The types of sound sources are divided into point source, line source and area source according to their geometric shapes. Different sound sources have different acoustic radiation modes.

When the size of the sound source is relatively small relative to the wavelength or propagation distance of the sound wave, and the directivity of the sound source is not strong, it can be approximately regarded as a point sound source, such as a person talking and clapping hands, which radiates outward in the form of spherical wave. Usually we often hear the noise of trains and a large number of motor vehicles on the road. We can regard it as a linear sound source composed of many point sound sources, which radiate noise in the form of approximate cylindrical wave. The plane sound source radiates outward by plane wave, and the function of sound energy radiated on the plane is equal everywhere, as shown in the figure below. No matter where the listener stands in front of the speaker wall, he hears the same sound pressure. At present, this kind of sound source is relatively less used.

In our professional sound reinforcement system, the representative ones are line array speaker system and conventional speaker system (non-standard name of pure point source speaker system). When I first entered the industry, Xiaobian always thought that the line array was the line sound source. With the deepening of learning, I realized that this was not the case. When we review and understand the content mentioned in the previous paragraph, we can see that the acoustic wave front pattern radiated by the line array loudspeaker system is similar to the cylindrical wave, which is similar to the linear sound source. However, when it reaches a certain critical distance, it gradually changes from the cylindrical wave to the spherical wave and radiates outward, just like the conventional loudspeaker system.

If the line array loudspeaker system doubles the distance within the critical distance, the sound pressure level will be reduced by 3dB for attenuation. If it doubles the distance outside the critical distance, the sound pressure level will be reduced by 6dB for attenuation. The distance from 3dB to 6dB is called the critical distance. The longer the line array is, the farther the cylindrical wave will diverge into spherical wave. The radiation of any sound source with limited size in free space will always diverge from its wave front, and the cylindrical wave will eventually diverge into spherical wave, that is, from "the sound pressure level decreases by 3dB when the distance is doubled" to "the sound pressure level decreases by 6dB when the distance is doubled". Therefore, the line array is not completely a line source.