Sound

Clap your hands and, a fraction of a second later, someone across the room hears it. Nothing solid crossed the gap between you. What travelled was a vibration, passed from one tiny piece of air to the next. Sound is energy on the move, and once you understand how it travels, a great deal about music, echoes, thunder and even animal senses starts to make sense.

Vibrations and waves

Every sound starts with something vibrating: a guitar string, your vocal cords, a drum skin, a loudspeaker cone. As the object moves forward, it squashes the air molecules in front of it together. As it moves back, it leaves a region where the air is spread out. These bands of squashed and stretched air, called compressions and rarefactions, spread outwards like ripples on a pond. This is a sound wave.

The air itself does not travel with the sound. Each molecule just jiggles back and forth, bumping its neighbours and passing the energy on. When the wave reaches your ear, it makes your eardrum vibrate in the same pattern, and your brain turns those vibrations into what you hear.

Speed of sound

Because sound needs something to travel through, it cannot cross empty space. In air it moves at about 343 metres per second, roughly a kilometre every three seconds. It travels faster through liquids and faster still through solids, where the particles are packed closely together: about four times faster in water and fifteen times faster in steel.

Light is almost a million times faster than sound. That is why you see lightning before you hear thunder. Counting the seconds between flash and rumble and dividing by three gives a rough distance to the storm in kilometres.

Pitch and loudness

Two features describe most sounds. Pitch is how high or low a sound is, and it depends on frequency: how many vibrations pass each second, measured in hertz. A small, tight string vibrates quickly and makes a high note; a long, loose string vibrates slowly and makes a low one. Humans hear from about 20 hertz up to about 20,000 hertz, though the top of that range drops with age.

Loudness depends on how much energy the wave carries, which we see as the height of the wave on a graph. It is measured in decibels. The scale climbs quickly: every 10 decibels means ten times the energy. A whisper is about 30 decibels, normal talking about 60, a busy road about 85 and a rock concert well over 100. Long exposure to sounds above about 85 decibels can permanently damage the delicate hair cells in the inner ear.

Echoes and sonar

Sound bounces off hard surfaces, producing an echo. Large empty halls and caves are echoey because sound reflects from bare walls; curtains, carpets and furniture soak it up. Concert halls are carefully designed so that reflections make music sound rich without turning into a muddle.

Some animals use echoes to see with sound. Bats send out high-pitched calls and listen for the echoes to find moths in the dark. Dolphins use clicks to sense fish and objects underwater. People copied the idea with sonar, which ships use to map the seafloor and find shoals of fish.

Beyond our hearing

Sounds too high for humans to hear are called ultrasound. Dogs, bats and dolphins can hear them. Doctors use ultrasound to make images of babies in the womb and of organs, because the echoes from different tissues build up a picture without any harmful radiation. Sounds too low to hear, called infrasound, are made by earthquakes, volcanoes and elephants, which can call to each other across several kilometres.

Why it matters

Sound lets us talk, make music and sense danger. Understanding it has given us telephones, microphones, hearing aids, noise-cancelling headphones and medical scanners. It also reminds us to protect our hearing, one of the senses that connects us most closely to other people.