Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 107

In practice many real engineering sources (such as machines and vehicles) are mounted or situated on hard reflecting ground and concrete surfaces. If we can assume that the source of sound power W radiates only to a half‐space solid angle 2π, and no power is absorbed by the hard surface (Figure 3.10), then

(3.52)

where L_W is the sound power level of the source and r is the distance in metres.

Figure 3.10 Source above a rigid surface.

In this discussion we have assumed that the sound source radiates the same sound intensity in all directions; that is, it is omnidirectional. If the source of sound power W becomes directional, the mean square sound pressure in Eqs. (3.48) and (3.46) will vary with direction, and the sound power W can only be obtained from Eqs. (3.41) and (3.47) by measuring either the mean‐square pressure (p²_rms) all over a surface enclosing the source (in the far acoustic field, the far field) and integrating Eq. (3.47) over the surface, or by measuring the intensity all over the surface in the near or far acoustic field and integrating over the surface (Eq. (3.41)). We shall discuss source directivity in Section 3.9.