Sound travels through air at about 343 meters per second at 20°C, but longitudinal sound waves can travel through steel at roughly 5,000 to 6,000 meters per second, depending on the type of steel.

That enormous difference reveals something important: the speed of sound is not a property of sound alone. It depends on the material carrying the wave.

Stiffness helps the disturbance travel

Sound is a mechanical wave. As particles in a material are displaced, forces between them pass that disturbance to neighboring particles. How quickly this happens depends strongly on the material's elastic properties. A material that strongly resists compression or deformation can transmit a disturbance rapidly.

That helps explain why sound travels much faster through steel than through air. Water falls between them, with sound traveling at roughly 1,480 meters per second under typical conditions. But stiffness is only half of the story.

Density matters too

The speed of sound depends on a combination of a material's elasticity and its density. In simplified terms, greater stiffness tends to increase wave speed, while greater density tends to reduce it, all else held constant.

The important point is that these properties must be considered together. A dense material does not automatically carry sound slowly, just as a light material does not automatically carry it quickly.

Different kinds of waves also require different elastic properties in the calculation. In fluids, sound speed is related to bulk modulus and density. In solids, the situation is more complicated because solids can support both longitudinal and transverse waves.

Solids Are Often Fast, but Not Always

It is tempting to remember a simple rule: sound travels fastest in solids, slower in liquids, and slowest in gases. That pattern works for many familiar materials, but it is not a universal law.

The deeper reason is how the material responds mechanically. Gases are highly compressible, so pressure disturbances generally travel relatively slowly through them. Liquids resist compression much more strongly, allowing sound to travel faster. Many solids are extremely stiff, producing still higher wave speeds. But simply knowing that something is a solid is not enough. Soft polymers, for example, can have much lower sound speeds than stiff metals. The material's actual elastic properties and density are what matter.

Temperature shifts the numbers

The speed of sound can also change when a material's conditions change. In air, temperature has a particularly noticeable effect. At 0°C, sound travels at about 331 meters per second. At 20°C, it travels at roughly 343 meters per second.

Near ordinary temperatures, a useful approximation is an increase of about 0.6 meters per second for every 1°C rise in temperature. The underlying explanation of gases is better understood through thermodynamics than by simply saying that warmer molecules “move faster.” For an ideal gas, the sound speed depends on temperature as well as on properties such as the gas's molecular mass and heat capacity ratio.

In the atmosphere, temperature differences can even cause sound waves to refract, changing how far or in what direction sound is heard.

It Shows Up in Everyday Technology

These differences matter far beyond the physics classroom. Sonar systems depend on the speed of sound in water, which varies with factors such as temperature, salinity, and pressure. Those variations can bend sound paths and affect distance measurements.

Medical ultrasound also depends on an assumed sound speed. Diagnostic systems commonly use 1,540 meters per second as the average speed for soft tissue, even though actual speeds vary among tissues. The system uses the time it takes for echoes to return to estimate the locations of structures. Differences between the assumed and actual sound speed can therefore contribute to image-positioning errors and other artifacts.

The same principle appears in engineering and materials testing, where sound and ultrasound can help reveal thickness, cracks, internal defects, and differences between materials.

The speed of sound is not one fixed number. It emerges from the physical properties of the material that carries the wave, as well as from conditions such as temperature. That is why a vibration can travel through a steel rail far faster than sound traveling through the surrounding air, even though both began with the same event.