Modal content of noise generated by a coaxial jet in a pipe

Noise generated by air flow through a coaxial obstruction in a long, straight pipe was studied for the modal characteristics of the noise field inside the pipe and downstream of the restriction. The measured modal pressure spectra were converted to modal power spectra and integrated over the frequency range 200-6000 Hz. The acoustic efficiency levels (acoustic power normalized by jet kinetic energy flow), when plotted vs. jet Mach number, showed a strong dependence on the ratio of restriction diameter to pipe diameter. The acoustic efficiency levels of the nozzles and orifices agreed closely, when the comparison was made using nozzle exit plane and orifice vena contracta conditions.