The Speed of Sound: How Fast Does Sound Travel?

The speed of sound is the distance a sound wave — a mechanical vibration of pressure carried through a medium — travels per unit of time. In dry air at 20°C (68°F) and standard atmospheric pressure, sound travels at 343 meters per second (m/s), which is approximately 1,235 kilometers per hour (km/h) or 767 miles per hour (mph). Unlike light, sound cannot travel through empty space; its speed depends entirely on the physical properties — elasticity and density — of the material it moves through.

This single number, 343 m/s, is one of the most cited constants in acoustics and aviation, but it is not fixed. It changes with temperature, humidity, altitude, and — most dramatically — with the medium itself. Sound moves roughly 4.3 times faster through water than through air, and more than 17 times faster through steel. This article explains why, with verified reference values, formulas, and the physics behind them.

How Fast Does Sound Travel in Air?

At 20°C (68°F), in dry air at sea-level atmospheric pressure, the speed of sound is 343 m/s (1,235 km/h; 767 mph). This is the figure most commonly quoted in acoustics, audio engineering, and physics textbooks, and it is the value used as the reference condition in most sound intensity and acoustic impedance calculations.

A closely related but slightly different reference value comes from aviation: the International Standard Atmosphere (ISA) defines sea-level conditions at 15°C, giving a speed of sound of 340.3 m/s (1,225 km/h; 761 mph). This is the value aircraft instruments use as the baseline for Mach number calculations. Neither figure is “more correct” than the other — they simply describe air at two different reference temperatures, 20°C versus 15°C. This is the first clue to the most important variable affecting the speed of sound in air: temperature, covered in detail below.

For quick reference, converted and rounded:

• 343 m/s ≈ 1,235 km/h ≈ 767 mph ≈ 1,125 ft/s (dry air, 20°C, 1 atm)
• 340.3 m/s ≈ 1,225 km/h ≈ 761 mph (ISA reference, 15°C, 1 atm)

Both values assume dry air. Humid air is slightly less dense on a per-molecule basis (water vapor, at molar mass ~18 g/mol, is lighter than the N₂/O₂ mix of dry air at ~29 g/mol), which very slightly raises the speed of sound — by roughly 0.1–0.6 m/s per 10% relative humidity change at room temperature, a small but measurable effect used in precision acoustic metrology.

Why Does the Speed of Sound Depend on the Medium?

Sound is a longitudinal wave: a chain reaction of compressions and rarefactions in which particles push against their neighbors and pass the disturbance along. How quickly that chain reaction propagates depends on two competing physical properties of the medium:

1. Elasticity (stiffness) — how strongly the medium resists being compressed and how quickly it springs back. A stiffer medium pushes back on its neighboring particles faster, propagating the disturbance more quickly.
2. Density (inertia) — how much mass must be accelerated by that elastic restoring force. A denser medium has more inertia, which slows the wave down.

The general relationship, derived from Newton’s laws applied to a compressible continuum, is:

c = √(B / ρ)

where c is the speed of sound, B is the (adiabatic) bulk modulus of elasticity in pascals, and ρ is the density of the medium in kg/m³. This is the master formula for fluids (gases and liquids). For solids, the relevant stiffness term is Young’s modulus E (for a thin rod) or the combined bulk-and-shear modulus for wave propagation through a large solid mass:

c = √(E / ρ) (thin solid rod, longitudinal wave)

c_L = √[(K + 4G/3) / ρ] (longitudinal wave in a bulk solid, K = bulk modulus, G = shear modulus)

The key insight — and the source of most confusion about this topic — is that stiffness typically increases much faster than density as you move from gas to liquid to solid. Steel is about 7.7 times denser than water, but its resistance to compression (its bulk/Young’s modulus) is on the order of 25,000 times greater. The stiffness term wins by a wide margin, so sound travels faster in steel than in water, and faster in water than in air, even though the density ranking runs in the opposite direction. This is the direct physical link between speed of sound and acoustic impedance (Z = ρc), which governs how much sound energy reflects or transmits at a boundary between two media.

