The speed of sound depends primarily on the material it travels through and the temperature of that material. Under standard sea-level conditions (15°C, or 59°F), sound moves through air at about 340 meters per second, roughly 760 miles per hour. Change the medium, raise the temperature, or alter the composition of a gas, and that number shifts considerably.
Temperature: The Dominant Factor in Air
In air and other gases, temperature is the single biggest influence on how fast sound travels. The relationship is straightforward: sound speed equals a constant multiplied by the square root of the absolute temperature. Warmer air means faster-moving molecules, which pass vibrations along more quickly. At 0°C, sound in dry air travels at about 331 m/s. At 20°C, it climbs to roughly 343 m/s. That’s a gain of about 0.6 m/s for every degree Celsius.
This is why sound behaves differently on a cold winter morning than on a hot summer afternoon. It’s also critical in aviation: as a plane climbs through the troposphere, air temperature drops at a fairly steady rate (about 6.5°C per kilometer), which means the local speed of sound decreases with altitude. The Mach number pilots rely on is simply their airspeed divided by the local speed of sound, so the same jet velocity represents a higher Mach number at cruising altitude than at sea level.
The Medium Itself: Stiffness vs. Density
Sound is a mechanical wave, and its speed in any material comes down to a tug-of-war between two properties: how stiff (or hard to compress) the material is, and how dense it is. Stiffer materials push vibrations forward more forcefully; denser materials have more mass to move, which slows things down. The general formula is the square root of the material’s stiffness divided by its density.
This explains a pattern that surprises many people: sound travels faster in solids and liquids than in air, even though solids and liquids are far denser. The reason is that solids and liquids are also vastly stiffer, and the stiffness wins out. Some rough comparisons at 20°C:
- Dry air: ~343 m/s
- Fresh water: ~1,482 m/s
- Seawater (3.5% salinity): ~1,522 m/s
- Stainless steel: ~5,790 m/s
Steel is about 17 times faster than air. Water is about four times faster. In each case, the material’s resistance to compression far outweighs the drag of its extra mass.
Gas Composition and Molecular Weight
Not all gases carry sound at the same speed, even at identical temperatures. The key variable is the molecular weight of the gas. Lighter molecules accelerate more easily when a pressure wave passes through, so sound travels faster in gases made of small, light molecules. Helium, with a molar mass about seven times lighter than air’s average, carries sound at roughly 1,007 m/s at room temperature. That’s nearly three times faster than in regular air, and it’s the reason your voice sounds high-pitched after inhaling helium: the higher sound speed shifts the resonant frequencies of your vocal tract upward.
Heavier gases do the opposite. Sulfur hexafluoride, sometimes used in demonstrations, has a molar mass about five times that of air. Sound crawls through it at only about 134 m/s, making your voice sound comically deep.
Humidity: A Smaller but Real Effect
Adding moisture to air makes sound travel slightly faster. This seems counterintuitive if you think of water as “heavy,” but water vapor molecules (molecular weight 18) are actually lighter than the nitrogen (28) and oxygen (32) molecules they displace. Humid air is, on average, a lighter gas mixture than dry air, so sound speeds up.
The effect is modest. Below about 30% relative humidity, the change is small and nonlinear. Above 30%, the increase becomes roughly linear with moisture content. At typical indoor conditions the difference between bone-dry air and very humid air might be 1 to 2 m/s. It’s measurable, and it matters in precision acoustics, but for everyday purposes temperature dwarfs it.
Why Pressure Doesn’t Matter (in Gases)
One of the most common misconceptions is that higher air pressure should make sound faster. In an ideal gas, it doesn’t. Here’s why: raising the pressure does increase the gas’s stiffness, which would speed sound up. But it also increases the density by exactly the same proportion, which would slow sound down. The two effects cancel perfectly. Sound at the top of a mountain and at sea level travels at the same speed, provided the temperature is the same. In practice, mountaintop air is colder, so sound is slower there, but the lower pressure itself is not the reason.
Sound Speed in the Ocean
Underwater acoustics involve three interacting variables: temperature, salinity, and depth (which corresponds to pressure). Unlike in air, pressure does matter in liquids because water isn’t an ideal gas and doesn’t compress the same way. Higher pressure at greater depths makes seawater stiffer without a proportional increase in density, so sound speeds up.
Temperature and salinity also push sound speed around. Warmer water transmits sound faster, and saltier water is slightly stiffer than fresh water. In the upper ocean, temperature typically dominates: as you descend from the warm surface through the thermocline, sound slows down. Below the thermocline, temperature stabilizes and the increasing pressure takes over, causing sound speed to climb again. The result is a minimum-speed layer, usually between 600 and 1,200 meters deep, called the SOFAR channel. Sound waves naturally bend toward this layer and can travel enormous distances with little energy loss, a phenomenon the military and marine researchers have exploited for decades.
Practical Takeaways
If you’re trying to estimate how fast sound will travel in a given situation, start with the medium. Solids transmit sound fastest, then liquids, then gases. Within a gas, focus on temperature: every 1°C increase in air temperature adds roughly 0.6 m/s. Gas composition matters if you’re working with something other than normal air. Humidity and altitude have real but secondary effects, and in an ideal gas, pressure alone changes nothing. In water, all three variables (temperature, salinity, and depth) are in play, and their interaction creates the layered sound-speed profiles that define underwater acoustics.