energy into the cochlear fluid.
Inside the inner ear, the pressure waves in the fluid bend thousands of microscopic hair cells. Different positions along the coiled cochlea respond to different frequencies, with high-pitched sounds detected near the entrance and low-pitched sounds detected deeper in. When these hair cells bend, they generate electrical signals that travel along the auditory nerve to the brain, where they’re interpreted as the sounds you hear.
Why Sound Cannot Travel Through a Vacuum
Because sound is a mechanical wave, it requires particles to carry it. No particles, no sound. This has been demonstrated since the 1600s with a classic experiment: place a ringing bell inside a glass jar and pump the air out. As the air thins, the bell’s sound fades until it becomes completely silent, even though you can still see it vibrating. Let the air back in, and the sound returns immediately.
This is why space is silent. The near-perfect vacuum between stars and planets contains too few particles to sustain a pressure wave. Astronauts on spacewalks communicate by radio (electromagnetic waves, which need no medium) rather than by shouting, because their voices have no air to carry them.