Classic theories for linear monopole noise generated by fluid injection have been established and can be found in several literature references. A simple example is the analytical expression of peak sound pressure at far field, which is shown to be proportional to the derivative of the mass flow rate history. However, in cases of high-intensity (impulsive) noise and for near-field regions, these theories become inapplicable. It was found that when the gas injection reached a certain level of intensity, a shock wave could form in the near field and the transient flow field evolved quickly with time. It is well known that a discontinuous mass flow rate history is a feature of a moving shock wave; therefore, the derivatives do not mathematically exist. With the presence of flow discontinuity, a different theory should be established or a different means of study should be used to investigate the properties of monopole impulse noise. By performing direct aeroacoustic simulations, the evolution and propagation of an impulse noise generated by a high-intensity monopole source were numerically investigated. The effects of a few key parameters of the monopole source on the leading pressure wave were determined. It was determined that monopole impulse noise is a physical process sandwiched between the linear acoustic waves generated by a pulsating sphere and the blast waves generated by weapons. Monopole impulse noise involves the degeneration of the shock waves into intense sound waves.

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