Comments on convective amplification of sound sources in flows

The paper gives several general comments on convective amplification of sound sources in moving fluids. It is commonly believed that, together with refraction in inhomogeneous media, convective amplification has the major effect on the far-field directivities of the sources located inside the flow. The effect is mathematically interpreted to be caused by the convection terms on the left-hand sides of the acoustic equations, such as the classical convective wave equation. It is argued here that this interpretation is inaccurate and that convective amplification should not be treated as a propagation phenomenon in a similar manner as refraction. Moreover, it seems that in certain practical cases, such as low Mach number jet noise, convective amplification has only a secondary effect on the far-field directivity of aerodynamic sources, compared to, for example, diffraction from the solid bodies, which are usually located in their vicinity