Waves can be classified into different categories depending on the dimensions in which they can travel. When they travel along just one dimension they are called ** one-dimensional** (for instance, waves propagating along a rope or a string and sound waves propagating through a cylinder containing a gas).

A wave that travels in two dimensions is called **two-dimensional**, such as the waves shown in the picture below produced by a dropping an object into still water.

The waves produced by a musical instrument are **three-dimensional waves** because they can travel in the three dimensions of space.

In order to visualize the propagation of a wave it is useful to introduce the concept of **wavefront**.

**Wavefront** is a line (or surface) on which every point has the same phase.

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For a one dimensional wave, the wavefront would be just a point; for a two-dimensional wave such as the one shown in the picture above, wavefronts are circumferences. The wavefronts of a sound wave propagating along a homogeneous and isotropic medium is a sphere centered at the source. This is why such waves are called **spherical waves**.

The distance between two consecutive wavefronts is called the **wavelength** λ of the wave. The **rays** (in red in the figure below) are arrows pointing in the direction waves are moving, and are always perpendicular to the wavefronts.

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##### Plane wave

At great distances from the source, a small part of a spherical wavefront can be approximated by a plane, as shown in the figure below. In this case we have a **plane wave**.

The mathematical description of a plane wave is the same as that of a one-dimensional harmonic wave :

where A is the amplitude, ω the angular frequency and k the wavenumber.

Check out the page intensity of a sound wave to know more about the energy carried by a wave.

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