Shock formation and spectrum alteration in spherically diverging finite amplitude waves

It has been suggested that the characteristic farfield spectrum of a supersonic jet could be the result of nonlinear acoustic phenomena. Evidence from previous studies indicate that this may be the case; however, sound power levels comparable to those generated by an actual supersonic jet were not obtained. In this study, a 10 000-W acoustic source was installed in an anechoic chamber. Band limited random noise and dual frequency pure tones were generated. Spectrum alteration as a function of propagation distance and acoustic shock formation were observed for both cases. For the band limited random noise case, a continuous high-frequency spectrum with a shape inversely proportional to frequency was obtained in the farfield regardless of the initial spectrum shape. For dual frequency pure tone excitation a strong parametric effect was observed with sum and difference frequency components comparable in level to those of the primary frequencies. [Work supported by NASA Langley Research Center.]