For gases specifically, the bulk modulus is proportional to pressure and the adiabatic index, and combining this with the ideal gas law gives a more practical form:

c = √(γRT / M)

where γ (gamma) is the adiabatic index (≈1.4 for the diatomic gases N₂ and O₂ that make up dry air), R is the universal gas constant (8.314 J/(mol·K)), T is the absolute temperature in kelvin, and M is the molar mass of the gas (≈0.02897 kg/mol for dry air). Two important consequences fall out of this equation:

• Speed of sound in a gas depends on temperature, not on pressure, at a fixed composition. Doubling atmospheric pressure at constant temperature also doubles the density, and the two effects cancel out in the ratio B/ρ. This is a common misconception worth stating plainly: air pressure changes with weather or altitude do not directly change the speed of sound — only the accompanying temperature change does.
• Lighter gas molecules carry sound faster. Helium (molar mass ≈4 g/mol) has a speed of sound around 972 m/s at 0°C — nearly three times that of air — despite having lower density than air, because the 1/√M term dominates. Carbon dioxide (molar mass ≈44 g/mol), heavier than air, carries sound more slowly, around 259 m/s at 0°C. This is also why inhaling helium raises the pitch of a person’s voice: the resonant frequencies of the vocal tract scale with the local speed of sound.

Does Sound Travel Faster in Water or Air?

Sound travels faster in water than in air — about 4.3 times faster. In fresh water at 20°C, the speed of sound is approximately 1,481 m/s (5,332 km/h; 3,314 mph), compared with 343 m/s in air at the same temperature. In seawater, the average speed is even higher, around 1,500 m/s, because dissolved salts increase the water’s bulk modulus and density in a way that slightly favors a higher propagation speed overall (values recorded by NOAA’s ocean acoustics program range roughly 1,450–1,570 m/s depending on temperature, salinity, and depth/pressure).

This is consistently one of the most counterintuitive facts in basic acoustics: people often assume sound should move more slowly through a denser substance like water. But per the c = √(B/ρ) relationship above, water’s incompressibility (its very high bulk modulus, roughly 2.2 GPa versus about 0.000142 GPa for air) far outweighs its higher density (about 1,000 kg/m³ versus about 1.2 kg/m³ for air). The net result is a much faster wave speed. This is why whales and submarines can communicate and detect objects across tens or hundreds of kilometers of ocean — water is a remarkably efficient, low-loss medium for long-range acoustic transmission, one reason underwater Doppler effect sonar systems work at such extended ranges.

Does Sound Travel Faster Through Solids?

Yes — solids generally carry sound faster than either liquids or gases, for the same reason liquids beat gases: dramatically higher stiffness relative to density. In structural steel, the speed of sound (longitudinal wave) is approximately 5,960 m/s — over 17 times faster than in air and about 4 times faster than in water. In diamond, the stiffest naturally occurring bulk material, the speed of sound reaches roughly 12,000 m/s, the highest value recorded for any material, a direct consequence of the exceptionally strong, short covalent carbon-carbon bonds that give diamond both extreme stiffness and relatively low mass per bond.

This solid-state behavior is why pressing an ear to a railway track lets you hear an approaching train long before the sound arrives through the air, and why knocking on a long metal pipe produces a signal at the far end almost instantaneously compared to shouting the same distance. It is also the physical basis of ultrasonic non-destructive testing, in which engineers send high-frequency ultrasound pulses through metal components and time their echoes to detect internal flaws.

Table 1: Speed of Sound Across Media

All solid and liquid values above represent longitudinal (compression) wave speeds; actual figures vary somewhat with alloy composition, grain direction, temperature, and measurement method.

How Does Temperature Affect the Speed of Sound?

