There is considerable interest in the question of whether coherent structures that govern the mixing process in turbulent jets also affect the radiation of acoustic energy. This study indicates that below a Reynolds number of 10^5, where coherent structures are most easily observed, the spectral properties of jet noise change significantly. The relative acoustic power is lower in this Reynolds number range and the noise spectra scale with Helmholtz number rather than Strouhal number. There appears to be definite compressibility effects on the large scale structure and flow development. Similar Mach number effects on the fine scale turbulence are not noted.