Sound radiation from exponentially growing and decaying surface waves

A simplified model problem is used to illustrate some of the parameters controlling the radiation of sound into an ambient medium due to the growth and subsequent decay of subsonic travelling waves, such as may occur via non-linear interactions in turbulent free shear flows. It is shown that substantial sound may be generated by apparently subsonic modes as a result of their growth and decay characteristics. Low frequency modes that saturate over a short timescale are the most effective radiators.