In air, the speed of sound rises with temperature because warmer air molecules move faster on average, transmitting the compression-rarefaction chain of a sound wave more quickly. The commonly used linear approximation, valid across the ordinary range of atmospheric temperatures, is:

c ≈ 331.3 + 0.606 × T (m/s), where T is the air temperature in degrees Celsius.

This approximation is a linearization of the exact ideal-gas relationship c = √(γRT/M) around 0°C, and it matches that formula closely across normal weather and room-temperature ranges: at T = 0°C it gives 331.3 m/s (the standard reference value for dry air at 0°C, 1 atm), and at T = 20°C it gives 331.3 + 0.606(20) = 343.4 m/s, matching the commonly cited 343 m/s figure. At T = 15°C it gives 340.3 m/s — matching the ISA standard atmosphere value exactly, which is a strong internal cross-check of the formula’s accuracy.

Table 2: Speed of Sound in Air vs. Temperature

Because temperature normally decreases with altitude, the speed of sound also decreases with altitude — not because of the drop in air pressure itself, but because of the associated drop in temperature. At the cruising altitude of a commercial jet (around 11,000 m / 36,000 ft, in the lower stratosphere), the standard atmosphere temperature is about −56.5°C, and the local speed of sound falls to roughly 295 m/s (about 1,062 km/h / 660 mph) — noticeably lower than the 343 m/s figure at sea level. This is precisely why aircraft speed is expressed in Mach number relative to the local speed of sound rather than as a fixed absolute figure.

What Is Mach 1?

Mach 1 is the speed of sound in the local medium at the moment and altitude in question — it is not a single fixed velocity, but a ratio. The Mach number (named after physicist Ernst Mach) is defined as:

Mach number = object’s speed ÷ local speed of sound

An object traveling at exactly the local speed of sound is traveling at Mach 1. At sea level in air at 20°C, Mach 1 equals 343 m/s (1,235 km/h; 767 mph). At a commercial jet’s cruising altitude, where the air is much colder, Mach 1 corresponds to only about 295 m/s (660 mph) — which is why a jet cruising at “Mach 0.85” is physically moving at roughly 0.85 × 660 mph ≈ 560 mph true airspeed, even though the ratio to the local speed of sound is unchanged.

Flight regimes are commonly categorized by Mach number: subsonic (Mach < 1), transonic (roughly Mach 0.8–1.2, where shock waves begin forming over parts of an aircraft), supersonic (Mach 1–5), and hypersonic (Mach > 5). A classic real-world illustration of crossing Mach 1 is the sonic boom: when an aircraft exceeds the local speed of sound, it outruns the pressure waves it generates, which pile up into a shockwave that reaches the ground as a sharp, thunderclap-like report — the same physical principle behind the crack of a bullwhip, whose tip briefly exceeds Mach 1.

Speed of Sound vs. Speed of Light: The Thunder-Lightning Example

One of the most familiar everyday demonstrations of the finite, comparatively slow speed of sound is the delay between seeing lightning and hearing thunder. Light travels at roughly 300,000 km/s (186,000 miles/s) — so fast that a flash of lightning is, for all practical human-scale purposes, seen instantaneously. Sound, at only about 343 m/s, takes roughly 3 seconds to travel 1 km (or about 5 seconds per mile). Counting the seconds between the flash and the boom and dividing by three (in metric terms) gives a rough distance to the storm in kilometers — a simple, direct consequence of the numeric value discussed throughout this article.

Does Sound Travel Faster or Slower Than the Speed of Light?

Sound is always dramatically slower than light. Even the fastest known bulk material speed of sound (diamond, ~12,000 m/s) is still roughly 25,000 times slower than the speed of light in a vacuum (299,792,458 m/s). Sound and light are also fundamentally different phenomena: light is an electromagnetic wave that requires no medium and can cross a vacuum, while sound is a mechanical (pressure) wave that absolutely requires a medium — a fact relevant to the myth addressed below.

