Sound waves start from the sound source and propagate in a certain direction in the same medium. The envelope surface of each point reached by the sound wave at the same time is called the wave front. A wave whose wavefront is concentric spherical is called a spherical wave. It is emitted by a point sound source. When the scale of the sound source is much smaller than the wavelength of the sound wave radiated by it, it can be regarded as a point sound source. The wave front is a coaxial cylindrical wave, called a cylindrical wave. It is emitted by a line sound source. If many single point sound sources that are close together are arranged in a straight line, a line sound source is formed. A wave whose wavefront is a parallel plane perpendicular to the direction of propagation is called a plane wave. It is emitted by a surface sound source. Near a large vibrating surface, the sound wave is close to a plane wave. If many sound sources that are very close together are placed on a flat surface, it is also similar to a plane wave sound source.
We often use sound rays to indicate the direction of sound waves. In an isotropic medium, sound rays and wave fronts are perpendicular to each other.