Long description of figure 1

This plot shows the pressure spectrum for operation of the test compressor at a blade passing frequency of 1024 Hz. The vertical axis is pressure amplitude ranging from 100 to 200 dB, and the horizontal axis is frequency from 0 to 2200 Hz. Spikes in amplitude are observed at 1024 and 2048 Hz, corresponding to one and two times the blade passing frequency. A wider, lower-amplitude peak, or hump, in pressure amplitude is seen near 600 Hz, or roughly one-half the blade passing frequency. This hump is characteristic of rotating instability.

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