Translations:The wave equation/12/en

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Notice that the amplitude of any spherical wavefront is a function of its distance from the center because the radial term serves as an extrinsic attenuation factor. This extrinsic attenuation factor is a direct consequence of energy conservation, a phenomenon known as geometric spreading. That is, unlike a plane wave, a spherical wave decreases in amplitude as it expands and travels away from its source, thereby changing its profile. As a spherical wavefront propagates outward, its radius increases. Far enough away from the source, a small area of the wavefront therefore will resemble closely a portion of a plane wave (Figure 5).