Sounds are pressure waves of air. If there wasn't any air, we wouldn't be able to hear sounds. There's no sound in space. We hear sounds because our ears are sensitive to these pressure waves.

Perhaps the easiest type of sound wave to understand is a short, sudden event like a clap. When you clap your hands, the air that was between your hands is pushed aside. A hand clap is a short event that causes a single pressure wave that quickly dies out. The image above shows the waveform for a typical hand clap.

When a sound is made the air pressure in the space near sound object, air molecules are temporarily compressed into less space. The high pressure pushes the air molecules outwards in all directions at the speed of sound, which is about 340 meters per second. When the pressure wave reaches your ear, it pushes on your eardrum slightly, causing you to hear the sound.


In waveforms, the horizontal axis represents time, and the vertical axis is for pressure. The initial high pressure is followed by low pressure, but the oscillation quickly dies out.

Periodic periodic wave.
When you ring a bell, after the initial strike (which is a little like a hand clap), the sound comes from the vibration of the bell. While the bell is still ringing, it vibrates at a particular frequency, depending on the size and shape of the bell, and this causes the nearby air to vibrate with the same frequency. This causes pressure waves of air to travel outwards from the bell, again at the speed of sound.

Pressure waves from continuous vibration