  This paper clarifies the nature of sound spectra within subsonic jets. Three
problems, of increasing complexity, are presented. Firstly, a point source is
placed in a two-dimensional plug flow and the sound field is obtained
analytically. Secondly, a point source is embedded in a diverging axisymmetric
jet and the sound field is obtained by solving the linearised Euler equations.
Finally, an analysis of the acoustic waves propagating through a turbulent jet
obtained by direct numerical simulation is presented. In each problem, the
pressure or density field are analysed in the frequency-wavenumber domain. It
is found that acoustic waves can be classified into three main
frequency-dependent groups. A physical justification is provided for this
classification. The main conclusion is that, at low Strouhal numbers, acoustic
waves satisfy the d'Alembertian dispersion relation.
