F-22A aircrafts usually are operated at four engine power settings.They are the idle,intermediate power,military power
and afterburner.There is a direct correlation between engine power and the rate of fuel burn.At idle power,a significant
component of radiated sound comes from the tones and narrow broadband peaks of turbo machinery noise.This is not of
interest to the present investigation.At intermediate power,jet noise becomes the dominant noise of the engine.Noise
spectral data at intermediate power,military power and afterburner at 901 and 1301 are available in Wall et al. [3]. Fig. 12
shows the noise spectra in the 1301 direction.Obviously,the noise level increases significantly at higher power settings.The
dotted curve in this figure is the similarity noise spectrum of the large turbulence structures of laboratory jets(see Fig. 2).
The dotted curve matches the intermediate power noise spectrum very well.This suggests that at intermediate power,the
noise of an F-22A aircraft is mainly turbulent mixing noise,the same as a laboratory jet. Fig. 13 shows an attempt to compare the standard large turbulence structures noise spectrum with the measured noise spectra at military power and afterburner. The agreement is poor. This indicates that the large turbulence structures in the jet flow are some what different from those of standard laboratory jets. One reason is that laboratory jets are comprised entirely of gases. The jet flow of an F-22 jet at afterburner, on the other hand, is laden with fuel drop lets. This affects the turbulent mixing process of the jet and consequently the radiated noise .It is to be noted that at the higher power settings the noise spectra have double peaks.This is a telltale sign that there could be more than one dominant noise component as the rate of fuel burn increases significantly.