An experimental study is conducted investigating the characteristics of tones due to ‘guided jet waves’ in the forward arc of jet noise radiation fields. These spectral peaks disappear in the far acoustic field in the aft and sideline directions, which is why they went unnoticed in decades of jet noise measurements. However, it is clearly shown in the present study that they radiate in the forward arc (shallow upstream polar locations in the approximate range θ < 45°). Jet noise spectra in the forward arc, data on which had been lacking in the literature, are not smooth but are characterised by these peaks. This is found for high subsonic to supersonic jets up to the highest jet Mach number covered in the experiment ( M J ≈ 1.9), for round and rectangular as well as convergent–divergent nozzles. In heated jets, these spectral peaks appear to get weakened especially around transonic conditions, whereas they clearly persist in supersonic conditions.
Zaman et al. (Fri,) studied this question.