Myth vs. Fact

Myth: “Sound travels faster in a vacuum, since there’s nothing in the way.”
Fact: False. Sound cannot travel through a vacuum at all. Sound is a mechanical wave — it needs a physical medium of particles to compress and transmit the vibration. In a perfect vacuum, there is no matter to carry the wave, so the “speed of sound” there is not merely slow, it is undefined/zero. This is why explosions in space are silent in physically accurate depictions — a detail famously (and deliberately) ignored in most films.

Myth: “Sound is slower in water than in air, because water is thicker/denser.”
Fact: False — the opposite is true. Sound travels roughly 4.3 times faster in water (~1,481 m/s) than in air (~343 m/s), because water’s much higher stiffness (bulk modulus) outweighs its higher density in the c = √(B/ρ) relationship. Density alone does not determine wave speed; the ratio of stiffness to density does.

Myth: “Higher air pressure makes sound travel faster.”
Fact: False, for a fixed gas composition. In the ideal-gas approximation, the speed of sound in air depends on temperature, not on atmospheric pressure — pressure changes and density changes cancel out in the underlying formula. Weather-related “pressure effects” on sound propagation distance are really about temperature gradients and wind, not static pressure itself.

Why Does the Speed of Sound Matter?

The speed of sound is not an isolated trivia figure — it is a load-bearing constant that connects several other core acoustic quantities. It links frequency and wavelength directly through the wave equation λ = c / f (wavelength equals speed divided by frequency), meaning that as the speed of sound changes with temperature, the wavelength of a given fixed-frequency tone changes too — a real effect in outdoor sound systems and musical instrument tuning across seasons. It also underlies acoustic impedance (Z = ρc), which determines how much sound energy reflects versus transmits at a boundary between two media (for example, air to water, or air to a wall) — a governing principle in architectural sound insulation and underwater acoustics alike. The speed of sound is also the reference velocity used in Doppler shift calculations (the Doppler effect), in the geometric spreading described by the inverse-square law, and in how sound intensity and the physical scale of longitudinal pressure waves are calculated and measured in decibels.

Frequently Asked Questions

What is the speed of sound in mph and km/h?
In dry air at 20°C, the speed of sound is 343 m/s, which converts to approximately 1,235 km/h or 767 mph. At the ISA reference temperature of 15°C, it is 340.3 m/s (about 1,225 km/h / 761 mph).

Does sound travel faster in water or air?
Sound travels faster in water. At 20°C, sound moves at roughly 1,481 m/s in fresh water versus 343 m/s in air — about 4.3 times faster — because water’s high stiffness (bulk modulus) far outweighs its higher density.

What is the speed of sound in steel?
The speed of sound (longitudinal wave) in structural steel is approximately 5,960 m/s, more than 17 times faster than in air, due to steel’s very high elastic stiffness relative to its density.

What is Mach 1 in mph?
At sea level in air at 20°C, Mach 1 is about 767 mph (343 m/s). Mach 1 is not a fixed speed everywhere, though — it equals the local speed of sound, which is lower at high, cold altitudes (around 660 mph / 295 m/s near typical jet cruising altitude).

Why does sound travel faster in solids than in gases, if solids are denser?
Because wave speed depends on the ratio of stiffness to density (c = √(B/ρ) or √(E/ρ)), not on density alone. Solids are far stiffer (more resistant to compression) than gases — often by several orders of magnitude — and that stiffness advantage outweighs their higher density, resulting in a faster wave speed overall.

Does altitude affect the speed of sound?
Yes, indirectly. The speed of sound in air depends on temperature, and temperature typically falls with increasing altitude. At commercial jet cruising altitude (around 11 km), the speed of sound drops to roughly 295 m/s, compared with 343 m/s at sea level on a 20°C day.

Is the speed of sound the same as the speed of a sonic boom?
The sonic boom is the audible shockwave produced when an object exceeds the local speed of sound (Mach 1); the boom itself then propagates outward from the shock at the ordinary local speed of sound.