Sound is a vibration
Sound begins with something vibrating: a guitar string, a loudspeaker cone, vocal cords or a drum skin. As the object moves back and forth, it pushes on the surrounding air. The air molecules are squeezed together in one place and spread apart in the next. This pattern of squeezes and stretches, called compressions and rarefactions, moves outward from the source.
An important detail is that the air itself does not travel from the source to your ear. Each molecule only jiggles back and forth a tiny distance. What moves is the disturbance, passing from molecule to molecule, rather like a wave traveling through a crowd doing “the wave” while each person stays in place. Sound waves of this kind are called longitudinal waves, because the jiggling is in the same direction as the wave’s travel. This differs from ripples on water, where the motion is mainly up and down.
When the pressure changes reach your ear, they make your eardrum vibrate, and the rest of the process is described in the article on the hearing process.
Why sound needs a medium
Because sound is a disturbance passed between particles, it needs something made of particles to travel through. That material is called a medium, and it can be a gas, a liquid or a solid. In a vacuum, where there are essentially no particles, sound cannot travel. This is why, in space, there is no sound in the way we experience it on Earth, and astronauts outside a spacecraft communicate by radio, which uses electromagnetic waves that do not need a medium. Light, treated in the science of light, is the contrast: it crosses empty space easily.
Speed of sound
The speed of sound depends on the medium and its conditions. Rounded typical values are:
- Air at about 20 degrees Celsius: roughly 343 meters per second.
- Water: roughly 1,500 meters per second.
- Steel: roughly 5,000 meters per second.
Sound tends to travel faster in solids and liquids than in gases because the particles are closer together and respond more quickly to each other’s push. In air, the speed rises a little as the temperature rises.
This is why lightning and thunder arrive at different times. Light reaches you almost instantly, while sound takes about 3 seconds to cover each kilometer (1,000 ÷ 343 is about 2.9). Counting the seconds between the flash and the thunderclap and dividing by 3 gives a rough distance to the strike in kilometers. The physics behind a lightning strike itself is a separate topic.
Pitch and frequency
Pitch is how high or low a sound seems. It is determined by frequency, the number of vibrations per second, measured in hertz (Hz). A fast vibration gives a high pitch, as with a whistle, and a slow one gives a low pitch, as with a bass drum. Shortening a guitar string by pressing a fret raises its frequency and so its pitch.
Humans with typical hearing can detect frequencies from about 20 Hz to about 20,000 Hz, although the upper limit tends to fall with age. Sounds below this range are called infrasound, and above it, ultrasound. Some animals respond to frequencies outside the human range. Bats, for example, use ultrasound to navigate.
Volume and amplitude
Loudness depends mainly on amplitude, which is the size of the pressure variations in the wave. A larger vibration carries more energy and sounds louder. Loudness is commonly measured in decibels, a logarithmic scale. That means each increase of 10 decibels corresponds to a tenfold increase in sound intensity, although to the ear it seems only about twice as loud. Because of the scale, a small number of decibels can represent a large difference in energy.
Very loud sounds, particularly if heard for long periods, can damage hearing. Ear protection is advisable in noisy environments, and anyone concerned about their hearing should speak with a qualified professional.
Why sounds differ
Two instruments playing the same note at the same loudness still sound different. This is because most sounds are mixtures of several frequencies: a main pitch plus higher overtones in various proportions. The mix gives each instrument or voice its distinct quality, known as timbre.
What happens to sound on the way
Sound waves can be reflected, absorbed and bent. An echo is a reflection from a hard surface that is far enough away for you to hear it separately. Soft materials such as curtains, carpet and foam absorb sound and reduce echoes, which is why a bare room sounds more echoey than a furnished one. Sound also spreads out as it travels, so it fades with distance.
The same physics operates on a vast scale in the ground. Vibrations from an earthquake travel through the Earth as waves, and the study of how those waves move helps scientists learn about the planet’s interior, a subject covered in earthquake causes.