Sound surrounds us all the time: voices, music, traffic, birdsong, the hum of a refrigerator. Yet sound itself is invisible. It is a traveling disturbance, a wave of pressure moving through air, water or solid materials. The physics of sound explains why you see lightning before you hear thunder, why a siren's pitch drops as it passes, why your voice echoes in a canyon and why there is no sound in space. This explainer describes how sound is created, how it travels and the fascinating effects that arise along the way.
Sound is a vibration
All sounds begin with something vibrating: a guitar string, a drum skin, your vocal cords, a loudspeaker cone. As the object vibrates, it pushes and pulls on the surrounding medium, usually air.
When a vibrating surface moves outward, it pushes air molecules closer together, creating a region of higher pressure called a compression. When it moves back, it leaves a region of lower pressure called a rarefaction. These alternating compressions and rarefactions travel outward from the source as a sound wave.
Importantly, the air itself does not travel from the source to your ear. Each air molecule only moves back and forth a tiny distance, bumping into its neighbors and passing the disturbance along, much like a wave moving through a line of people in a stadium.
Longitudinal waves
Sound waves in air are longitudinal waves: the molecules vibrate back and forth in the same direction that the wave travels. This is different from transverse waves, like waves on a rope, where the motion is perpendicular to the direction of travel. In solids, sound can travel as both longitudinal and transverse waves.
Describing a sound wave
Sound waves have several key properties:
- Frequency: The number of compressions passing a point each second, measured in hertz (Hz). Higher frequency means higher pitch.
- Wavelength: The distance between two neighboring compressions. Higher frequency sounds have shorter wavelengths.
- Amplitude: The size of the pressure changes. Larger amplitude means louder sound.
- Speed: How fast the wave travels through the medium.
Frequency, wavelength and speed are related by a simple equation: speed equals frequency times wavelength.
Humans can typically hear frequencies between about 20 Hz and 20,000 Hz. Sounds below this range are called infrasound, which elephants and whales use to communicate over long distances. Sounds above it are called ultrasound, which bats and dolphins use for echolocation and doctors use for medical imaging.
The speed of sound
The speed of sound depends on the medium it travels through and on its temperature.
| Medium | Approximate speed of sound |
|---|---|
| Air at 68 °F (20 °C) | about 767 mph (343 m/s) |
| Water | about 3,300 mph (1,480 m/s) |
| Steel | about 13,000 mph (about 5,900 m/s) |
Sound travels faster in liquids and solids than in air because their molecules are more tightly connected and pass vibrations along more quickly. The stiffness of a material matters more than its density.
In air, sound travels faster at higher temperatures because warmer air molecules move faster. The speed increases by about 0.6 m/s for every degree Celsius.
Lightning and thunder
Light travels about 874,000 times faster than sound in air. When lightning strikes, you see the flash almost instantly, but the sound of thunder takes time to arrive. Sound travels about 1 mile in 5 seconds, or about 1 kilometer in 3 seconds. By counting the seconds between the flash and the thunder, you can estimate how far away the lightning is.
No sound in space
Sound needs a medium to travel through. In the near-vacuum of space, there are almost no molecules to carry vibrations, so sound cannot travel. Explosions in space would be silent, despite what movies show. Astronauts communicate by radio, which uses electromagnetic waves that can travel through a vacuum.
Reflection: echoes
When sound waves hit a hard surface, such as a cliff, wall or building, they bounce back. This is reflection. If the reflected sound reaches your ears at least about a tenth of a second after the original sound, you hear it as a separate echo. Since sound travels about 34 meters in a tenth of a second, the reflecting surface must be at least about 17 meters away, because the sound has to travel there and back.
In smaller rooms, reflected sounds arrive too quickly to be heard separately. Instead, they blend together and prolong the sound, an effect called reverberation. Concert halls are carefully designed to have the right amount of reverberation for music. Soft materials like curtains, carpets and acoustic panels absorb sound and reduce reverberation.
Echolocation and sonar
Bats, dolphins and some whales use echoes to navigate and hunt. They emit high-frequency sounds and listen for the echoes, judging the distance, size and even texture of objects by the time and character of the returning sound. People use the same principle in sonar, which uses sound to map the seafloor and detect submarines, and in medical ultrasound, which creates images of organs and unborn babies using high-frequency sound waves.
Refraction and diffraction
- Refraction: Sound waves bend when they pass through layers of air with different temperatures, because sound travels at different speeds in each layer. On calm evenings, when the air near the ground is cooler than the air above, sound can bend back toward the ground and travel unusually far. That is why sounds across a lake can be heard clearly at night.
- Diffraction: Sound waves bend around obstacles and spread through openings. Because sound wavelengths are similar in size to everyday objects, you can hear someone talking around a corner even when you cannot see them. Low-frequency sounds, with longer wavelengths, diffract more easily, which is why you hear the bass of music from a neighboring room more than the high notes.
Resonance
Every object has natural frequencies at which it tends to vibrate. If a sound wave matches one of these frequencies, it can make the object vibrate strongly. This is called resonance.
Musical instruments use resonance to amplify sounds. The body of a guitar or violin resonates with the vibrating strings, making the sound louder and richer. The air inside a flute or organ pipe resonates at specific frequencies, producing musical notes. A singer who hits exactly the right note can, under special conditions, make a thin wine glass vibrate so strongly that it shatters.
The Doppler effect
You have probably noticed that the siren of an ambulance sounds higher in pitch as it approaches and lower as it moves away. This is the Doppler effect, named after the Austrian physicist Christian Doppler, who described it in 1842.
As the ambulance moves toward you, each sound wave is emitted from a position slightly closer to you than the previous one. The waves bunch up, so their wavelength becomes shorter and their frequency higher. As the ambulance moves away, the waves spread out, so the frequency is lower.
The Doppler effect is used in many technologies: police radar guns measure the speed of cars, weather radar measures wind speeds in storms, doctors measure blood flow with Doppler ultrasound, and astronomers measure how fast stars and galaxies are moving toward or away from us using a similar effect with light.
Sonic booms
When an airplane flies faster than the speed of sound, it moves faster than the sound waves it creates. The waves pile up into a cone-shaped shock wave that trails behind the aircraft. When this shock wave passes over people on the ground, they hear a loud sonic boom, like a sharp clap of thunder. A sonic boom is not a single event when the plane breaks the sound barrier; it travels continuously with the aircraft as long as it flies faster than sound.
The crack of a whip is a small sonic boom: the tip of the whip moves faster than the speed of sound.
Questions readers ask
Why does sound travel faster in water than in air? Water molecules are more tightly connected, so vibrations pass between them more quickly.
Why is there no sound in space? Sound needs a medium, such as air or water, to travel through. Space is nearly a vacuum.
What causes an echo? Sound waves reflect off a distant hard surface and return to your ears after a noticeable delay.
Why does a siren change pitch as it passes? The Doppler effect: sound waves bunch up as the source approaches and spread out as it moves away.
What is a sonic boom? A loud sound created by the shock wave of an object traveling faster than the speed of sound.
In short
Sound is a wave of pressure created by vibrating objects and carried by the molecules of air, water or solids. It travels at about 767 miles per hour (343 meters per second) in air at room temperature, much faster in water and steel, and not at all in the vacuum of space. Sound reflects as echoes, bends around obstacles, resonates in instruments and shifts in pitch with the Doppler effect. When objects move faster than sound, they create sonic booms. All around us, sound is physics in motion, carrying information, music and emotion from one place to another.