  The radiating part of a circular acoustic source is determined on the basis
of an exact analysis of the radiation properties of a source with angular
dependence $\exp \J n\theta$ and arbitrary radial dependence. It is found that
the number of degrees of freedom in the radiated field is no greater than
$k-n$, where $k$ is the wavenumber. The radiating part of the source at low
frequency is the wavenumber. The radiating part of the source at low frequency
is explicitly stated and used to analyze noise cancellation. The results are
applied to the identification of sources in jet noise and an explanation for
the low order structure of jet noise fields is proposed.
