Waves and sounds

Waves

When we throw a rock into the water we can see growing circles on the water surface. The same can also be seen when water drips onto a water surface.

Figure: Circles on a water surface caused by water dripping

Although these circles grow outwards from the place where a rock or water drop has fallen into the water, the water itself does not move that way. At each place, the water surface is only moving slightly up and down from its equilibrium position. Up-down movements from an equilibrium position are called vibrations.  The vibrations of the water surface particles are coordinated in such a way that the circles on the water surface are growing. 

Each water particle on the surface starts to imitate the vibrations of the neighboring, already vibrating particle, with a short delay. This causes the traveling of any disturbance on the water surface outward from the place where it originated. In physics, such an appearance is called a wave. If we photograph a water surface in the presence of a wave, we can see that it has crests and troughs. The crests are the places where the circles on the water surface are high while troughs are the places where they are low. The length between two crests is called a wavelength and the height of the crests is called amplitude.

Figure: The cross section of a water surface in the presence of a wave

Sounds

Sound is a wave. What we hear are the vibrations of the particles of air. The source of sound disturbs the surrounding particles of air, making them vibrate. This disturbance travels to our ears. The source of sound can be a musical instrument or a chalk that scratches a blackboard. While we find some sounds pleasant, others sound unpleasant. We find random vibrations of air unpleasant and call them noise.