Discrete tones in subsonic jet engine test cells

Jet Engine Test Cells used in testing large engines are often plagued by the generation of discrete frequency tones of up to 170 decibels necessitating emergency stoppage of tests to prevent infrastructure damage. The mechanism(s) behind these tones is not well understood and they present a significant issue to airlines, engine manufacturers' and test cell operators. This work provides an overview of typical u-shaped jet engine test cells before briefly presenting the mechanisms of jet impingement, edge tones, diffuser noise and room acoustics as well as the possibility of each of these mechanisms being a source of discrete tone noise in jet engine test cells. The limitations of noise prediction models applied to jet engine test cells is discussed for each aforementioned mechanism and where possible noise predictions are made. Select literature relating to test cell tone generation and super resonance is presented before examining duct resonance and finding it to be a likely source.